Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Report No. R20/50 ISBN 978-1-99-002707-9 (print) 978-1-99-002708-6 (web)

Tina Bayer Adrian Meredith

September 2020 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Report No. R20/50 ISBN 978-1-99-002707-9 (print) 978-1-99-002708-6 (web)

Tina Bayer Adrian Meredith

September 2020 Name Date Prepared by: Tina Bayer & Adrian Meredith May 2019 Internal reviewed by: Graeme Clarke June 2019 & August 2020 External review by: David Kelly- Cawthron Institute July 2019 Approved by: Tim Davie October 2020 Director Science Group

Report No. R20/50 ISBN 978-1-99-002707-9 (print) 978-1-99-002708-6 (web)

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Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Executive summary

Background: Canterbury’s high-country lakes are highly valued for their biodiversity values and cultural significance, as well as recreation and visual amenities. Several of our high-country lakes are still relatively undisturbed ecosystems with significantly intact ecological values. However, with increasing development and land use intensification, as well as changes in climate, some of our lakes have undergone, or are likely to undergo, significant changes in level regimes, water quality, and ecological condition.

The problem: Before establishing the high-country lakes monitoring programme in 2005, we had limited knowledge about the state of our high-country lakes and could not consistently assess potential changes in lake water quality and lake ecological condition.

What we did: We sampled more than 30 of our high-country lakes each summer (December-May) from 2005 (or 2008) to 2019 for nutrients, algal biomass and turbidity. The lakes were also surveyed for their ecological condition, i.e. state of the community of aquatic plants (macrophytes) and presence of exotic weed beds, at regular intervals. This report presents a summary of the state and trends in water quality in Canterbury’s high-country lakes with respect to Trophic Level Index (TLI), total nitrogen (TN), total phosphorus (TP), phytoplankton biomass (chl-a) and turbidity, as well as condition of the aquatic macrophyte community (LakeSPI) and recreational water quality (E. coli). Where information on lake cultural health was available, it was also included.

What we found: Our large, deep lakes generally have very good water quality. Of the small to medium sized lakes, the majority (20 out of 25 lakes) have nutrient concentrations or algal biomass that are now above the objectives set in the Canterbury Land and Water Regional Plan. While the condition of the five large, low-nutrient lakes (such as Lakes Ōhau and Sumner) has remained steady or improved, the water quality of 10 of the 25 smaller lakes is deteriorating (i.e. concentrations of either nutrients, phytoplankton biomass and/or TLI were increasing). Many lakes with deteriorating water quality are located in the Ashburton Lakes Area and the Upper Waimakariri. Lakes of concern include Lakes Pearson, Grasmere, Denny, Clearwater, Heron and Kellands Pond. Lake Denny, Kellands Pond and the Māori Lakes in particular have significant agricultural sources of nutrients in their catchments.

In about half of the surveyed lakes the macrophyte community was in high or excellent condition (according to LakeSPI classification). Slightly more lakes are improving in ecological condition than deteriorating. Popular swimming lakes that were monitored for recreational water quality were generally safe to swim in. Out of 11 lakes currently assessed for cultural health only two were found to be in poor cultural health, the remainder was approximately equally divided between moderate and good.

What does it mean? Increasing trends in nutrients, turbidity and/or phytoplankton biomass, and the failure of many smaller lakes to meet the water quality objectives set in the Canterbury Land and Water Regional Plan, highlight the importance to further minimise or reduce nutrient and sediment losses to lakes, particularly in the Sensitive Lake Zones outlined in the Canterbury Land and Water Regional Plan.

This report also highlights some limitations of our current high-country lake monitoring programme. We recommend a review of the high-country lakes monitoring programme to identify opportunities to address these limitations.

Environment Canterbury Technical Report i Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

How we have considered climate change: Water quality in our high-country lakes is influenced by local climate, and climate change is likely to impact long-term water quality and aquatic ecosystem health. However, this report only analyses 15 years of data, and thus, it is not an assessment of the long-term impacts of climate change on our high-country lakes.

ii Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table of contents

Executive summary ...... i

1 Introduction ...... 1

2 Methods ...... 2

3 Limitations of current sampling programme ...... 15

4 High-country lakes state and trends ...... 16 4.1 Regional climate summary, 2004-2019 ...... 16 4.2 Hurunui – Waiau Water Management Zone ...... 17 4.2.1 Loch Katrine – Waitetemoroiti ...... 17 4.2.2 Lake Sumner - Hoka Kura ...... 21 4.2.3 Lake Taylor ...... 25 4.2.4 Lake Sheppard ...... 29 4.2.5 Lake Marion ...... 31 4.2.6 Lake Mason ...... 33 4.3 Selwyn – Waihora Water Management Zone ...... 35 4.3.1 Lake Sarah ...... 39 4.3.2 Lake Grasmere ...... 43 4.3.3 Lake Pearson - Moana Rua ...... 49 4.3.4 Lake Hawdon ...... 54 4.3.5 Lake Lyndon ...... 58 4.3.6 Lake Georgina...... 62 4.3.7 Lake Ida ...... 66 4.3.8 Lake Selfe ...... 70 4.3.9 Lake Coleridge ...... 74 4.3.10 Lake Evelyn ...... 80 4.3.11 Lake Catherine ...... 82 4.3.12 Lake Henrietta ...... 83 4.4 Ashburton Management Water Zone...... 85 4.4.1 Lake Emily ...... 89 4.4.2 Māori Lake (East/Front) - Ō Tū Wharekai (East) ...... 92 4.4.3 Māori Lake (West/Back) - Ō Tū Wharekai (West) ...... 97 4.4.4 Lake Denny ...... 100 4.4.5 Lake Heron - Ō Tū Roto ...... 104 4.4.6 Lake Emma – Kirihonuhonu ...... 108 4.4.7 Lake Camp – Ōtautari ...... 112 4.4.8 Lake Clearwater - Te Puna a Taka ...... 117 4.5 Orari Temuka Opihi Pareora Water Management Zone ...... 122 4.5.1 Lake Opuha ...... 122 4.6 Upper Waitaki Water Management Zone ...... 124 4.6.1 Lake McGregor - Whakarukomoana or Otetoto ...... 125 4.6.2 Lake Alexandrina – Takamoana or Te Kaupururu ...... 127 4.6.3 Lake Tekapo/Takapō ...... 133 4.6.4 Lake Pukaki ...... 138 4.6.5 Lake Middleton ...... 142 4.6.6 Lake Ōhau ...... 144 4.6.7 Lake Benmore ...... 148 4.6.8 Lake Aviemore ...... 158 4.6.9 Kellands Pond ...... 163

Environment Canterbury Technical Report iii Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

5 Regional summary of state and trends ...... 166 5.1 Regional state of lake water quality ...... 166 5.1.1 Comparison to LWRP objectives ...... 166 5.1.2 NPS-FM 2014 Attribute State ...... 174 5.1.3 Nutrient limitation ...... 177 5.1.4 Turbidity ...... 178 5.1.5 Lake water colour ...... 179 5.1.6 Presence of the exotic organism Lindavia, and the effect of “lake snow” .... 181 5.1.7 Cultural health ...... 182 5.2 Regional trends in water quality ...... 183 5.2.1 Trends in phytoplankton biomass, turbidity and nutrients ...... 183 5.2.2 Patterns in Trophic Level Index ...... 187 5.3 Summary of high-country lakes state and trends ...... 189

6 Conclusions ...... 190

7 Recommendations ...... 191

8 References ...... 192

Appendix 1: Protocol for sampling Lindavia intermedia ...... 197

Appendix 2: Lake catchment proportional cover by LCDB v4.0 (Landcare Research 2012) land cover categories for 27 Canterbury high- country lakes ...... 199

Appendix 3: Regional climate summary ...... 200

Appendix 4: Genera of benthic mats identified in Smith et al., 2011 ...... 207

iv Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

List of Figures

Figure 2-1: Forel-Ule Scale, colour distribution ...... 6 Figure 2-2: High-country lakes sampling locations – Overview including zone boundaries ...... 7 Figure 2-3: Maps of lakes sampled in the Hurunui-Waiau Water Management Zone ...... 8 Figure 2-4: Maps of lakes sampled in the Selwyn-Waihora Water Management Zone ...... 8 Figure 2-5: Maps of lakes sampled in the Selwyn-Waihora Water Management Zone near Lake Coleridge ...... 9 Figure 2-6: Maps of lakes sampled in the Ashburton Water Management Zone ...... 10 Figure 2-7: Maps of lakes sampled in the Upper Waitaki Water Management Zone – Lakes Tekapo, Alexandrina, Pukaki, Ōhau, Middleton, and Kellands Pond...... 11 Figure 2-8: Maps of lakes sampled in the Upper Waitaki Water Management Zone – Lakes Benmore and Aviemore ...... 12 Figure 4-1: Sampling location in Loch Katrine ...... 17 Figure 4-2: TP, TN, chl-a concentrations and turbidity in Loch Katrine from 2005-2019 ...... 19 Figure 4-3: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Loch Katrine, 2005-2019...... 20 Figure 4-4: Landsat derived water colour data for Loch Katrine, 2013-2018 ...... 21 Figure 4-5: Sampling location in Lake Sumner (SQ30079) ...... 21 Figure 4-6: TP, TN, chl-a concentrations and turbidity in Lake Sumner from 2005-2019 ...... 23 Figure 4-7: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Sumner, 2005-2019...... 24 Figure 4-8: Landsat derived water colour data for Lake Sumner, 2013-2018 ...... 25 Figure 4-9: Sampling locations in Lake Taylor (bottom) and Lake Sheppard (top) ...... 25 Figure 4-10: TP, TN, chl-a concentrations and turbidity in Lake Taylor from 2005-2017 ...... 27 Figure 4-11: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Taylor, 2005-2019...... 28 Figure 4-12: Landsat derived water colour data for Lake Taylor, 2013-2018 ...... 29 Figure 4-13: Landsat derived water colour data for Lake Sheppard, 2013-2018 ...... 30 Figure 4-14: Sampling location in Lake Marion ...... 31 Figure 4-15: Landsat derived water colour data for Lake Marion, 2013-2018 ...... 32 Figure 4-16: Sampling location in Lake Mason (Little lake Mason and Lake Mason) ...... 33 Figure 4-17: Landsat derived water colour data for Lake Mason (Northern basin, LID 39297), 2013-2018 ...... 34 Figure 4-18: Land Cover Classification (LCDB 4) near the Waimakariri lakes...... 37 Figure 4-19: Land Cover Classification (LCDB 4) near Lake Coleridge (Ryton Lakes) ...... 38 Figure 4-20: Sampling location in Lake Sarah ...... 39 Figure 4-21: TP, TN, chl-a concentrations and turbidity in Lake Sarah from 2005-2019 ...... 41 Figure 4-22: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Sarah, 2005-2019...... 41 Figure 4-23: Landsat derived water colour data for Lake Sarah, 2013-2018 ...... 42 Figure 4-24: Sampling location in Lake Grasmere ...... 43 Figure 4-25: Temperature profile of Lake Grasmere December 1988 ...... 44 Figure 4-26: Bottom and surface temperature in Lake Grasmere, 1988-1990 ...... 44 Figure 4-27: TP, TN, chl-a concentrations and turbidity in Lake Grasmere from 2005-2019 ...... 46 Figure 4-28: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Grasmere, 2005-2019...... 47 Figure 4-29: Landsat derived water colour data for Lake Grasmere, 2013-2018...... 48

Environment Canterbury Technical Report v Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-30: Lake Grasmere in March 2018 showing high turbidity after significant discharge from Ribbonwood Stream ...... 48 Figure 4-31: Sampling location in Lake Pearson ...... 49 Figure 4-32: TP, TN, chl-a concentrations and turbidity in Lake Pearson from 2005-2019 ...... 51 Figure 4-33: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Pearson, 2005-2019...... 51 Figure 4-34: Landsat derived water colour data for Lake Pearson, 2013-2018 ...... 52 Figure 4-35: Sampling location in Lake Hawdon ...... 54 Figure 4-36: TP, TN, chl-a concentrations and turbidity in Lake Hawdon from 2005-2019 ...... 55 Figure 4-37: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Hawdon, 2005-2019...... 56 Figure 4-38: Landsat derived water colour data for Lake Hawdon, 2013-2018 ...... 57 Figure 4-39: Sampling location in Lake Lyndon...... 58 Figure 4-40: TP, TN, chl-a concentrations and turbidity in Lake Lyndon from 2005-2019 ...... 60 Figure 4-41: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Lyndon, 2005-2019...... 60 Figure 4-42: Landsat derived water colour data for Lake Lyndon, 2013-2018 ...... 61 Figure 4-43: Sampling location in Lake Georgina ...... 62 Figure 4-44: TP, TN, chl-a concentrations and turbidity in Lake Georgina from 2005-2019 ...... 64 Figure 4-45: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Georgina, 2005-2019...... 64 Figure 4-46: Landsat derived water colour data for Lake Georgina, 2013-2018 ...... 65 Figure 4-47: Sampling location in Lake Ida ...... 66 Figure 4-48: TP, TN, chl-a concentrations and turbidity in Lake Ida from 2005-2019 ...... 68 Figure 4-49: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Ida, 2005-2019 ...... 68 Figure 4-50: Landsat derived water colour data for Lake Ida, 2013-2018 ...... 69 Figure 4-51: Sampling location in Lake Selfe ...... 70 Figure 4-52: TP, TN, chl-a concentrations and turbidity in Lake Selfe from 2005-2019...... 72 Figure 4-53: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Selfe, 2005-2019...... 72 Figure 4-54: Landsat derived water colour data for Lake Selfe, 2013-2018 ...... 73 Figure 4-55: Sampling location in Lake Coleridge (SQ31045) ...... 74 Figure 4-56: Depth distribution of temperature (top) and chl-a (bottom) in Lake Coleridge, 1993- 1994 ...... 76 Figure 4-57: TP, TN, chl-a concentrations and turbidity in Lake Coleridge from 2005-2019 ...... 77 Figure 4-58: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Coleridge, 2005-2019...... 78 Figure 4-59: Landsat derived water colour data for Lake Coleridge, 2013-2018 ...... 79 Figure 4-60: Sampling location in Lake Evelyn ...... 80 Figure 4-61: Landsat derived water colour data for Lake Evelyn, 2013-2018 ...... 81 Figure 4-62: Sampling locations in Lake Catherine ...... 82 Figure 4-63: Landsat derived water colour data for Lake Catherine, 2013-2018 ...... 83 Figure 4-64: Sampling locations in Lake Henrietta ...... 83 Figure 4-65: Landsat derived water colour data for Lake Henrietta, 2013-2018 ...... 84 Figure 4-66: Land Cover Classification (LCDB 4) in the Ashburton Lakes basin (Lakes Heron, Emily and Māori Lakes) ...... 86 Figure 4-67: Land Cover Classification (LCDB 4) in the Ashburton Lakes basin (Lakes Clearwater, Camp, Emma and Denny)...... 87 Figure 4-68: Public conservation land in the Ashburton Lakes Basin...... 88 vi Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-69: Sampling location in Lake Emily...... 89 Figure 4-70: TP, TN, chl-a concentrations and turbidity in Lake Emily from 2008-2019 ...... 90 Figure 4-71: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Emily, 2008-2019...... 91 Figure 4-72: Landsat derived water colour data for Lake Emily, 2013-2018 ...... 92 Figure 4-73: Sampling location in the Māori Lakes ...... 92 Figure 4-74: TP, TN, chl-a concentrations and turbidity in Māori Lake (East), 2008-2017 ...... 94 Figure 4-75: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in the East Māori Lake, 2005-2019...... 95 Figure 4-76: Landsat derived water colour data for Māori Lake (East), 2013-2018 ...... 96 Figure 4-77: TP, TN, chl-a concentrations and turbidity in Māori Lake (West), 2008-2017 ...... 98 Figure 4-78: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Māori Lake (West), 2008-2019...... 98 Figure 4-79: Landsat derived water colour data for Māori Lake (West), 2013-2018 ...... 99 Figure 4-80: Sampling location in Lake Denny ...... 100 Figure 4-81: TP, TN, chl-a concentrations and turbidity in Lake Denny, 2013-2019...... 101 Figure 4-82: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Denny, 2013-2019...... 101 Figure 4-83: Landsat derived water colour data for Lake Denny 2013-2018 ...... 102 Figure 4-84: Lake Denny (right) and Lake Emma (left) in March 2018 ...... 103 Figure 4-85: Sampling location in Lake Heron...... 104 Figure 4-86: TP, TN, chl-a concentrations and turbidity in Lake Heron 2005-2019 ...... 105 Figure 4-87: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Heron, 2005-2017...... 106 Figure 4-88: Landsat derived water colour data for Lake Heron, 2013-2018 ...... 107 Figure 4-89: Lake Heron lake level variation from January 2007 to 2016 ...... 107 Figure 4-90: Sampling location in Lake Emma ...... 108 Figure 4-91: TP, TN, chl-a concentrations and turbidity in Lake Emma 2005-2019 ...... 109 Figure 4-92: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Emma, 2005-2019...... 110 Figure 4-93: Landsat derived water colour data for Lake Emma, 2013-2018 ...... 111 Figure 4-94: Sampling location in Lake Camp (bottom) and Lake Clearwater (top) ...... 112 Figure 4-95: TP, TN, chl-a concentrations and turbidity in Lake Camp 2005-2019 ...... 114 Figure 4-96: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Camp, 2004-2019...... 115 Figure 4-97: Landsat derived water colour data for Lake Camp, 2013-2018 ...... 116 Figure 4-98: E. coli in Lake Camp 2004-2019 ...... 116 Figure 4-99: TP, TN, chl-a concentrations and turbidity in Lake Clearwater from 2005-2019 ...... 118 Figure 4-100: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Clearwater, 2004-2019...... 118 Figure 4-101: Landsat derived water colour data for Lake Clearwater, 2013-2018 ...... 119 Figure 4-102: E. coli in Lake Clearwater 2004-2019 ...... 120 Figure 4-103: Schematic of estimated loads in Lake Clearwater waterways ...... 121 Figure 4-104: Sampling location in Lake Opuha...... 122 Figure 4-124: E. coli counts at Lake Opuha, 2013-2019 ...... 123 Figure 4-104: Sampling locations in Lake McGregor ...... 125 Figure 4-105: Landsat derived water colour data for Lake McGregor, 2013-2018...... 126 Figure 4-106: Cultural health assessment of Lake McGregor ...... 126 Figure 4-107: Sampling locations in Lakes Alexandrina (left) and Tekapo (right) ...... 127

Environment Canterbury Technical Report vii Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-108: TP, TN, chl-a concentrations and turbidity in Lake Alexandrina from 2005-2019 ...... 129 Figure 4-109: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Alexandrina, 2005-2017...... 130 Figure 4-110: Landsat derived water colour data for Lake Alexandrina, 2013-2018 ...... 131 Figure 4-111: E.coli counts at Lake Alexandrina at bottom huts, 2013-2019 ...... 131 Figure 4-112: Cultural health assessment of the outlet of Lake Alexandrina ...... 132 Figure 4-113: TP, TN, chl-a concentrations and turbidity in Lake Tekapo from 2005-2019 ...... 134 Figure 4-114: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Tekapo, 2005-2017...... 135 Figure 4-115: Landsat derived water colour data for Lake Tekapo, 2013-2018 ...... 136 Figure 4-116: E. coli counts at Lake Tekapo beach, 2002-2019 ...... 136 Figure 4-117: Lake level of Lake Tekapo, 2004-2017...... 137 Figure 4-118: Sampling location in Lake Pukaki...... 138 Figure 4-119: TP, TN, chl-a concentrations and turbidity in Lake Pukaki from 2006-2019 ...... 139 Figure 4-120: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Pukaki, 2005-2019...... 140 Figure 4-121: Landsat derived water colour data for Lake Pukaki, 2013-2018 ...... 140 Figure 4-122: Lake level variation of Lake Pukaki, 2004-2017...... 141 Figure 4-123: Sampling location in Lakes Middleton ...... 142 Figure 4-124: E. coli counts at Lake Middleton, 2013-2019 ...... 143 Figure 4-125: Landsat derived water colour data for Lake Middleton, 2013-2018 ...... 143 Figure 4-126: Sampling location in Lake Ōhau (SQ32909) ...... 144 Figure 4-127: TP, TN, chl-a concentrations and turbidity in Lake Ōhau from 2006-2019 ...... 145 Figure 4-128: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Ōhau, 2006-2017...... 146 Figure 4-129: Landsat derived water colour data for Lake Ōhau, 2013-2018 ...... 147 Figure 4-130: Lake level of Lake Ōhau, 2005-2017...... 148 Figure 4-131: Sampling locations in Lake Benmore: near dam (bottom), Haldon Arm (top) and Ahuriri Arm (left) ...... 148 Figure 4-132: TP, TN, chl-a concentrations and turbidity in Lake Benmore from 2010-2019 ...... 152 Figure 4-133: Inter-annual variation TLI of Lake Benmore, 2006-2019 ...... 154 Figure 4-134: Flow in the Ahuriri River at South Diadem from 2005-2019 ...... 154 Figure 4-135: Landsat derived water colour data for Lake Benmore, 2013-2018 ...... 155 Figure 4-136: E. coli counts in Lake Benmore at Pumpkin Bay (top) and Sailors Cutting (bottom), 2002-2019 ...... 156 Figure 4-137: Lake level variation of Lake Benmore, 2004-2017 ...... 156 Figure 4-138: Trend in Dissolved Inorganic Nitrogen (DIN – mg/L) over time in the Ahuriri River at Ben Omar ...... 157 Figure 4-139: Sampling location in Lake Aviemore ...... 158 Figure 4-140: TP, TN, chl-a concentrations and turbidity in Lake Aviemore from 2010-2019 ...... 159 Figure 4-141: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual mean results in Lake Aviemore, 2010-2017...... 159 Figure 4-142: Landsat derived water colour data for Lake Aviemore, 2013-2018 (n = 78): ...... 160 Figure 4-143: E. coli counts in Lake Aviemore at Campground (top), Waitangi (middle) and Loch Laird (bottom), 2002-2019 ...... 161 Figure 4-144: Lake level variation in Lake Aviemore, 2005-2017...... 162 Figure 4-145: Sampling locations in Kellands Pond ...... 163 Figure 4-146: TP, TN, chl-a concentrations and turbidity in Kellands Pond from 2004-2017 ...... 164

viii Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 5-1: Trophic Level Index of 35 high-country lakes in Canterbury sorted on a basis of TLI (average from 2015-2019) ...... 168 Figure 5-2: 5 year mean chl-a concentrations in 35 high-country lakes in Canterbury (2015- 2019) ...... 170 Figure 5-3: Mean total nitrogen concentrations in 35 high-country lakes in Canterbury (2015- 2019) ...... 171 Figure 5-4: Mean total phosphorus concentrations in 35 high-country lakes in Canterbury (2015- 2019) ...... 171 Figure 5-5: Range and median of chl-a concentrations (µg/L) in 35 high-country lakes in Canterbury (2015-2019) ...... 174 Figure 5-6: Range and median of total nitrogen concentrations (µg/L) in seasonally stratified high-country lakes in Canterbury (2015-2019) ...... 176 Figure 5-7: Range and median of total nitrogen concentrations (µg/L) in polymictic high-country lakes in Canterbury (2015-2019) ...... 176 Figure 5-8: Range and median of total phosphorus concentrations (µg/L) in 35 high-country lakes in Canterbury (2015-2019) ...... 177 Figure 5-9: Range and median of TN:TP ratio for 35 high-country lake sites in Canterbury (2013- 2017)...... 178 Figure 5-10: Range and median of turbidity at 35 high-country lakes in Canterbury (2013-2017) .. 179 Figure 5-11: Range and median of Landsat-derived dominant wavelength in selected Canterbury lakes (2013-2018) ...... 180 Figure 5-12: Trends chl-a concentrations in 25 high-country lakes in Canterbury ...... 184 Figure 5-13: Trends TN concentrations in 25 high-country lakes in Canterbury ...... 185 Figure 5-14: Trends in TP concentrations in 25 high-country lakes in Canterbury ...... 185 Figure 5-15: Trends in turbidity in 25 high-country lakes in Canterbury ...... 186

Environment Canterbury Technical Report ix Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

List of Tables

Table 2-1: Description of trophic states ...... 2 Table 2-2: LakeSPI scores and categories ...... 3 Table 2-3: List of high-country lakes monitored...... 13 Table 4-1: Trophic Level Index and attribute states for Loch Katrine from 2005-2019 ...... 18 Table 4-2: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Loch Katrine ...... 19 Table 4-3: Loch Katrine, LakeSPI overview, 1987-2016 ...... 20 Table 4-4: Trophic Level Index and attribute states for Lake Sumner from 2005-2019 ...... 22 Table 4-5: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Sumner ...... 23 Table 4-6: Lake Sumner, LakeSPI overview, 1987-2016 ...... 24 Table 4-7: Trophic Level Index and attribute states for Lake Taylor from 2005-2019 ...... 26 Table 4-8: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Taylor ...... 27 Table 4-9: Lake Taylor, LakeSPI overview, 1987-2016 ...... 28 Table 4-10: Trophic Level Index and attribute states for from 2010-2017 ...... 29 Table 4-11: Lake Sheppard, LakeSPI overview, 1987-2016 ...... 30 Table 4-12: Trophic Level Index and attribute states for from 2010-2019 ...... 31 Table 4-13: Trophic Level Index and attribute states for Lake Mason from 2010-2017 ...... 33 Table 4-14: Lake Mason, LakeSPI overview, 2011-2016 ...... 34 Table 4-15: Trophic Level Index and attribute states for Lake Sarah from 2005-2019 ...... 40 Table 4-16: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Sarah ...... 40 Table 4-17: Lake Sarah, LakeSPI overview ...... 42 Table 4-18: Trophic Level Index and attribute states for Lake Grasmere from 2005-2019 ...... 45 Table 4-19: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Grasmere ...... 46 Table 4-20: Lake Grasmere, LakeSPI overview ...... 47 Table 4-21: Trophic Level Index and attribute states for Lake Pearson from 2005-2019 ...... 50 Table 4-22: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Pearson ...... 51 Table 4-23: Lake Pearson, LakeSPI overview, 1984-2013 ...... 52 Table 4-24: Trophic Level Index and attribute states for Lake Hawdon from 2005-2019 ...... 55 Table 4-25: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Hawdon ...... 56 Table 4-26: Lake Hawdon, LakeSPI overview...... 56 Table 4-27: Trophic Level Index and attribute states for Lake Lyndon from 2005-2019 ...... 59 Table 4-28: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Lyndon...... 59 Table 4-29: Lake Lyndon, LakeSPI overview, 1984-2013 ...... 61 Table 4-30: Trophic Level Index and attribute states for Lake Georgina from 2005-2019 ...... 63 Table 4-31: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Georgina ...... 63 Table 4-32: Lake Georgina, LakeSPI overview, 1984-2013 ...... 65 Table 4-33: Trophic Level Index and attribute states for Lake Ida from 2005-2019 ...... 67 Table 4-34: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Ida ...... 67 Table 4-35: Lake Ida, LakeSPI overview ...... 69 Table 4-36: Trophic Level Index and attribute states for Lake Selfe from 2005-2019 ...... 71 Table 4-37: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Selfe ...... 71 Table 4-38: Lake Selfe, LakeSPI overview, 2010-2014 ...... 73 Table 4-39: Trophic Level Index and attribute states for Lake Coleridge from 2005-2019 ...... 76 Table 4-40: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Coleridge ...... 77 Table 4-41: Lake Coleridge, LakeSPI overview, 1978-2014 ...... 78 Table 4-42: Trophic Level Index and attribute states for Lake Evelyn from 2014-2019 ...... 80

x Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-43: Lake Evelyn, LakeSPI overview, 2010-2014 ...... 81 Table 4-44: Trophic Level Index and attribute states for Lake Catherine from 2014-2019 ...... 82 Table 4-45: Trophic Level Index and attribute states for Lake Henrietta from 2014-2019 ...... 84 Table 4-46: Trophic Level Index and attribute states for from 2005-2019 ...... 90 Table 4-47: Seasonal Mann-Kendall Trend Test Results, 2008-2019, Lake Emily...... 90 Table 4-48: Lake Emily, LakeSPI overview, 2007-2017 ...... 91 Table 4-49: Trophic Level Index and attribute states for the Māori Lake (East) from 2008-2019 .... 93 Table 4-50: Seasonal Mann-Kendall Trend Test Results, 2008-2019, Front (East) Māori Lake ...... 94 Table 4-51: Māori Lake (East), LakeSPI overview, 2007-2017 ...... 95 Table 4-52: Trophic Level Index and attribute states for from 2005-2019, Māori Lake West ...... 97 Table 4-53: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Māori Lake West ...... 97 Table 4-54: Māori Lake (West), LakeSPI overview, 2007-2017 ...... 99 Table 4-55: Trophic Level Index and attribute states for Lake Denny from 2013-2019 ...... 100 Table 4-56: Lake Denny, LakeSPI overview, 2007-2017 ...... 102 Table 4-57: Trophic Level Index and attribute states for from 2005-2019 ...... 105 Table 4-58: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Heron...... 106 Table 4-59: Lake Heron, LakeSPI overview, 1982-2017 ...... 106 Table 4-60: Trophic Level Index and attribute states for Lake Emma from 2005-2019 ...... 109 Table 4-61: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Emma ...... 110 Table 4-62: Lake Emma, LakeSPI overview, 2007-2017 ...... 111 Table 4-63: Trophic Level Index and attribute states for Lake Camp from 2005-2019 ...... 113 Table 4-64: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Camp ...... 114 Table 4-65: Lake Camp, LakeSPI overview, 1982-2012 ...... 115 Table 4-66: Trophic Level Index and attribute states for Lake Clearwater from 2005-2019 ...... 117 Table 4-67: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Clearwater ...... 118 Table 4-68: Lake Clearwater, LakeSPI overview, 1982-2017 ...... 119 Table 4-69: Trophic Level Index and attribute states for Lake Opuha from 2016-2019 ...... 123 Table 4-69: Trophic Level Index and attribute states for Lake McGregor from 2012-2013 ...... 125 Table 4-70: Lake McGregor, LakeSPI overview, 1982-2017 ...... 126 Table 4-71: Mean nutrient concentrations in Lake Alexandrina, 1978-2017 ...... 128 Table 4-72: Trophic Level Index and attribute states for Lake Alexandrina from 2005-2019 ...... 128 Table 4-73: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Alexandrina ...... 129 Table 4-74: Lake Alexandrina, LakeSPI overview, 1982-2017 ...... 130 Table 4-75: Trophic Level Index and attribute states for Lake Tekapo from 2005-2019 ...... 133 Table 4-76: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Tekapo ...... 134 Table 4-77: Lake Tekapo, LakeSPI overview 2012-2017 ...... 135 Table 4-78: Trophic Level Index and attribute states for Lake Pukaki from 2006-2019 ...... 139 Table 4-79: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Pukaki...... 140 Table 4-80: Trophic Level Index and attribute states for Lake Middleton from 2012-2018 ...... 142 Table 4-81: Lake Middleton, LakeSPI overview, 2012-2017 ...... 143 Table 4-82: Trophic Level Index and attribute states for Lake Ōhau from 2006-2019 ...... 145 Table 4-83: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Ōhau...... 146 Table 4-84: Lake Ōhau, LakeSPI overview, 1982-2017 ...... 147 Table 4-85: Trophic Level Index and attribute states for Lake Benmore (Ahuriri Arm) from 2010- 2019 ...... 150 Table 4-86: Trophic Level Index and attribute states for Lake Benmore (Near Dam) from 2010- 2019 ...... 150

Environment Canterbury Technical Report xi Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-87: Trophic Level Index and attribute states for Lake Benmore (Haldon Arm) from 2006- 2019 ...... 151 Table 4-88: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (Ahuriri Arm) ...... 151 Table 4-89: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (near dam) 151 Table 4-90: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (Haldon Arm) ...... 152 Table 4-91: Lake Benmore, LakeSPI overview ...... 155 Table 4-92: Trophic Level Index and attribute states for Lake Aviemore from 2005-2019 ...... 158 Table 4-93: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Aviemore ...... 159 Table 4-94: Lake Aviemore, LakeSPI overview, 2012-2017 ...... 160 Table 4-95: Overview of Kellands Pond TLI status from 2012 to 2017 ...... 164 Table 4-96: Attribute states for Kellands Pond in 2012 and 2013 ...... 164 Table 4-97: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Kellands Pond, shore ...... 164 Table 4-98: Kellands Pond, LakeSPI overview, 2012-2015 ...... 165 Table 5-1: Do lakes meet LWRP objectives? ...... 166 Table 5-2: Summary of frequency and magnitude (severity) of lake TLI score exceeding LWRP objectives ...... 169 Table 5-3: Summary of recreational water quality grades, 2015-2019 ...... 172 Table 5-4: LakeSPI Regional Overview ...... 173 Table 5-5: Attribute states for 5-year average from 2015 to 2019...... 175 Table 5-6: Lake snow presence in Canterbury high-country lakes ...... 182 Table 5-7: Summary of lake cultural health assessments ...... 183 Table 5-8: Trends in water quality 2007-2019 ...... 187

xii Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

1 Introduction Canterbury’s high-country lakes are highly valued and unique ecosystems with high biodiversity values. Several lakes have been recognised as ‘high naturalness water bodies’ in the Canterbury Land and Water Regional Plan. Many lakes are significant to the local iwi as mahinga kai and travelling areas, and are highly valued for their visual amenity values. A large number of lakes also have high recreational use including camping, fishing, swimming and boating. Several have large bach communities associated with them. In the past decades many lake catchments have undergone changes, including increasing tourism and recreational use, intensification in farming and irrigation, potential reduced water yields due to surface or ground- water abstraction and hydropower schemes. The lakes are also affected by local or regional scale climatic patterns and changes in climate. However, before establishing the high-country lakes monitoring programme in 2005, we had little consistent information about the state of these lakes and were not able to reliably comment or report on changes in lake water quality prior to this time. Prior to this the only consistent lakes dataset was from the NIWA lakes program (1992 to 1996) that aided the development of the Trophic Lake Index process (TLI), but only included a small number of our high- country lakes.

Lakes monitored in our programme range from small, shallow lakes such as Lakes Sarah, Hawdon and Emily, through to large, deep lakes such as Lakes Sumner, Coleridge, Tekapo and Ōhau. Across the region there is a wide range of lake water quality due to differences in lake morphology, hydrodynamics and catchment land cover. Water residence times in the lakes ranges from days to years. While the large, deep lakes have a long, stable summer stratification, shallow lakes are usually polymictic. Water residence times and lake stratification patterns are important drivers of lake water quality. Some lakes are situated in catchments recently highly development for pastoral agriculture, such as Kellands Pond and Lake Denny, while other such as Lakes Sumner and Marion are located in catchments with relatively natural native vegetation. The different water quality objectives set in the Canterbury Land and Water Regional Plan to some extent reflect the variability in catchment land use, lake morphology, residence times and seasonal stratification patterns. For the purpose of this report (and the Canterbury Land and Water Regional Plan) a high-country lake is located at an elevation greater than 400 m above mean sea level (Norton & Snelder, 2003). Natural lakes with a surface area greater than 8 km2 are ‘large high- country lakes’ (Hayward et al., 2009).

The purpose of this report is to summarise water quality and ecological state and trends observed in our high-country lakes since the establishment of the helicopter based sampling programme in 2005. We first present data on a lake-by-lake basis, then provide a regional summary of water quality, the ecological condition of the aquatic plant community, and an assessment against current national standards and the regional plan. Lakes included are situated in four water management zones, the Hurunui – Waiau, Ashburton, Selwyn – Waihora, and Upper Waitaki Water Management Zones. A summary is also provided for each water management zone.

Environment Canterbury Technical Report 1 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

2 Methods

Sampling Between December 2004 and April 2019 up to 34 high-country lakes were sampled monthly from late spring to autumn (between December and May) by grab sample from a low hovering helicopter. Samples were analysed for total phosphorus (TP), total nitrogen (TN), turbidity and phytoplankton biomass (as chlorophyll a). Samples were collected by securing a 1L bottle in a holder weighted such that it filled slowly while the bottle descended rapidly to a 10 metre depth. This ensured a depth integrated sample of approximately the top 10 m of water depth was achieved. Flags on the sampling rope visually indicated 5 and 10 metre depth for the operator to assist in rapid and accurate deployment. In lakes shallower than 10 m a sample from the first one metres of depth was obtained to avoid water contamination with bottom sediments or macrophytes. Sampling frequency aimed for 5 consecutive monthly samples per year. While we aimed to collect samples in monthly (4-weekly) intervals from December to April, weather and logistical considerations did not always permit this, and in some years samples from December could not be obtained and occasionally the last sample was delayed and obtained in May. The rationale behind the program design can be found in Meredith (2004).

Water quality indicators The Trophic Level Index (Table 2-3) was calculated according to the methods of Burns et al. (1999), however Secchi disk readings were not included as they could not be carried out from the helicopter. This meant that TLI values were obtained from three parameters (TLI3) rather than four parameters (TLI4). Turbidity was measured as a surrogate of Secchi disk readings to enable some comment on lake clarity.

Omission of such clarity readings from TLI was not considered a significant issue, as water clarity in many high-country lakes in Canterbury is impacted by glacial flour inputs, which has no implicit relationship to impacts of eutrophication or to trophic state. The TLI is based upon the four trophic indicators being strongly correlated or related to the same trophic processes. Therefore, with Secchi depth being strongly influenced by inorganic clarity drivers, it is preferable for this parameter to be excluded from TLI assessments in many Canterbury lakes. The annual TLI was calculated for the water year (i.e. July to June inclusive). We first calculated annual averages of TP, TN and chl-a, then converted these to an annual TLI score.

Table 2-1: Description of trophic states

TLI Tropic state General description <1 Ultra-microtrophic Practically pure, very clean, often have glacial sources 1-2 Microtrophic Very clean, often have glacial sources, very low nutrient concentrations 2-3 Oligotrophic Clear and blue, with low levels of nutrients and algae 3-4 Mesotrophic Moderate levels of nutrients and algae 4-5 Eutrophic Green and murky, with higher amounts of nutrients and algae 5-6 Supertrophic Very high nutrient enrichment and high algae growth >6 Hypertrophic Saturated in nutrients, highly fertile, excessive algae growth

LakeSPI surveys were carried out by the National Institute for Water and Atmospheric Research (NIWA) on behalf of Environment Canterbury in the summer months, according to the method outlined in Clayton & Edwards (2006). LakeSPI surveys assess the ecological condition of a lake’s submerged plant community. It involves SCUBA diving at five representative sites within a lake and recording the maximum depth of native and invasive submerged vegetation, the presence of native plant community types, the cover and height of key invasive plant species and the ratio of native to invasive species and the nature of invasive cover. This information is ‘scored’ to generate numerical values for native condition, invasive condition and overall lake condition.” (Clayton & Edwards, 2006). The Native Condition Index assesses the diversity and quality of indigenous plant communities. The higher this

2 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

index, the better the ecological condition of the lake. The Invasive Condition Index describes the degree of impact by invasive weed species. The higher this index, the poorer the lake condition. The LakeSPI Index is a summary index (Table 2-2), including both native condition and invasive condition.

Table 2-2: LakeSPI scores and categories Score LakeSPI category > 75% Excellent >50-75% High >20-50% Moderate >0-20% Poor 0% Non-vegetated

Statistical analyses Data transformation For trend analyses, values below detection limit sample values were transformed as recommended by Ballentine et al. (2012):  Values below the detection limit were replaced with one half of their individual detection limit if less than 40% of all data points were below detection limit or if there was no change in detection limit.  Values below the detection limit were replaced with the value of half the highest detection limit for all data points below detection limit if there were multiple detection limits, and between 40 and 70% of all data points were below the detection limit,  If more than 70% of data points were below the detection limit, then no trend analysis was carried out for this parameter.

For calculations of lake trophic level index (TLI) all data points below the detection limit were replaced with the value of half the detection limit. Consideration of raw (uncensored) data indicated that TP values in Lake Benmore were closer to the detection limit than half the detection limit. Consequently, TP results in Lake Benmore below the detection limit were assigned the detection limit value.

We also removed several data points as outliers from calculation of Trophic Level Index (TLI) and trend analyses because they fell outside of the normal probability data distribution and expected range. These data points were: a) TN, chl-a, TP Lake Marion in January 2017 were discarded as probable contamination or sampling error b) TP in  Benmore Haldon, 27/02/2012, 280 µg/L  Benmore Haldon, 16/03/2017, 74 µg/L  Lake Coleridge, 8/02/2013, 161 µg/L  Lake Sumner, 21/03/2014, 59 µg/L

Temporal trend analyses Statistical analyses of temporal trends were conducted with Time Trends (version 6.10, NIWA). We conducted a de-seasonalised trend analysis (seasonal Mann-Kendall trend tests) for chl-a, TP and TN. Trends were considered significant for p ≤ 0.05. Due to changes in sampling methodology, samples taken prior to 2007 were not included in the temporal trend analysis. Prior to this date, samples were not consistently taken over the top 10 m, but closer to or at the lake surface. Since 2007 they were taken consistently over the top 10 m in lakes deeper than 10 m. TLI scores calculated from the samples taken at/near the surface for many lakes were significantly lower in 2005 and 2006 compared with subsequent years data sampled over an integrated 10 m depth. Therefore, while it is possible that other climatic or anthropogenic factors influenced these observed differences, consistency in sampling methodology was considered critical when attempting to determine if lake state has changed over time.

Environment Canterbury Technical Report 3 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Boxplots of the data distributions were generated using the Python package ‘seaborn’. The box represents the 25th to 75th percentile (first to third quartile). Outliers (indicated as individual points) are observations that fall below Q1 − 1.5 IQR (interquartile range) or above Q3 + 1.5 IQR (i.e. the range indicated by the whiskers).

Statutory assessments Lake water quality was assessed according to a) Trophic Level Index (TLI)(Burns et al., 1999), LakeSPI (Lake Submerged Plant Indicators, Clayton & Edwards 2006), and microbial data (where available) as outlined the in Canterbury Land and Water Regional Plan (LWRP). b) Median annual concentration of TN, TP, chl-a, and E. coli (where available) according to the National Policy Statement – Freshwater (NPS-FM) (Ministry for the Environment, 2014).

Limitations of statutory assessments TLI scores presented here are based on seasonal data (Dec-May), not monthly sampling as recommended by Burns et al. (1999). However, since objectives for the LWRP were mostly based on the existing seasonal data, we consider a comparison of seasonal TLI to LWRP objectives as reasonable. The Canterbury LWRP does not clearly specify the amount of data points needed for TLI calculations for high-country lakes, or when samples should be taken. However, it specifically refers to Burns et al. (2000) for TLI calculations for coastal Lakes Ellesmere and Forsyth. We advise some caution in interpreting these trophic assessments as an indication of annual average state.

The results presented in this report are based on seasonal data (i.e. Dec-May), not annual data as suggested in the NPS-FM (2014). Thus, any assignment of attribute grades needs to be interpreted accordingly. As our results likely cover the most productive season in a proportion of the lakes sampled (and do not include winter samples when chl-a is usually lowest), the chl-a attribute grade is likely to be a conservative assessment. However, some nutrient-poor stratified lakes are likely to have an autumn or winter peak of chl-a. In addition, data from Lakes Alexandrina and Pearson (Burn & Rutherford, 1998) indicate that TP and TN concentrations do not necessarily follow the same annual pattern as chl-a. That is, if we had sampled the lakes for 12 months a year instead of only from December to May, annual averages would likely be lower for chl-a for shallow, mesotrophic lakes, but not necessarily deep nutrient-poor lakes and for TN and TP. While attribute grades (and TLI scores) obtained from monthly sampling are likely to be lower in shallow, mesotrophic lakes, we cannot confidently extrapolate our data without collecting monthly data. Furthermore, the current sampling program samples the top 10 m of the water column for most of our lakes, and a surface sample is taken for our shallow lakes. Phytoplankton production is seldom uniform throughout the water column, and it may be that the current sampling protocol does not produce a sample representative of the bulk of the water column.

Lindavia intermedia/Lake snow Along with the LakeSPI surveys, NIWA also carried out horizontal and vertical hauls using a 40 µm mesh size zooplankton net to test for the presence of Lindavia intermedia cells, the centric diatom that is thought to be responsible for ‘lake snow’ formation, as outlined in Appendix 1. While this method is likely to capture Lindavia cells that have been clustered in aggregates, single cells in lakes with low cell densities of Lindavia intermedia may not be retained. Thus, while it is reliable in confirming the presence of Lindavia intermedia, this method may yield ‘false negatives’ if cells were present but well dispersed.

Contact recreation Suitability for contact recreation was assessed against the Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas (Ministry of Health and Ministry for the Environment 2003) as outlined in the Canterbury LWRP. An attribute grade according to NPS-FM 2014 was also assigned for each year and the 5-year average from 2013 to 2017.

Lake colour Lake water colour is reported in this report as dominant wavelength and Forel-Ule (FU) index and was calculated from remote sensed from Landsat 8 OLI data. Data and graphs were supplied by Moritz Lehmann, University of Waikato. Unlike our helicopter-based water quality monitoring data, remote

4 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

sensed data is not restricted to the summer months (only to cloud-free days). Therefore, water colour data supplements our seasonal water quality data. Lake water colour varies between blue for the purest high-country lakes, to green for productive lakes, to brown or yellow moderately tannin-stained lakes, and highly turbid lakes with large amounts of dissolved or suspended inorganic material.

The Forel-Ule (FU) Index is an oceanography method to describe the colour of bodies of water, and has a long history in both limnology and oceanography (Novoa et al., 2013). It is similar to the Munsell colour scale, but designed specifically for water bodies. The FU Index scale ranges from 1-21; with 1 = blue, 10 = green, 21 = brown. Ranges between 1 and 7 indicate predominately blue water, 8-14 green and 15-21 brownish water (Figure 2-1). It was calculated from dominant wavelength according to table 6 in Novoa et al. (2013). The relationship between FU scale and dominant wavelength is also shown in Figure 2-1.

The dominant wavelength (nm) is the closest ‘pure’ colour to the whole polychromatic spectrum reflected by the lake water. A ‘pure’ colour is monochromatic light specified by a single wavelength. The dominant wavelength was calculated after van der Woerd and Wernand (2018) from Landsat 8 OLI data for a 3 by 3 pixel area near the centre of each lake (Lehmann et al., in preparation). In summary, Landsat remote sensing data is analysed to yield chromaticity coordinates in the International Commission of Illumination (CIE) XYZ colour space. Then the dominant wavelength can be calculated by drawing a line between the reference white point and the chromatic coordinates, and extrapolating it to a pure colour locus.

The chromaticity diagram (‘horseshoe diagram’ in water colour graphs) “shows the colour space perceptible by the human eye based on the red, green and blue colour primaries (CIE 1932). Each x, y coordinate is calculated from a weighted integral of the upwelling light spectrum. The data shown is derived from radiance reflectance measured by Landsat 8 OLI in four bands within the visible spectrum of light. Therefore, the colour at the locations of the dots represent the best estimate of the true colour of the lake at the time of observation (Lehmann et al., in preparation).

Outliers in the dominant wavelength are identified in the plots as red crosses. The criterion for outlier is a distance of greater than 3 standard deviations from the mean. Note that there is no assurance that the outliers are true. They may reflect unusual but real conditions.

Environment Canterbury Technical Report 5 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

https://www.eurekalert.org/multimedia/pub/web/113860_web.jpg Wernard et al., 2010

Figure 2-1: Forel-Ule Scale, colour distribution

Cultural health Assessments of lake (or outlet stream) cultural health have been made for several high-country lakes. Information was available for the Waitaki catchment (Tipa and Associates & Williams, 2015), the Ashburton Lakes Area (Te Rūnanga o Arowhenua et al., 2010), the Hurunui catchment (Lenihan, 2011), and from an online map resource provided by Statistics New Zealand (https://statisticsnz.shinyapps.io/cultural_health/). Significantly more information is available in those reports than is presented here.

Results were reported as Cultural Health Index (CHI), or Takiwā assessments, and methods varied among reports. Cultural health assessments are a tool to measure several factors of cultural importance to Māori, including mahinga kai (customary food gathering) status. A summary of CHI methodology is given in Tipa and Associates & Williams (2015). Takiwā site assessments are similar CHI assessments, but also include an assessment of land resources over the entire site, as well as pest and weed presence (Te Rūnanga o Arowhenua et al., 2010). The grading and scoring system are also different.

Monitoring sites A list of monitored lakes, lake attributes, and time record of monitoring is shown in Table 2-3, with geographical distributions shown in Figures 2-2 through 2-8.

6 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-2: High-country lakes sampling locations – Overview including zone boundaries

Environment Canterbury Technical Report 7 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-3: Maps of lakes sampled in the Hurunui-Waiau Water Management Zone

Figure 2-4: Maps of lakes sampled in the Selwyn-Waihora Water Management Zone (Waimakariri Lakes)

8 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-5: Maps of lakes sampled in the Selwyn-Waihora Water Management Zone near Lake Coleridge

Environment Canterbury Technical Report 9 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-6: Maps of lakes sampled in the Ashburton Water Management Zone

10 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-7: Maps of lakes sampled in the Upper Waitaki Water Management Zone – Lakes Tekapo, Alexandrina, Pukaki, Ōhau, Middleton, and Kellands Pond

Environment Canterbury Technical Report 11 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 2-8: Maps of lakes sampled in the Upper Waitaki Water Management Zone – Lakes Benmore and Aviemore

12 Environment Canterbury Technical Report

Report Technical Canterbury Environment Table 2-3: List of high-country lakes monitored Canterbury high Canterbury Lake Monitoring Lake Max. Altitude Area Res. Res. Depth Size Zone Seasonal NPS-FM since Type depth (m) (km2) (FENZ) (Kelly) Stratification stratification (m) (y) (y) category Loch Katrine Dec-04 Glacial 28 520 0.78 0.74 3.2 Deep Small Hurunui - Waiau Stratifies Seasonal

Lake Sumner Dec-04 Glacial 134.5 505 13.73 1.14 Deep Large Hurunui - Waiau Stable Seasonal Lake Taylor Dec-04 Glacial 40.5 579 2.07 3.01 1.7 Deep Medium Hurunui - Waiau Stratifies Seasonal - Lake Sheppard Dec-09 Glacial 21 578 1.1 1.28 0.56 Deep Medium Hurunui - Waiau Stratifies Seasonal programme monitoring lakes country

Lake Marion Dec-09 Glacial 19 680 0.1 0.61 Deep Small Hurunui - Waiau Stratifies Seasonal Lake Mason Dec-09 Glacial 38.5 675 0.53 0.68 0.7 Deep Small Hurunui - Waiau Stratifies Seasonal Lake Sarah Dec-04 Glacial 6.7 575 0.22 0.57 0.3 Shallow Small Selwyn –Waihora None Polymictic Lake Grasmere Dec-04 Glacial 15 589 0.62 0.27 0.3 Intermediate Small Selwyn –Waihora Intermittent Polymictic Lake Pearson Dec-04 Glacial 17 603 2.02 1.13 0.37 Intermediate Medium Selwyn - Waihora Intermittent Polymictic Lake Hawdon Dec-04 Glacial 4 571 0.35 0.45 0.42 Shallow Small Selwyn - Waihora None Polymictic

Lake Lyndon Dec-04 Glacial 18.3 826 0.88 0.16 0.52 Intermediate Small Selwyn - Waihora Stratifies Seasonal Lake Georgina Dec-04 Glacial 10 537 0.17 0.23 0.09 Shallow Small Selwyn - Waihora None or Polymictic intermittent Lake Ida Dec-04 Glacial 9 676 0.1 0.12 0.06 Shallow Small Selwyn - Waihora None of Polymictic intermittent Lake Selfe Dec-04 Glacial 30 569 0.65 1.90 0.74 Deep Small Selwyn - Waihora Stratifies Seasonal

Lake Coleridge Dec-04 Glacial 200 450 36.88 21.35 Deep Large Selwyn - Waihora Stable Seasonal –

Lake Evelyn Dec-13 Glacial 3.2 592 0.17 0.57 0.03 Deep Small Selwyn - Waihora Stratifies Seasonal 2005 trends, and state Lake Catherine Dec-13 Glacial 6 667 0.18 0.72 0.18 Shallow Small Selwyn - Waihora Stratifies Seasonal Lake Henrietta Dec-13 Glacial 5 572 0.04 0.06 Shallow Small Selwyn - Waihora Stratifies Seasonal Lake Emily Jan-08 Glacial 2.3 674 0.19 0.77 0.07 Shallow Small Ashburton None Polymictic Māori Lake East Jan-08 Glacial 1.3 626 0.09 0.0007 Shallow Small Ashburton None Polymictic Māori Lake West Jan-08 Glacial 1.8 629 0.1 0.0004 Shallow Small Ashburton None Polymictic Lake Denny Jan-13 Glacial 2.1 678 0.05 0.039 0.005 Shallow Small Ashburton None Polymictic Lake Heron Dec-04 Glacial 37 692 6.95 0.97 0.88 Deep Medium Ashburton Stratifies Seasonal -

Lake Emma Dec-04 Glacial 3 640 1.67 0.11 Shallow Medium Ashburton None Polymictic 2019 Lake Camp Dec-04 Glacial 13 676 0.44 0.66 0.54 Intermediate Small Ashburton Intermittent Polymictic

13

14 Lake Monitoring Lake Max. Altitude Area Res. Res. Depth Size Zone Seasonal NPS-FM since Type depth (m) (km2) (FENZ) (Kelly) Stratification stratification (m) (y) (y) category Canterbury Lake Clearwater Dec-04 Glacial 18 675 1.97 0.48 0.34 Intermediate Medium Ashburton Intermittent Polymictic Lake McGregor Dec-11 Glacial 25 710 0.37 2.85 0.19 Deep Small Upper Waitaki Stratifies Seasonal Lake Middleton Dec-11 Glacial 4 525 0.23 0.66 0.1 Shallow Small Upper Waitaki None Polymictic

Lake Alexandrina Dec-04 Glacial 27 711 6.46 3.54 Deep Medium Upper Waitaki Stable Seasonal high

Lake Tekapo Dec-04 Glacial 120 686 96.59 2.19 Deep Large Upper Waitaki Stable Seasonal - Lake Pukaki Jan-06 Glacial 70 478 172.74 0.95 Deep Large Upper Waitaki Stable Seasonal programme monitoring lakes country

Lake Ōhau Jan-06 Glacial 129 455 59.27 1.49 Deep Large Upper Waitaki Stable Seasonal Lake Benmore Jan-06 Reservoir 91 347 75.85 0.36 Deep Large Upper Waitaki Stable Seasonal

Haldon Arm Jan-06 Reservoir 50 347 0.16* Deep Large Upper Waitaki Stable Seasonal Ahuriri Arm Jan-06 Reservoir 30 347 0.21* Deep Large Upper Waitaki Stable Seasonal

Lake Aviemore Dec-09 Reservoir 62 300 28.11 0.23 Deep Large Upper Waitaki Stratifies Seasonal Kellands Pond Dec-11 Artificial 6.5 456 0.19 2.60 0.03 Shallow Small Upper Waitaki None Polymictic Lake Opuha Dec-16 Artificial 40 390 7.1 Deep Medium Orari Temuka Stable, but Seasonal Opihi Pareora artificially de-

(OTOP) stratified

Stratification: Stable – stable summer stratification with autumn turnover; intermittent – lake has been reported to be stratified for periods in summer, but not Environment Canterbury Technical Report Technical Canterbury Environment consistently; stratifies – we assume either stable or intermittent stratification based on lake depth and/or one-off sampling, none - lake frequently mixes to bottom in summer.

Residence times: FENZ = Department of Conservations ‘Freshwater Ecosystems of New Zealand (FENZ) database (http://www.doc.govt.nz/our- –

work/freshwater-ecosystems-of-new-zealand/), Kelly = Kelly et al. (2014), *Norton et al. (2009) 2005 trends, and state

- 2019

Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

3 Limitations of current sampling programme While our current high-country lakes monitoring programme supplies valuable information on state and trends in our high-country lakes, there are several areas in which the programme could be improved: In particular, the programme does not meet all reporting requirements as outlined in the LWRP and the NPS-FM 2014. To fully meet the LWRP requirements, sampling for dissolved oxygen, temperature, and colour should be included. To fully meet NPS-FM 2014 requirements, all samples should be taken monthly, and NH4-N, cyanobacteria, and E. coli sampled as additional parameters.

However, it is not a prudent use of funding to frequently monitor clear, microtrophic lakes for cyanobacteria, E. coli, and ammonia toxicity. Potentially toxic cyanobacteria are unlikely to grow in significant numbers in these lakes, faecal contamination is limited, and dissolved nutrients are frequently below lab detection limits. By contrast, smaller and more accessible lakes that have a mesotrophic or eutrophic nutrient status, as well as lakes with high recreational use, the monitoring of E. coli and cyanobacteria could provide valuable information.

Our dataset did not allow for an assessment of dissolved oxygen concentrations, Munsell water colour, and temperature, and only for a limited assessment of ammonia toxicity. For the microtrophic lakes and for most oligotrophic lakes, dissolved nutrient concentrations are likely to be below or close to the laboratory minimum detection limit. In fact, samples were analysed for DRP, NNN and NH4-N in 2006, and for DRP and NNN a large number of results were below detection limit (Meredith & Wilks, 2006). Therefore, dissolved nutrients were not included in monitoring after 2006. Although no data was collected for ammonia since 2006, ammonia toxicity is unlikely to be an issue in most high-country lakes: Even if all nitrogen (TN) was present as ammoniacal nitrogen 21 lakes would fall in ammonia (toxicity) attribute grade A or B under the NPS-FM 2014. In Kellands Pond, where we measured both TN and NH4-N, a maximum of 30% of TN was present as NH4-N in 2003-2012. Thus, if we assumed that just over 50% of TN was present as ammonia (i.e. a maximum acceptable concentration of TN of 500 µg/L to stay in band B), all lakes except Lakes Denny and Georgina would be in band A or B, and none of the lakes monitored would be below the national bottom line (see Figure 5-6).

To comply better with the Burns et al. (1999) TLI protocol and to obtain nationally comparable TLI results, samples for chl-a, TP and TN should be taken monthly. Seasonal sampling (Dec to May) may also be missing peak chl-a in some years in some lakes, but capturing it in others. Thus, it is difficult to compare TLIs between years and lakes. It is worth noting that this problem is only reduced, not eliminated, when collecting monthly data.

Monthly sampling of high-country lakes in Canterbury by helicopter is not logistically feasible due to the lack of reliable ‘weather windows’ in spring, autumn and winter as well as health and safety considerations. Monthly sampling would require boat access to all lakes, and would significantly increase costs and staff time required. However, sampling by boat would have the advantage that the Burns et al. (1999) protocol could be implemented better. Improvements would include the ability to take samples that are a better representation of the conditions in the epilimnion and the hypolimnion, and a greater range of field measurements and observations could be made. Under the current sampling method, the hypolimnion is not sampled at all, and the 10 m depth integrated sample may not represent mixed layer conditions. In lakes with a deep thermocline (25-45 m in the large, deep lakes) the 10 m sample only covers approximately one third of the mixed layer. In shallower lakes however, the thermocline may be located between 5-10 m, so the 10 m depth integrated sample may include the thermocline or even the top of the hypolimnion. Thus, our current method may compare samples representative of surface condition with samples representative of the mixed layer and samples of the thermocline conditions. This ‘mismatch’ can occur between lakes, but also between months for the same lake. This type of sampling is contradictory to the intent of trophic state monitoring which is to compare mixed layer conditions throughout the year.

Additionally, the statistical power of trend detection depends on the number of datapoint collected, so monthly monitoring would increase the power of trend analysis and enable earlier and more confident trend detection.

Environment Canterbury Technical Report 15 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

The lack of long-term data on lake water temperatures, near bottom dissolved oxygen concentrations, mixing patterns, lake levels, and winter phytoplankton biomass highlight the need for inclusion of continuous water quality monitoring and depth profiles in our monitoring. Priorities for additional future (continuous) monitoring include  Lake level in Lake Emma  Chl-a, turbidity and bottom dissolved oxygen in Lake Pearson  Chl-a in Lake Heron  Chl-a in Lake Benmore (both Ahuriri Arm & Haldon Arm)  Bottom dissolved oxygen in Lake Lyndon, Camp and Alexandrina

4 High-country lakes state and trends This chapter contains a lake-by-lake overview of state and trends in Trophic Level Index, nutrients, turbidity and chl-a concentrations between December 2004 and April 2017. It also presents the lake ecological condition based on the LakeSPI protocol (Clayton & Edwards, 2006), and cultural health assessments where available.

4.1 Regional climate summary, 2004-2019 In addition to changes in land use, land use intensification and other factors, variations in climate can drive changes and variability in lake water quality. For instance, high rainfall can increase sediment influx and thus turbidity and nutrient availability, and frequent high winds can de-stabilise or deepen summer thermoclines. A deepening of the mixed layer can both reduce light availability and increase nutrient availability for phytoplankton growth On the other hand periods of calm, settled weather can results in a more stable and shallow stratification, resulting in high light availability, but possibly reducing nutrient supply .

Due to considerable differences in climate across the Canterbury region, the regional summary in Appendix 2 includes data from six meteorological stations that were chosen to represent the five Water Management Zone (WMZ) in which the lakes are located.

1. Lake Taylor (rainfall only) – Hurunui-Waiau-WMZ 2. Arthurs Pass - Waimakariri Lakes (SelwynWaihora-WMZ) 3. Snowdon – Lake Coleridge (Selwyn-Waihora-WMZ) 4. Mt Potts (since 2009) - Ashburton-WMZ 5. Lake Tekapo - Upper Waitaki-WMZ 6. Pukaki (wind speed only) - Upper Waitaki-WMZ

Parameters summarized are 1. Total annual rainfall 2. Summer rainfall (December to April) 3. Summer air temperature (January to March) 4. Number of days of wind speed above 12 m/s between January and April

While there were variations across the region, some broad patterns were apparent throughout the region: 1. Total annual rainfall was below average in 2005 and 2007. 2. Summer rainfall increased from 2007 to 2011 in Arthurs Pass and Lake Taylor. 3. The years from 2009 to 2011 had above average summer rainfall. 4. Number of windy days in summer was above average in 2010. 5. The summer of 2016/2017 was wet and cool. 6. 2018 had high annual rainfall in most areas and the summer 2018/2019 was also unusually wet at Tekapo and Lake Taylor.

16 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.2 Hurunui – Waiau Water Management Zone The lakes monitored in the Hurunui – Waiau Water Management Zone are located in, or near, the Lake Sumner Forest Park (Figure 2-2). The park adjoins Lake Sumner and Lake Mason, and includes Lake Marion. Lake Marion is a relatively undisturbed lake in a native forest catchment.

All the Hurunui lakes are valued as wildlife habitats, and many also for sports fishing. Land-uses in the Lake Sumner area are extensive pastoral farming and recreation, including game-bird shooting, fishing, boating, camping and walking. There are only a few permanent settlements in the upper Hurunui catchment. The protection of Lake Sumner and associated high-country lakes is addressed in the Mahaanui Iwi Management Plan (2013). This management plan highlights the need for protection of the lakes outside conservation estate from sedimentation caused by stock access and forestry activity on lake margins.

Water quality in the monitored lakes was generally good, and lake ecological condition was high or moderate. Only Lake Sheppard failed to meet its objectives for both lake TLI and LakeSPI. With the exception of Lake Marion, lake size and catchment land use were good predictors for lake water quality. Lake Sumner, the largest lake in the area, located in a catchment dominated by native vegetation, was microtrophic or oligotrophic. Lake Taylor and Loch Katrine, which are deep lakes in catchments with low producing grassland, were on the threshold between oligotrophic and mesotrophic. Lake Sheppard, with the most intensive land use of the sampled lakes, had the second highest TLI score. Lake Marion is surrounded by native forest, and was already classified as mesotrophic in the 1970s (Spencer, 1977). Elevated nutrients and phytoplankton biomass in this lake are possibly linked to organic inputs from surrounding forest, as well as hydrodynamic characterises such as long residence time and a small catchment area that may encourage nutrient recycling.

There were no significant trends in lake water quality other than increasing turbidity between 2007 and 2019. Lakes Sumner, Taylor, and Loch Katrine experienced a period of increased chl-a and/or nutrients between 2007 and 2010, with highest levels observed between 2009 and 2010. Summer rainfall generally increased between 2007 and 2011, resulting in a possible increase in nutrient inputs, which may be linked to the observed pattern in nutrient concentrations and TLI.

4.2.1 Loch Katrine – Waitetemoroiti

Figure 4-1: Sampling location in Loch Katrine – aerial photograph (EcanMaps)

Environment Canterbury Technical Report 17 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Loch Katrine (Figure 4-1) is a small lake with a surface area of 0.75 km2. Its maximum depth is 28 m. The lake drains to Lake Sumner at the North-Western end through a wetland and narrow channel. It can separate into two basins at low lake levels. A previously unauthorised bach community is located on the eastern shore of the lake, near the inflow. With the implementation of the Loch Katrine Management Plan in 1998, a number of huts have been removed and replaced with new huts available for booking by the public. A total of ten new huts can be built under the management plan. Loch Katrine acts as a boat harbour for both Loch Katrine and the adjacent Lake Sumner. Both this bach community and relatively high boat traffic are potential nutrient sources to Loch Katrine.

Loch Katrine is framed by beech forest on one side, and low producing grassland on the other, and its beaches are mostly steep shingle. Nearly half of the catchment land cover is native forest, 7% highly producing exotic grassland and 16% low producing grassland (Table in Appendix 2 from Kelly et al., 2014). Recreational use of the lake includes boating and brown trout fishing.

Nutrients, Phytoplankton biomass and Turbidity The lake generally had low concentrations of TP and TN (Table 4-1). Mean annual nutrient concentrations were usually below the LWRP limits, but phytoplankton biomass (measured as chl-a) as assigned to the B band in some years and exceeded the plan limit in half of the years. Total nitrogen, total phosphorus and chl-a were elevated between 2008 and 2010 (Figure 4-2). However, there were no significant overall trends in TN, TP or chl-a in Loch Katrine between 2007 and 2019 (Table 4-2). Loch Katrine is a clear lake with low turbidity (Figure 4-2) however, turbidity increased slightly but significantly between 2007 and 2019 (with a very small a mean annual increase of 0.04 NTU, Table 4-2).

Trophic Level Index Loch Katrine was oligotrophic in most years between 2005 and 2019 (Table 4-1, Figure 4-3). In 2008 and 2009 the lake was graded mesotrophic, and did not meet the LWRP objectives. TLI increased from 2006 to 2009, then decreased between 2009 and 2014 (Figure 4-3). These fluctuations were visible in all three TLI3 parameters, but appear to be most strongly driven by direct changes in phytoplankton biomass (chl-a) (Figure 4-3).

Table 4-1: Trophic Level Index and attribute states (NPS-FM 2014) for Loch Katrine from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.23 OLIGO Yes 75 4 1.5 1.6 A A A A 2006 2.19 OLIGO Yes 87 3 0.9 3.8 A A A A 2007 2.78 OLIGO Yes 110 3 4.0 10.1 A A B B 2008 3.11 MESO No 165 6 4.1 6.4 B A B A 2009 3.26 MESO No 140 7 4.7 11.1 A A B B 2010 2.95 OLIGO Yes 140 7 1.9 7.1 A A A A 2011 2.75 OLIGO Yes 110 6.5 1.7 2.8 A A A A 2012 2.66 OLIGO Yes 120 6 1.6 2.1 A A A A 2013 2.45 OLIGO Yes 117 4 0.7 2.6 A A A A 2014 2.35 OLIGO Yes 122 2 1.0 4.2 A A A A 2015 2.62 OLIGO Yes 125 2 1.5 2.5 A A A A 2016 2.54 OLIGO Yes 99 4 2.5 3.3 A A B A 2017 2.71 OLIGO Yes 122 2 3.2 5.2 A A B A 2018 2.63 OLIGO Yes 116 6 1.8 2.2 A A A A 2019 2.73 OLIGO Yes 121 4 4.6 5.9 A A B A 5 year 2.64 OLIGO Yes 117 3.6 2.7 5.9 A A B A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

18 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-2: TP, TN, chl-a concentrations and turbidity in Loch Katrine from 2005-2019

Table 4-2: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Loch Katrine

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 62 -39 987 -1.21 0.23 0 0 -0.14 to 0.00 0.50 TN 62 -20 1045.33 -0.59 0.56 -0.46 -0.38 -1.92 to 0.75 decreasing 0.74 chl-a 62 10 1061.33 0.28 0.78 0.01 0.52 -0.10 to 0.10 increasing 0.59 Turbidity 62 131 1088.33 3.94 0.00 0.04 6.3 0.02 to 0.06 increasing 1

Environment Canterbury Technical Report 19 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-3: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Loch Katrine, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Lake ecological condition in Loch Katrine is moderate, with a high impact of invasive species (Elodea sp.) and no significant changes since 1987 (Table 4-3, de Winton, Champion & Sutherland (2011)).

Table 4-3: Loch Katrine, LakeSPI overview, 1987-2016 1987 2011 2016 % Change indicated? Change LakeSPI 45 48 47 -1 No Native Condition 48 51 54 3 No Invasive Condition 57 54 53 -1 No Condition Moderate Moderate Moderate NA No Invasive species Elodea, Elodea, Elodea, NA No present Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 533 nm, i.e. green (Figure 4-144). The dominant wavelength was variable, between 488 nm (blue-green) and 573 (yellow-orange). In other words, the lake colour frequently fell into the ‘green’ range (as indicated by a reading on the FU scale of 8-14) (Figure 4-144). Green reflectance can mean either high chlorophyll, or moderate tannin staining from the beech forest inflows which absorb the blue and red wavelengths (leaving green). The latter is more likely for Loch Katrine which is in a catchment with abundant native forest and had low measured chl-a concentrations over the summer months. The lake also had clear ‘blue periods’ as indicated by FU scores below 7, this could either reflect low chlorophyll, or very low CDOM inputs that could be associated with dry periods and low inputs from the forest catchment.

20 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-4: Landsat derived water colour data for Loch Katrine, 2013-2018 (n = 28): Distribution of Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary While the lake temporarily did not meet the TLI objective set in the LWRP in 2008 and 2009, Loch Katrine is predominantly oligotrophic and TLI objectives were met in most years. The pattern of increasing TP and particularly TN between 2004 and 2010 seems to match the pattern of increasing (summer) rainfall in this period (with the exception of 2006) (Figure A3-5). Besides natural sources, the septic tanks from the old bach community were a possible source of phosphorus and nitrogen in the catchment. A number of huts have been removed in the last decade which may have reduced the nutrient load to the lake. Lake macrophyte community condition remains only moderate due to the presence of some less harmful exotic species (Elodea canadensis), but appears stable over the monitoring record. Except for a slight increasing trend in turbidity which is also seen in most other small to medium sized high-country lakes, there is no indication for deterioration of water quality in Loch Katrine.

4.2.2 Lake Sumner - Hoka Kura

Figure 4-5: Sampling location in Lake Sumner (SQ30079) – aerial photograph (EcanMaps)

Lake Sumner (Figure 4-5) is a large, deep lake adjacent to the Lake Sumner Forest Park. Its catchment is mostly forested. The lake covers an area of 11.8 km2 and is 134.5 m deep at its deepest point. Lake Sumner is located on the mainstem of the Hurunui River and due to its large size readily assimilates most headwater flood events. Therefore, lake levels can vary by up to 3.5 m depending on inflow

Environment Canterbury Technical Report 21 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

volumes (Cromarty & Scott, 1995). The lake basin orientation is exposed to North-Westerly winds along the main axis of its fetch (over 9 km) indicating a high potential for wind driven seiches and wind mixing.

Nutrients, Phytoplankton biomass and Turbidity Concentrations of TP, TN and chl-a were low throughout the sampling period (Table 4-4). There were no significant trends in TN, TP or chl-a between 2007 and 2017. However, as for Loch Katrine, TP and chl-a increased from 2008-2010, then decreased from 2011 to 2013 (Figure 4-6, Table 4-4).

Turbidity was generally low, but with isolated high turbidity events. High turbidity events did not appear to be correlated with TN, TP or chl-a, and are therefore likely due to inorganic suspended sediment. Floods in the upper catchment have been reported to cause minor discoloration in the lake (Cromarty & Scott, 1995). There was no trend in turbidity between 2007 and 2019 (Figure 4-6; Table 4-5).

Trophic Level Index Lake Sumner is a large high-country lake and therefore has a TLI objective target of ‘microtrophic’ (not exceeding 2) under the LWRP. The lake was oligotrophic between 2008 and 2012, but microtrophic outside this period (Figure 4-7). TLI increased from 2005-2009, then decreased from 2010 to 2013 (Figure 4-7), driven mainly by increases in TP and chl-a over this period (Figure 4-7).

Table 4-4: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Sumner from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a- Chl-a- Chl-a- Chl-a- Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 1.32 MICRO Yes 51 2 0.4 1.0 A A A A 2006 1.53 MICRO Yes 49 1.5 0.9 1.2 A A A A 2007 1.60 MICRO Yes 37 2 1.3 1.8 A A A A 2008 2.03 OLIGO No 60 3 1.5 1.6 A A A A 2009 2.26 OLIGO No 50 5.5 1.8 2.3 A A A A 2010 2.52 OLIGO No 70 10 2.0 2.4 A A A A 2011 2.18 OLIGO No 60 7 0.8 1.6 A A A A 2012 2.13 OLIGO No 80 5 0.6 1.5 A A A A 2013 1.60 MICRO Yes 52 2 0.2 0.8 A A A A 2014 1.64 MICRO Yes 53 2 0.6 1.9 A A A A 2015 1.83 MICRO Yes 66 2 0.8 1.1 A A A A 2016 1.67 MICRO Yes 50 2 0.9 1.7 A A A A 2017 1.98 MICRO Yes 50 2 1.3 2.9 A A A A 2018 1.78 MICRO Yes 54 2 0.5 0.8 A A A A 2019 1.71 MICRO Yes 55 2 1.3 1.8 A A A A 5 year 1.79 MICRO Yes 55 2 1.0 2.9 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

22 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-6: TP, TN, chl-a concentrations and turbidity in Lake Sumner from 2005-2019

Table 4-5: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Sumner

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 63 -35 1100.33 -1.02 0.31 -0.33 -0.59 -1.16 to 0.38 decreasing 0.84 chl-a 63 -51 1124.33 -1.49 0.14 -0.03 -3.71 -0.07 to 0.00 decreasing 0.93 Turbidity 63 41 1119.67 1.2 0.23 0.01 2.4 0.00 to 0.03 increasing 0.88

Environment Canterbury Technical Report 23 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-7: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Sumner, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Sumner was high in 2016 (Table 4-6), with only two invasive plants present: Elodea and water buttercup (de Winton et al., 2011).

Table 4-6: Lake Sumner, LakeSPI overview, 1987-2016 1987 2011 2016 % Change Change indicated LakeSPI 60 62 61 -1 No Native Condition 60 63 65 2 No Invasive Condition 36 36 39 3 No Condition High High High NA No Invasive species Elodea, Elodea, Elodea, NA No present Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 499 nm, i.e. blue-green (Figure 4-8). FU scores were generally stable around 4-8 with little change, indicating low algal biomass. Occasional scores above 8 possibly reflect inputs of DOM from flood flows and surrounding beach forest.

24 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-8: Landsat derived water colour data for Lake Sumner, 2013-2018 (n = 25): Dominant wavelength and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Sumner was microtrophic over most years of monitoring, with TLI scores below the LWRP objective. However, a slightly elevated period of higher nutrient status occurred between 2008 and 2012 where the lake was oligotrophic. Patterns in TLI, TP and chl-a appear to coincide with patterns in higher summer rainfall at the Lake Taylor meteorological station. The summers months of 2009 to 2011 were also relatively windy, which may have resulted in a deeper mixed layer or more frequent disturbances of the thermocline, thereby increasing available nutrients in the mixed layer. Our data indicates no trends in water quality between 2007 and 2019. Condition of the aquatic macrophyte community continues to be high, but falls short of the LWRP objective of ‘excellent’, which would require a lower coverage of exotic species in the lake.

4.2.3 Lake Taylor

Figure 4-9: Sampling locations in Lake Taylor (bottom) and Lake Sheppard (top) – aerial photograph (EcanMaps)

Environment Canterbury Technical Report 25 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake Taylor (Figure 4-9) is a medium sized (area = 1.85 km2), deep (40.5 m) lake near, but not connected to Lake Sumner. Its catchment is a mix of high and low productivity exotic grassland, native forest and tussock (Appendix 2). The lake drains into the Hurunui River downstream of the outlet from Lake Sumner. Lake Taylor has previously been described as the “most accessible of all the Sumner Lakes” for recreation (Cromarty & Scott, 1995).

Nutrients, Phytoplankton biomass, Turbidity Lake Taylor had consistently low nutrient and chl-a concentrations between 2006 and 2019 (Table 4-7). There were no significant trends in TN, TP or chl-a in Lake Taylor between 2007 and 2019 (Table 4-8). Unlike some of the other Hurunui Lakes, there were no periods of elevated nutrient concentrations over the 12-year monitoring record, but TN shows some slow gradual increase. As in Lake Sumner, chl-a in Lake Taylor increased between 2006 and 2009, then decreased until 2013 and has subsequently increased again to 2017 (Figure 4-10).Turbidity in the lake was generally low (Figure 4-10), with only one instance of elevated turbidity. There was a very small but significant increasing trend in turbidity, however increase over 12 years amounts to less than 0.25 NTU and turbidity remains low (Figure 5-10).

Trophic Level Index Lake Taylor was oligotrophic in all years (Table 4-7). As in Lake Sumner, TLI in Lake Taylor increased between 2006 and 2009, then decreased until 2013 (Figure 4-11).

Table 4-7: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Taylor from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a- Chl-a- Chl-a- Chl-a- Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.00 OLIGO Yes 66 3.5 1.1 1.6 A A A A 2006 2.13 OLIGO Yes 91 3.5 0.9 1.2 A A A A 2007 2.25 OLIGO Yes 100 4 1.3 1.5 A A A A 2008 2.39 OLIGO Yes 90 4 1.2 2.9 A A A A 2009 2.64 OLIGO Yes 105 6 2.2 2.7 A A B A 2010 2.46 OLIGO Yes 110 5 1.0 1.8 A A A A 2011 2.49 OLIGO Yes 105 8 1.3 1.4 A A A A 2012 2.40 OLIGO Yes 100 6 0.9 1.6 A A A A 2013 2.24 OLIGO Yes 100 11 0.3 0.9 A B A A 2014 2.46 OLIGO Yes 98 5 1.7 3.2 A A A A 2015 2.34 OLIGO Yes 92 2 0.8 6.6 A A A A 2016 2.59 OLIGO Yes 104 7 1.2 2.2 A A A A 2017 2.57 OLIGO Yes 96 6 1.1 3.7 A A A A 2018 2.39 OLIGO Yes 109 5 1.1 2.7 A A A A 2019 2.65 OLIGO Yes 98 4 2.0 3.5 A A A A 5 year 2.51 OLIGO Yes 100 4.8 1.2 6.6 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

26 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-10: TP, TN, chl-a concentrations and turbidity in Lake Taylor from 2005-2017

Table 4-8: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Taylor

ndall

Probability Parameter Sample size Ke statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 62 17 1011.67 0.5 0.61 0 0 0.00 to 0.17 0.5 TN 62 25 1060.33 0.74 0.46 0.45 0.45 -0.47 to 1.22 increasing 0.8 chl-a 62 16 1058 0.46 0.64 0.01 0.91 -0.02 to 0.05 increasing 0.74 Turbidity 60 110 940 3.56 0.00 0.02 4.99 0.01 to 0.03 increasing 1

Environment Canterbury Technical Report 27 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-11: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Taylor, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Taylor was high in 2016, with a possible improvement between 2011 and 2016 (Table 4-9). The native condition was somewhat impacted by Elodea (de Winton et al., 2011).

Table 4-9: Lake Taylor, LakeSPI overview, 1987-2016 1987 2011 2016 % Change Change indicated? LakeSPI 57 53 56 3 No Native Condition 57 58 63 5 Possible Invasive 40 52 47 -5 Possible Condition Condition High High High NA No Invasive species Elodea, Elodea, Elodea, NA No present Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 498 nm (i.e. blue-green). Like in Lake Sumner FU scores were generally stable, around 4-8, indicating low algal biomass. Occasionally results above 8 possibly reflect episodic inputs of DOM from flood flows and surrounding beach forest.

28 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-12: Landsat derived water colour data for Lake Taylor, 2013-2018 (n = 19): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Taylor was oligotrophic in all years, and in ‘high’ ecological condition, thereby meeting its LWRP objectives. While there was significant increase in turbidity, turbidity remains very low and changes are probably not yet likely to be visibly or ecologically significant. No other statistically significant changes in water quality were detected although there has been a slight increase in chl-a over the monitoring record.

4.2.4 Lake Sheppard Lake Sheppard (Figure 4-9) is adjacent to Lake Taylor, but is a shallower and smaller lake with a maximum depth of 21 m and a surface area of 1.15 km2. Its catchment also has a higher proportion of improved grassland (21%, see Appendix 2).

Nutrients, phytoplankton biomass and Trophic Level Index The lake was monitored in 2010/2011 and 2016/17. TN and chl-a were elevated in all years above the targets in the LWRP of oligotrophic (A-grade NPS-FM) (Table 4-10). The lake was assigned a mesotrophic status in all four years of monitoring (Table 4-10). Due to the limited data record, no trend analyses of nutrient status were possible for Lake Sheppard.

Table 4-10: Trophic Level Index and attribute states (NPS-FM 2014) for from 2010-2017

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a- Chl-a- Chl-a- Chl-a Year TLI Grade TN TP TN TP met? MED MAX MED - MAX 2010 3.30 MESO No 190 14 2.6 4.0 B B B A 2011 3.17 MESO No 175 11 2.5 3.1 B B B A 2016 3.27 MESO No 188 10 2.7 4.9 B A B A 2017 3.56 MESO No 198 10 5.0 8.0 B A B A MED = seasonal median concentration (Dec-May)

Environment Canterbury Technical Report 29 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake ecological condition: LakeSPI Ecological condition of Lake Sheppard was moderate across all years of monitoring between 1987 and 2016 (Table 4-11). In 2011, aquatic macrophyte community was dominated by the exotic species Elodea, with a modest depth of vegation and a high cover of attached algae (de Winton et al., 2011).

Table 4-11: Lake Sheppard, LakeSPI overview, 1987-2016 1987 2011 2016 % Change Change indicated? LakeSPI 40 34 34 0 No Native Condition 43 33 37 4 No Invasive Condition 64 72 67 -5 Possible Condition Moderate Moderate Moderate N/A No Invasive species Elodea, Elodea, Elodea, NA No present Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 550 nm, i.e. green (Figure 4-13). The FU index was frequently above 8, indicative of a green colour due to moderate algal biomass.

Figure 4-13: Landsat derived water colour data for Lake Sheppard, 2013-2018 (n = 26): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Sheppard was consistently moderately enriched (mesotrophic) and did not meet the LWRP targets for trophic status. Out of the lakes surveyed in the Hurunui-Waiau Water Management Zone, Lake Sheppard’s catchment is most intensively used for pastoral farming. The TLI for Lake Sheppard was higher than in the other Hurunui lakes (apart from Lake Marion).

30 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.2.5 Lake Marion

Figure 4-14: Sampling location in Lake Marion – aerial photograph (EcanMaps)

Lake Marion (Figure 4-14) is a small (0.1 km2), isolated lake, surrounded by native forest, within the Lake Sumner Forest Park. Its maximum depth is estimated to be 19 m, which in association with the very sheltered position would indicate that the lake seasonally stratifies. The lake is drained by Marion Stream, which flows to Lake Sumner. The lake is reported to be free from introduced fish, and protected as a ‘Faunal Reserve’ (Cromarty & Scott, 1995).

Nutrients, phytoplankton biomass and TLI Lake Marion had elevated concentrations of chl-a and TN (Table 4-12). Lake Marion was in a mesotrophic state when sampled in 2010/11, 2017 and 2019 (Table 4-12). In 2016 the lake was classed as eutrophic particularly due to very high TN, TP, chl-a and turbidity in April 2016. We are unsure why April 2016 exhibited such high sample values across all parameters.

Table 4-12: Trophic Level Index and attribute states (NPS-FM 2014) for from 2010-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2010 3.36 MESO No 240 9 2.5 4.0 B A B A 2011 3.49 MESO No 255 12 3.3 4.7 B B B A 2016 4.24 EUTRO No 330 16 2.8 15.0 B B B B 2017 3.53 MESO No 300 10 2.7 2.9 B A B A 2019 3.53 MESO No 270 12 3.4 5.0 B B B A 5 year 3.63 MESO No 300 13 3.0 15.0 B B B B MED = seasonal median concentration (Dec-May)

Water colour The median of the dominant wavelength between 2013 and 2018 was 528 nm, i.e. green (Figure 4-15). The distribution of the FU index indicates the occasional ‘brownish’ colour probably due to the influence

Environment Canterbury Technical Report 31 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

of floods or high DOM inputs from surrounding forest, as well as the presence of ‘blue’ clear water phases (FU below 5) and common ‘green’ coloration due to moderate phytoplankton biomass.

Figure 4-15: Landsat derived water colour data for Lake Marion, 2013-2018 (n = 15): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Despite Lake Marion being a relatively undistributed natural lake, concentrations of nutrients and chl-a were consistently elevated and representative of mesotrophic conditions in most years. In the 1970s, chl-a concentrations were reported to be between 3 and 4 µ/L (similar to 2010, 2011, 2017 and 2019), and the lake was then also classified as mesotrophic (Spencer, 1977). Among sources of nutrients may be natural organic matter from decomposing forest litter, similar to many lakes in Westland. While forest lakes are generally expected to have natural low productivity, some lakes rich in humic material can be highly productive, and the dissolved organic matter can sometimes contain considerable amount of bioavailable organic nitrogen (Jolly & Brown, 1975). Higher altitude and UV exposure may photo-oxidise dissolved organic matter, releasing these nutrients to phytoplankton. Hydrodynamic characterises such as long residence time, a deep basin, sheltered location and a small catchment area may also encourage nutrient recycling in the lake. In addition, Lake Marion is on a tramping track and thus not isolated from potential anthropogenic nutrient inputs. We remain unsure of the cause of the unusually high productivity of Lake Marion.

32 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.2.6 Lake Mason

Figure 4-16: Sampling location in Lake Mason (Little lake Mason and Lake Mason) – aerial photograph (EcanMaps)

Lake Mason (Figure 4-16) is situated southwest of Loch Katrine. The lake is 0.73 km2 in area and split in two independent lakes; a deep lake (38.5 m) and a much shallower second lake (1.9 m). They drain into each other. The sampling site is located in the larger and deeper lake. Lake Mason’s catchment is dominated by mountain beech forest and tussock grasslands (Appendix 2), although it also has grazing land and 4WD access to a significant accommodation hut on the lake shore

Nutrients, phytoplankton biomass and Trophic Level Index Lake Mason was in oligotrophic condition, with low nutrient concentrations (Table 4-13). Due to limited data no trend analyses were possible for this lake, but trophic level status was relatively stable over the 7-year time span of the monitoring data.

Table 4-13: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Mason from 2010- 2017 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2010 2.69 OLIGO Yes 100 A 3.75 A 2.4 B 3.1 A 2011 2.50 OLIGO Yes 90 A 6 A 1.5 A 1.9 A 2016 2.06 OLIGO Yes 94 A 2 A 1.0 A 1.6 A 2017 2.27 OLIGO Yes 95 A 2 A 1.6 A 2.3 A MED = seasonal median concentration (Dec-May)

Environment Canterbury Technical Report 33 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community was high in 2011 and 2016 (Table 4-14), with both Elodea and water buttercup present, but not forming continuous beds (de Winton et al., 2011). The LakeSPI assessments indicate a possible and probable improvement in invasive condition (Table 4-14). We are unsure of what the likely driver of this is and whether it is likely to be sustained in the longer term.

Table 4-14: Lake Mason, LakeSPI overview, 2011-2016 2011 2016 % Change Change indicated? LakeSPI 61 63 2 No Native Condition 63 58 -5 Possible Invasive Condition 37 25 -12 Probable Condition High High NA No Invasive species Elodea, Elodea, No No present Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 502 nm, i.e. green but the lake had frequent clear, ‘blue’ episodes and occasional brownish colouration probably due to floods or DOM from surrounding forest (Figure 4-17).

Figure 4-17: Landsat derived water colour data for Lake Mason (Northern basin, LID 39297), 2013-2018 (n = 19): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Mason is an oligotrophic lake in high ecological condition in a catchment dominated by native beech forest. The lake trophic status has remained in stable over the monitoring record and macrophyte status may be improving, although only limited data are available.

34 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3 Selwyn – Waihora Water Management Zone The lakes sampled in the Selwyn-Waihora Zone (Waimakariri and Rakaia River catchments) can be grouped into two clusters: Four small lakes in the upper catchment of the Waimakariri River (Lakes Pearson, Grasmere, Sarah, and Hawdon), and eight lakes (Lakes Lyndon, Georgina, Ida, Selfe, Coleridge, Evelyn, Catherine, Henrietta) in the Upper Rakaia River Catchment. Climate is generally warm in summer, and exposed to frequent strong North-Westerly winds, but cold in winter. While significant areas of surface ice can form on the smaller lakes in very cold winters, most rarely freeze completely (Cromarty & Scott, 1995). The exception to this was Lake Ida (a historic Canterbury Ice Skating facility) and historically parts of Lake Lyndon. These occur largely because of a sheltered or shady southerly aspect.

The Waimakariri lakes have a common geomorphology: They are mostly unmodified by dams or diversions, and inflow streams are either small or non-existent. However, Lake Pearson and Lake Grasmere have inflowing streams draining large areas, that, even though baseflows are low, can influence the lakes with large storm inflows (Craigieburn River and Ribbonwood Stream (Figure 4-18)). Most also drain via small, swampy streams or underground seepage (Cromarty & Scott 1995). These lakes are situated in mostly Crown land, University of Canterbury Endowment Lease land, and some private pastoral runs. Land cover is illustrated in Figure 4-18, Figure 4-19 and Appendix 2.

Lakes monitored near Lake Coleridge drain either to the Ryton River, which flows to Lake Coleridge, directly to Lake Coleridge (a reason why these lakes are often colloquially called the “Coleridge lakes”), or west to the Harper River. These lakes are located in a landscape with extensive pastoral farming as the predominant land use (usually 30-50% of catchments). Changes to farm ownership and development over the period of this monitoring (2005 to 2019) has seen significant increase in development of improved pastures draining into many of these lakes. Except for Lakes Lyndon, Selfe and Coleridge, these lakes are too shallow and wind-exposed to develop stable stratification.

Both lake clusters are home to a number of species of water birds, and important breeding and feeding areas for Great Crested Grebe (Cromarty & Scott, 1995). Many lakes were traditional Mahinga kai sources (mostly in the form of birds and fibres) for Māori people and were used by travelling Māori for seasonal harvests of mahinga kai. The lakes are also valued sites for their scenic value and recreational fisheries.

Six lakes were graded a ‘high’ or ‘excellent’ ecological status using the LakeSPI indicators, while four lakes failed to meet their LakeSPI objectives. Both Lakes Pearson and Grasmere are deteriorating in LakeSPI ecological condition, mostly likely due to reduced water clarity.

Most of the lakes were mesotrophic either periodically or throughout the sampling period, failing to meet their LWRP objectives. There is limited information on the historic trophic state of many of the high- country lakes, but Lakes Evelyn, Ida, Sarah, and Grasmere were classified as mesotrophic (based on chl-a, TN, TP, and bacterial production) in 1970s (Spencer, 1977). In this same report, Lakes Hawdon, Pearson, Lyndon, Georgina, Selfe, and Coleridge were described as oligotrophic. Timms (1983) also described Lakes Sarah and Grasmere as mesotrophic, and Lakes Lyndon and Hawdon as oligotrophic. Thus, the current mesotrophic state of Lakes Evelyn, Ida, Sarah and Grasmere may not necessarily be indicative of a major deterioration in water quality since the 1970s. However, phytoplankton biomass, turbidity, and/or nutrient concentrations have increased significantly in Lakes Pearson, Grasmere, Hawdon, Ida and Lyndon between 2007 and 2019, possibly reflecting increasing pressures on these lakes. These pressures may include recent pasture development and stocking strategies, and past or increased recreational uses (e.g. fishing, tourism). Positive actions include several toilet facilities constructed at many popular angling lakes.

To increase the chances that LWRP lake water quality objectives are met nutrient inputs to all lakes should be minimized. No further increase in nutrient loading should be permitted for these “sensitive” or ‘at risk” lakes such as Lakes Pearson and Grasmere where water quality objectives are frequently not met, and water quality is very likely deteriorating.

Several lakes in the Selwyn-Waihora-WMZ (Lakes Hawdon, Ida, Selfe) had high TLI and/or chl-a in summer 2017. This coincided with abnormally high rainfall from December 2016 to April 2017, cooler than normal air temperatures, and potentially above average wind speeds in the summer. While

Environment Canterbury Technical Report 35 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

nutrients inputs with high rainfall are a possible explanation of unusually high algal biomass, there may have been other unidentified factors may have driven these blooms.

Lakes Lyndon, Pearson and Ida all had increasing turbidity and chl-a- trends, along with at least one increasing nutrient. Nearly all of the lakes in the Selwyn-Waihora Zone have undergone a significant increase in turbidity between 2007 and 2017. This may possibly be linked to a significant increase in total annual rainfall (recorded at Arthurs Pass (2007-2016, probability 98%, p = 0.02)) and therefore higher sediment loads carried to these lakes. The Canterbury earthquakes in 2010/2011 (Christchurch) and 2016 (Kaikoura) should also not be discounted as having an influence on land stability and sediment runoff.

36 Environment Canterbury Technical Report

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Figure 4-19: Land Cover Classification (LCDB 4) near Lake Coleridge (Ryton Lakes) - 2019

Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.1 Lake Sarah

Figure 4-20: Sampling location in Lake Sarah – aerial photograph (EcanMaps)

Lake Sarah (Figure 4-20) is a shallow (6.7 m maximum depth), small lake (0.2 km2) located in the upper catchment of the Waimakariri River, near Lake Grasmere. It previously received all of the outlet flow from Lake Grasmere, until a cut-off diversion bypassed much of the Lake Grasmere outlet flows. However, Lake Sarah still receives a regular, though smaller, inflow from Grasmere Stream. Lake Sarah’s outlet stream flows into Grasmere Stream near Cass and eventually to the Cass and Waimakariri Rivers. Lake Sarah’s catchment is comprised of both high and producing grassland to the west and low yielding vegetation to the north and east (LCDB v 4.1, App 2). Because the lake is relatively shallow it is likely to be well mixed in summer. The lake is also adjoined by both a gravel road and the railway, and fishermen’s’ huts.

Nutrients, Phytoplankton biomass and Trophic Level Index TN was elevated, but TP and cha were mostly low with the exception of the years 2009/2010, 2013 and 2017/2018 (Table 4-15). Both nutrients (TN and TP) and chl-a were high in April 2011 (Figure 4-21). Between 2005 and 2017 both TN and TP increased significantly, but there were no significant trends in nutrients or phytoplankton biomass between 2007 and 2019 (Table 4-16, Figure 4-21). This apparent contradictory finding suggests that there may be episodic inputs of nutrients to Lake Sarah rather than consistently increasing inputs. This may result from episodes of pasture cultivation or development, or changing stocking strategies. Turbidity increased significantly between 2007 and 2019 (Table 4-16).

TLI in Lake Sarah fluctuated around the mesotrophic threshold, except for 2011 where the lake was eutrophic and had high concentrations of phytoplankton and nutrients in April (Table 4-15, Figure 4-22).

Environment Canterbury Technical Report 39 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-15: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Sarah from 2005- 2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.47 OLIGO Yes 140 4 0.8 1.3 A A A A 2006 2.77 OLIGO Yes 175 5.5 1.4 2.6 B A A A 2007 2.84 OLIGO Yes 180 6 1.6 2.5 B A A A 2008 2.82 OLIGO Yes 230 6 1.2 2.0 B A A A 2009 3.17 MESO No 240 10 1.8 2.1 B A A A 2010 3.42 MESO No 260 9 2.8 3.7 B A B A 2011 4.04 EUTRO No 245 10.5 2.5 9.2 B B B A 2012 2.88 OLIGO Yes 250 8 0.9 1.2 B A A A 2013 2.81 OLIGO Yes 250 27 0.1 0.5 B C A A 2014 3.19 MESO No 240 7 1.1 2.2 B A A A 2015 2.93 OLIGO Yes 240 7 1.2 2.2 B A A A 2016 2.94 OLIGO Yes 250 8 1.1 1.6 B A A A 2017 3.32 MESO No 300 12 1.5 2.0 B B A A 2018 3.59 MESO No 290 12 2.2 3.3 B B B A 2019 3.08 MESO No 210 7 1.8 3.5 B A A A 5 year 3.17 MESO No 258 9.2 1.6 3.5 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-16: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Sarah

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 62 -39 987 -1.21 0.23 0 0 -0.14 to 0.00 0.50 TN 62 -20 1045.33 -0.59 0.56 -0.46 -0.38 -1.92 to 0.75 decreasing 0.74 chl-a 62 10 1061.33 0.28 0.78 0.01 0.52 -0.10 to 0.10 increasing 0.59 Turbidity 62 131 1088.33 3.94 0.00 0.04 6.3 0.02 to 0.06 increasing 1

40 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-21: TP, TN, chl-a concentrations and turbidity in Lake Sarah from 2005-2019

Figure 4-22: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Sarah, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Sarah was moderate in 2013, and high in 2018 (Table 4-17). However, there was no statistically significant change, and lake condition is ‘stable’.

Environment Canterbury Technical Report 41 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-17: Lake Sarah, LakeSPI overview 2013 2018 % Change Change indicated? LakeSPI 46 51 5 No Native Condition 65 70 5 No Invasive Condition 70 62 -8 Possible Condition Moderate High Yes Invasive species Elodea Elodea Yes Juncus bulbosus Data from https://lakespi.niwa.co.nz/

Water colour Water colour in Lake Sarah was relatively variable with FU readings from 4 (blue) to 12 (green). The median of the dominant wavelength between 2013 and 2018 was 528 nm, i.e. green (Figure 4-23).

Figure 4-23: Landsat derived water colour data for Lake Sarah, 2013-2018 (n = 25): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The lake water level was particularly low in April 2011.

Summary Lake Sarah was eutrophic in 2011, mesotrophic in many years, but also met the oligotrophic TLI objective in several years. In 2010/11 parts of the catchment were ploughed and re-pastured. The spike in TP, TN, and chl-a (and thus TLI) in April 2011 may have been a result of this activity, or may also be linked to the low lake water level observed in April 2011 which may have increased sediment re- suspension. TLI score and concentrations of the individual TLI parameters seem to have stabilised since 2012, but have been close to or above the oligotrophic-mesotrophic boundary in recent years. Overall, Lake Sarah appears to exhibit episodic challenges to its nutrient status and condition. Grazing and pasture development of areas to the west are deserved of increased scrutiny.

42 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.2 Lake Grasmere

Figure 4-24: Sampling location in Lake Grasmere – aerial photograph (EcanMaps)

Lake Grasmere (Figure 4-24) is located in the upper catchment of the Waimakariri River, adjacent to State Highway 73. The lake is small (0.63 km2) and of intermediate depth (up to 14 m). Water enters the lake via surface storm flows or underground seepage from the extensive Ribbonwood Stream catchment, as well as surrounding fans and hill slopes. Near the lake outlet there are significant areas of strongly spring fed wetlands draining the fan to the west. Grasmere Stream drains the lake to downstream Lake Sarah, and significant wetland areas before discharging into the Cass River which flows to the Waimakariri River. Ribbonwood Stream is frequently dry, but when it flows after heavy rain the fine sediment discharged to the lake can give Lake Grasmere a ‘milky green’ colour.

The lake can (intermittently) thermally stratify in summer, and stratification can persist for several weeks causing appreciable depression of oxygen concentration in the bottom waters (Figure 4-26, Figure 4-25, Sanoamuang 1992; Environment Canterbury unpublished data December 2018-April 2019).

Lake Grasmere is easily accessible and is popular for fishing, picnicking and conservation tourism (kayaking). It is also a Wildlife Refuge and part of the shoreline (and nearby hillside) have Recreation Reserve or Scenic Reserve status. The lake has fringing wetlands to the South, South West, and North- West. Large, relatively unmodified reed-beds provide valuable vegetation and bird habitat (Cromarty & Scott, 1995) and large numbers of waterfowl often populate the lake.

Lake Grasmere’s catchment uses (pre-2014) included high (6%) and low (33%) producing grassland (LCDB v 4.1, App 2), some in close proximity to the lake. However, pasture areas to the west of the lake have been extensively developed and are more recently dominated by a high nitrogen producing lucerne crop. The land to the South is held under a University of Canterbury Endowment Lease.

Environment Canterbury Technical Report 43 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-25: Temperature profile of Lake Grasmere December 1988 (Sanoamuang, 1992)

Figure 4-26: Bottom and surface temperature in Lake Grasmere, 1988-1990 (Sanoamuang, 1992)

Nutrients Concentrations of TP were elevated most years, and highest from 2011 to 2013 (Table 4-18, Figure 4-27). TN concentrations increased between 2005 to 2009, peaked in 2011, and then persisted at the elevated state as they did not subsequently return to pre-2008 levels (Figure 4-27). This may be associated with the recent pasture development since 2009. TP increased significantly from 2005 to 2019 (Table 4-19) and this appears to be associated with periods of increase in both 2011 and more recently.

Phytoplankton biomass Concentrations of chl-a were also elevated (i.e. in the NPS B-band) most years (Table 4-18). Stout (1973) described the lake as having peak phytoplankton biomass over spring (October) and sustained

44 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

over summer or early autumn. Maximum concentrations of 14 µg/L recorded by Stout (1973) were slightly higher than peak concentrations measured 2005-2019 (Table 4-18). Concentrations of chl-a increased significantly from 2005 to 2019 (Table 4-19), but particularly over the past 5 years (2014 to 2019 (Figure 4-27)).

Trophic Level Index Lake Grasmere was oligotrophic from 2005-2007, but has been in a mesotrophic state since 2008 (Table 4-18). TLI increased steadily from 2005 to 2009, and peaked in 2011 (Figure 4-28) and the mesotrophic condition has been sustained since. The lake has been graded as mesotrophic in the past (Timms, 1983).

Turbidity Turbidity increased significantly between 2007 and 2019 (Table 4-19). The highest turbidity was recorded in early 2018 probably associated with a large rainfall event that turned the lake a ‘milky green’ colour in March 2018 (Figure 4-30). The significant increasing trend is likely to be influenced by the high readings in 2018, but a steadily increasing trend is also visible in the data (Fig 4-27).

Table 4-18: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Grasmere from 2005-2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.31 OLIGO Yes 82 3 1.0 3.6 A A A A 2006 2.58 OLIGO Yes 106 4 1.3 4.1 A A A A 2007 2.92 OLIGO Yes 107 8 2.3 7.4 A A B A 2008 3.05 MESO No 180 7 2.2 2.9 B A B A 2009 3.35 MESO No 160 12.5 3.3 6.4 A B B A 2010 3.21 MESO No 160 12 2.6 3.3 A B B A 2011 3.65 MESO No 150 14.5 2.4 11.0 A B B B 2012 3.36 MESO No 160 23 1.5 4.1 A C A A 2013 3.11 MESO No 146 16 1.1 1.6 A B A A 2014 3.11 MESO No 162 8 1.3 6.1 B A A A 2015 3.22 MESO No 155 11 2.3 6.7 A B B A 2016 3.26 MESO No 154 10 3.8 4.1 A A B A 2017 3.47 MESO No 153 12 4.2 4.3 A B B A 2018 3.49 MESO No 154 12 2.5 4.3 A B B A 2019 3.38 MESO No 155 12 4.4 5.5 A B B A 5 year 3.37 MESO No 154 11.4 3.4 6.7 A B B A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Environment Canterbury Technical Report 45 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-27: TP, TN, chl-a concentrations and turbidity in Lake Grasmere from 2005-2019

Table 4-19: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Grasmere

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 73 1075 2.2 0.03 0.33 3.03 0.00 to 0.64 increasing 0.99 TN 63 18 1106 0.51 0.61 0.6 0.39 -1.00 to 2.48 increasing 0.71 chl-a 63 79 1149 2.3 0.02 0.14 5.44 0.05 to 0.22 increasing 0.99 Turbidity 63 118 1120.67 3.5 0.00 0.05 5.15 0.02 to 0.06 increasing 1

46 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-28: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Grasmere, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Lake Grasmere was in a moderate ecological condition in 2013. Elodea was dominant and formed a closed canopy at all survey sites (Sutherland et al., 2013). Condition of the aquatic macrophyte community has deteriorated from 2013 to 2018, due to reduced extent of (native) vegetation and also linked to reductions in water clarity (de Winton, 2018).

Table 4-20: Lake Grasmere, LakeSPI overview 2013 2018 % Change Change indicated? LakeSPI 40 32 -8 Possible Native Condition 46 33 -13 Probable Invasive Condition 67 72 5 No Condition Moderate Moderate N/A No Invasive species Elodea Elodea No Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 524 nm, i.e. green (Figure 4-29). The lake is quite variable in colour and can have a ‘milky-green’ hue after heavy rainfall when Ribbonwood Creek discharges a lot of fine sediment into the lake. We observed such colour in March and November 2018 as well as in June 2019, along with high turbidity.

Environment Canterbury Technical Report 47 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-29: Landsat derived water colour data for Lake Grasmere, 2013-2018 (n = 36): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Figure 4-30: Lake Grasmere in March 2018 showing high turbidity after significant discharge from Ribbonwood Stream

Other observations Stock access to lake was reported in 2008 and 2020 despite the construction of a new fence in 2018/2019.

Summary Lake Grasmere is currently mesotrophic, exceeding the objective set in the LWRP. Turbidity, TP and chl-a are increasing, and the condition of the aquatic macrophyte community is deteriorating due in part to decreased visual clarity. Water quality in the lake appears to be affected by land use development in the catchment, changes in lake water levels, periodic glacial flour inputs from Ribbonwood Creek, intermittent stratification and other climate induced factors. Lake Grasmere is therefore a sensitive lake being influenced adversely by a number of issues. Thus it should be managed conservatively as expected by the “sensitive lake zoning” in the LWRP. To increase the chances of LWRP water quality objectives being met in the future, opportunities to limit or reduce nutrient inputs to the lake (in particular phosphorus) should be actioned. As a very minimum, no further increase in nutrient allocation should be permitted in this lake zone, in line with current LWRP rules.

48 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.3 Lake Pearson - Moana Rua

Figure 4-31: Sampling location in Lake Pearson – aerial photograph (EcanMaps)

Lake Pearson (Figure 4-31) is situated in upper catchment of the Waimakariri River, adjacent to State Highway 73. The lake is 17 m deep and has a surface area of 2.02 km2. It has two distinct basins, with a narrow, but permanent connection between them. Monitoring of both basins for the first few years of this programme indicated close similarity of water quality in both basins, indicating a high level of basin mixing. The turbid consequences of flood inputs to the south basin mixing rapidly into the northern basin also illustrated this. Therefore, monitoring of Lake Pearson was reduced to monitoring and reporting of only the larger northern basin.

Water levels in the lake can vary throughout the year by up to 1.5 m. Highest water levels are usually observed during the spring snowmelt, and lowest in winter following a dry summer (Cromarty & Scott, 1995) or summer (2018/2019 monitoring data). The Craigieburn River is the main inflow source to the lake. Lake Pearson drains to the Waimakariri River via Winding Creek, but often there is no surface water connection from the lake to Winding Creek. The lake performs an important function of buffering flood discharges and associated sediment loads from the Craigieburn River to Winding creek. The lake is also an important recreational fishery, with a range of salmonid species including a remnant population of Lake Char.

Uses of the lake shore include an adjacent highly accessible public campsite and picnic area alongside the state highway. Its catchment includes high and low producing grassland (LCDB v 4.1), used for farming of sheep, cattle and deer, some of it intensively (5%, App 2). Livestock access issues on the northern end of the lake were addressed with riparian fencing and reticulated stock water supplies in the 2000s, but the riparian environment remains unfenced on the southern end of the lake and is managed via stocking strategies. We receive regular adverse public observations of this strategy.

Lake Pearson was an important habitat for the nationally endangered Great Crested Grebe, although their numbers appear to have greatly decreased in recent years. Winding Creek is one of the major

Environment Canterbury Technical Report 49 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Waimakariri River spawning tributaries for Quinnat Salmon (Cromarty & Scott, 1995). The lake has four species of introduced salmonids, as well as a number of native fish species include two species of galaxiids, two species of bully and Long-finned Eels (Anguilla dieffenbachii) (Cromarty & Scott, 1995).

Nutrients TP and TN concentrations were low in 2005 but increased between 2005 and 2012 (Table 4-21). We observed a further, larger increase in nutrient concentrations, along with chl-a and turbidity, between 2013 and 2014 (Figure 4-32). The TP and TN increasing trends were highly significantly between 2007 and 2019 (Table 4-22).

Phytoplankton biomass Chl-a has shown an interesting bimodal pattern over the sampling period with a small peak in 2008- 2009 and higher, more sustained peak 2016-2019 (Table 4-21, Figure 4-32). Chl-a concentration overall increased significantly between 2007 and 2019 (Table 4-22). The pattern in chl-a mostly mirrors the patterns of TN and TP, in particular the step-change increase between 2014 and 2015.

Trophic Level Index The lake was mostly in oligotrophic condition between 2005 and 2013 (expect in 2008 and 2009) but has been mesotrophic since 2014 (Table 4-21). TLI increased between 2005 and 2009, and 2013 and 2017 (Figure 4-33). Therefore, the trophic state has also demonstrated this bimodal pattern, but with increasing height of peaks.

Table 4-21: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Pearson from 2005-2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 1.81 MICRO Yes 90 2 0.5 1.2 A A A A 2006 2.27 OLIGO Yes 102 3 1.6 2.1 A A A A 2007 3.02 OLIGO Yes 125 6 4.3 5.5 A A B A 2008 3.11 MESO No 160 6 4.1 4.8 A A B A 2009 3.13 MESO No 160 8 3.2 4.0 A A B A 2010 2.67 OLIGO Yes 140 2.5 1.4 2.0 A A A A 2011 2.85 OLIGO Yes 140 7.5 2.1 2.5 A A B A 2012 2.76 OLIGO Yes 160 6 1.1 3.6 A A A A 2013 2.64 OLIGO Yes 182 8 0.4 0.8 B A A A 2014 3.31 MESO No 310 11 0.9 5.3 B B A A 2015 3.99 MESO No 330 20 4.8 7.1 B B B A 2016 3.94 MESO No 300 17 6.3 7.0 B B C A 2017 3.80 MESO No 270 12 5.9 6.8 B B C A 2018 3.58 MESO No 210 11 5.4 6.2 B B C A 2019 3.88 MESO No 230 14 8.2 10.0 B B C A 5 year 3.84 MESO No 268 14.8 6.1 10.0 A B C A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

50 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-32: TP, TN, chl-a concentrations and turbidity in Lake Pearson from 2005-2019 Table 4-22: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Pearson

ity

Probabil Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction TP 63 179 1094.33 5.38 0 1 11.08 0.70 to 1.20 increasing 1 TN 63 172 1069.33 5.23 0 11.12 5.76 9.10 to 15.05 increasing 1 chl-a 63 342 1154 10.04 0 4.76 15.11 4.70 to 4.83 increasing 1 Turbidity 63 103 1149 3.01 0.00 0.35 9.68 0.18 to 0.48 increasing 1

Figure 4-33: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Pearson, 2005-2019. Red line is the LWRP objective/outcome

Environment Canterbury Technical Report 51 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Turbidity Turbidity trends increased significantly between 2007 and 2017 (Table 4-22) driven by a sharp increase in turbidity between 2012 and 2019.The initial ‘jump’ between May 2012 and January 2013 (Figure 4-32) was possibly linked to a large storm and flood in January 2013, which potentially disturbed bottom sediments. Rainfall recorded at Lake Grasmere station on 2nd January 2013 was 70.5 mm, the second highest daily rainfall measured between December 2005 and July 2017 (Environment Canterbury rainfall data). The lake was reported to have been a bright grey/green colour after the 2013 flood with floating mats of benthic vegetation observed afterwards.

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Pearson was high in 2013 and 2018, but between 2010 and 2018 the native condition index decreased significantly (Table 4-23). This decrease has been attributed to long-term decreased water clarity which also appears to have caused a reduction in vegetation depth by ca. 2m (de Winton, 2018).

Table 4-23: Lake Pearson, LakeSPI overview, 1984-2013 1984 2010 2013 2018 % Change Change indicated? LakeSPI 45 56 56 55 -1 No Native Condition 50 59 60 53 -7 Possible Invasive Condition 60 41 44 37 -7 Possible Condition Moderate High High High NA No Invasive species Elodea Elodea Elodea Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 552 nm, i.e. green (Figure 4-34). The FU index ranged between 5 and 15, indicating a large variability in water colour from clear blue to green-brown.

Figure 4-34: Landsat derived water colour data for Lake Pearson, 2013-2018 (n = 36): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The lake water level was recorded as particularly low in January 2006 and May 2015 associated with long sustained drought periods. The lake was reported to have been a bright grey/green colour after the 2013 flood with floating mats (of macrophytes and debris) observed afterwards by our staff. Between November 2018 and March 2019 the water level fluctuated by 1.8 m.

52 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Summary Water quality in Lake Pearson has deteriorated between 2005 and 2019, with increases or “step changes” in TN, TP, chl-a, turbidity between 2008 and 2009 and between 2013 and 2015. TN increased greatest between 2013 and 2014, TP and chl-a from 2014 to 2015 (Figure 4-32). Total nutrient concentration have increased significantly over the sampling period. Increased nutrient concentrations may be linked to increased external loading from surrounding land uses (and possibly in part delivered by large flood events in 2013), increased internal recycling of nutrients, increased sediment resuspension, or a combination of the above. Since the lake intermittently stratifies in summer and periodic low oxygen episodes in the hypolimnion occur, internal phosphorus release may contribute to the total phosphorus loading. Since the summer of 2015/2016 TN and TP concentrations seem to be decreasing again, but phytoplankton biomass and turbidity remain on an increasing trajectory, and the lake remains mesotrophic and is close to becoming eutrophic.

Condition of the aquatic macrophyte community has deteriorated between 2013 and 2018. There is anecdotal evidence that the flood in January 2013 damaged and reduced the extent of both deep periphyton mat layers, and shallower macrophyte beds. However, the LakeSPI survey in January 2013 (after the flood event) indicated no change in macrophyte cover at the surveyed sites. Elevated turbidity and decreased water clarity following the floods may have resulted in the reported loss of bed periphyton mats and macrophytes after the January 2013 survey. The Lake SPI survey in May 2018 indicates that the macrophyte community has not been able to recover and vegetation depth is reduced by ca 2m, due to a long-term reduction in water clarity. At these greater depths the integrity of the lake bed periphyton mats stabilising bed sediments and reducing nutrient recycling/resuspension will also be an important consideration.

We continue to get regular observations and complaints about the visible state of the water in Lake Pearson and its effect on recreational activities. These may be mediated via increased land use nutrient inputs, effects of major flood events, or both.

While deterioration of water quality in Lake Pearson may at least in part be due to natural disturbances, effects of forest clearance and pasture development/grazing intensity can reduce a lake’s ability to recover from natural disturbances (as seen in Lake Okataina (Kpodonu et al., 2019)). To increase the chances of LWRP water quality objectives being met in the future, nutrient inputs to the lake should be reduced, and no further increase in nutrient allocation should be permitted in this lake zone, in line with current LWRP “sensitive lake zone” rules.

To closely monitor phytoplankton biomass, turbidity and dissolved oxygen levels in the lake in the future, Environment Canterbury will install a continuous water quality monitoring station in Lake Pearson in 2020.

Environment Canterbury Technical Report 53 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.4 Lake Hawdon

Figure 4-35: Sampling location in Lake Hawdon – aerial photograph (EcanMaps)

Lake Hawdon (Figure 4-35) is a small (0.35 km2), shallow (maximum 4 m), isolated kettle-hole lake, situated ca. 10 km from State Highway 73. Its small catchment is dominated by native tussock and low producing grasslands (LCDB v 4.1), partly used for extensive grazing of sheep and, sometimes, cattle. Recreational use is limited to a popular trout fishery. The outlet of the lake is an intermittently maintained deepened channel, which has modified natural drainage of the wetland at Lake Hawdon's outlet. The lake experiences small fluctuations in lake level and occasional surface freezing. It is particularly exposed to high NW winds. The lake can support large numbers of waterfowl at times. Lake Hawdon was a seasonal food collecting area for Māori in the past.

Nutrients The lake was generally low in TP, but had relatively high levels of nitrogen (Table 4-24). There was no significant trend in TP, but an increasing trend in TN between 2007 and 2019 (Table 4-25).

Phytoplankton biomass Chlorophyll a concentrations were generally low, with the exception of a large peak in April 2017 (Table 4-24). There was a significant increase in chl-a concentrations between 2007 and 2019 (Table 4-25).

Trophic Level Index The lake was oligotrophic from 2005 to 2008, but then became increasingly mesotrophic through to 2019 (Table 4-24). It was graded eutrophic in 2017 due to an unusually large algal biomass in April 2017. TLI increased between 2005 and 2009, then fluctuated above and below the mesotrophic threshold before becoming consistently mesotrophic or above from 2015 to the present (Figure 4-37).

Turbidity Turbidity was high in April 2017, along with high TN, TP and chl-a. Turbidity trends increased significantly between 2007 and 2019 (Figure 5-10, Table 4-25), particularly after 2012.

Lake ecological condition: LakeSPI The impact of invasive or exotic weeds in Lake Hawdon has been limited, and condition of the aquatic macrophyte community was excellent in 2013 and 2018 (Table 4-26, Sutherland et al., 2013). There was no significant difference in overall condition, but the invasive condition reduced between 2013 and 2018 as no Elodea beds were detected in the latest survey. However, Elodea is still likely to be present in the lake (de Winton, 2018). The macrophyte community was representative of a community indicative

54 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

of a ‘low nutrient and pristine lake system’ (de Winton, 2018). This finding is somewhat in contradiction to the relatively high TN concentrations.

Table 4-24: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Hawdon from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.43 OLIGO Yes 260 4 0.4 0.9 B A A A 2006 2.60 OLIGO Yes 340 3 1.0 1.1 B A A A 2007 2.83 OLIGO Yes 420 4.5 1.0 1.3 C A A A 2008 2.98 OLIGO Yes 460 5 1.3 1.4 C A A A 2009 3.18 MESO No 435 5.5 1.4 2.2 C A A A 2010 2.65 OLIGO Yes 430 2.5 0.5 0.7 C A A A 2011 2.95 OLIGO Yes 395 7 0.9 1.1 C A A A 2012 3.19 MESO No 470 6 1.2 1.7 C A A A 2013 3.26 MESO No 490 7 0.5 3.8 C A A A 2014 2.70 OLIGO Yes 330 2 0.4 2.3 B A A A 2015 3.40 MESO No 490 5 1.2 8.0 C A A A 2016 3.05 MESO No 450 4 1.4 2.4 C A A A 2017 4.08 EUTRO No 440 7 1.3 20.0 C A A B 2018 3.27 MESO No 490 6 1.4 3.3 C A A A 2019 3.42 MESO No 525 5 2.9 4.7 C A B A 5 year 3.44 MESO No 479 5.4 1.6 20.0 B A A B MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Figure 4-36: TP, TN, chl-a concentrations and turbidity in Lake Hawdon from 2005-2019

Environment Canterbury Technical Report 55 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-25: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Hawdon

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 62 33 1041.67 0.99 0.32 0 0 0.00 to 0.29 0.5 TN 61 84 1008 2.61 0.01 5.71 1.3 2.83 to 10.28 increasing 1 chl-a 62 84 1088.67 2.52 0.01 0.05 4.55 0.01 to 0.10 increasing 0.99 Turbidity 62 129 1082.33 3.89 0.00 0.03 6.01 0.02 to 0.04 increasing 1

Figure 4-37: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Hawdon, 2005-2019. Red line is the LWRP objective/limit

Table 4-26: Lake Hawdon, LakeSPI overview

2013 2018 % Change Change indicated? LakeSPI 86 96 10 Probable Native Condition 89 91 2 No Invasive Condition 15 0 -15 Yes Condition Excellent Excellent N/A No Invasive species Elodea None Yes Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 542 nm, i.e. blue-green (Figure 4-38).

56 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-38: Landsat derived water colour data for Lake Hawdon, 2013-2018 (n = 26): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The lake water level was reported to be low in March 2006, February 2007, and January 2010 following sustained dry spells.

Summary Lake Hawdon trophic status has increased from an oligotrophic status, to fluctuating around the threshold between oligotrophic and mesotrophic, and is now consistently mesotrophic. This is considered mainly due to high TN concentrations. A single high nutrient and chl-a concentration sampling event in 2017 meant that the lake exceeded LWRP outcomes in 2017, but it is not clear what drove this event. Concentrations of TN are high compared with the other Waimakariri lakes, but TP and phytoplankton biomass were nearly always within the oligotrophic range. Data from the 1970s and 1980s (Environment Canterbury, unpublished data) also suggest that Lake Hawdon had elevated TN compared to nearby lakes. Because of the relatively high concentrations of TN, the lake is very likely to be functionally limited by phosphorus availability. It is beneficial that Lake Hawdon has a small and relatively undeveloped catchment, so inputs of phosphorus from land use activities appear unlikely. The lake macrophyte community remains in excellent condition, which a community representative of a ‘pristine’ lake system and with a very low impact of invasive species. It is unclear why the invasive macrophyte Elodea canadensis has reduced or disappeared from the lake.

Environment Canterbury Technical Report 57 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.5 Lake Lyndon

Figure 4-39: Sampling location in Lake Lyndon – aerial photograph (EcanMaps)

Lake Lyndon (Figure 4-39) is a small (0.88 km2), lake, with a maximum depth of 19 m. While it drains to the Rakaia catchment, and so is considered part of the Coleridge Lakes Complex, it is geographically isolated from the other lakes in the complex and is more commonly associated with the Waimakariri lakes. The lake has neither a permanent inflow nor outflow stream. High ground at the northern end of the lake separates the main lake from ‘Little Lake Lyndon’, a shallow extension of the lake that persists and connects to the main lake only during high water levels. This ‘Little Lake Lyndon’ extension can remain wet consistently in wetter climate periods (years) but is reported to have been mostly dry in recent years.

The lake’s catchment is relatively small, with mostly native shrub and tussock grassland. In recent years the majority of the lake catchment has been consolidated into conservation control. Lake Lyndon is more sheltered from the North West winds than many of the nearby Waimakariri Lakes, but more exposed to Easterly systems. It is also more shaded from the north, so is considered a colder and more settled lake. For this reason it exhibits a greater propensity to freeze, particularly on the northern end, and in cold winters it has historically been an accessible ice skating lake. Lake Lyndon is located in a relatively undeveloped catchment, with no permanent human settlement nearby. The lake is a trout fishery (stocked for recreational fishing) and supports moderate numbers of waterfowl. Lake Lyndon is immediately adjacent to the state highway near the top of Porters Pass so is a common place for travellers to stop. Recreational use of the lake includes fishing, picnics, camping, water-skiing, motor- boating and ice-skating in winter.

Nutrients Lake Lyndon had low concentrations of TP in all years, but elevated TN particularly from 2012 to 2019 (Table 4-27). TN increased significantly between 2007 and 2019 (Table 4-28).

Phytoplankton biomass Chl-a concentrations were generally low, and very low in 2005. Chl-a increased significantly between 2007 and 2019 (Table 4-28).

58 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Trophic Level Index Lake Lyndon was graded oligotrophic for most years between 2005 and 2015 (Table 4-27), but was graded as mesotrophic in recent years 2016, 2018 and 2019 (Figure 4-41).

Turbidity Turbidity was generally low, with one exception in 2012, However, turbidity trends increased significantly between 2007 and 2017 (Table 4-28).

Table 4-27: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Lyndon from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 1.83 MICRO Yes 100 3 0.5 0.7 A A A A 2006 2.72 OLIGO Yes 135 4 2.2 2.8 A A B A 2007 2.40 OLIGO Yes 130 3.5 1.5 1.8 A A A A 2008 2.56 OLIGO Yes 150 5 1.4 1.6 A A A A 2009 3.15 MESO No 160 9 3.2 4.3 A A B A 2010 2.82 OLIGO Yes 170 6 1.3 3.3 B A A A 2011 2.82 OLIGO Yes 150 6.5 2.0 2.5 A A B A 2012 2.83 OLIGO Yes 170 6 1.4 2.8 B A A A 2013 2.96 OLIGO Yes 163 8 1.7 4.6 B A A A 2014 2.62 OLIGO Yes 166 6 0.7 4.9 B A A A 2015 2.99 OLIGO Yes 170 6 2.7 3.6 B A B A 2016 3.13 MESO No 166 5 2.9 6.1 B A B A 2017 2.97 OLIGO Yes 172 6 3.2 3.4 B A B A 2018 3.16 MESO No 159 7 3.2 13.0 A A B B 2019 3.19 MESO No 200 6 5.8 7.0 B A C A 5 year 3.09 MESO No 173 6 3.6 13.0 B A B B MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-28: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Lyndon

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 62 36 1016.67 1.1 0.27 0.1 1.67 0.00 to 0.25 increasing 0.98 TN 62 123 1045 3.77 0 2.99 1.83 1.96 to 4.01 increasing 1 chl-a 62 103 1111 3.06 0 0.18 7.78 0.10 to 0.26 increasing 1 Turbidity 62 108 1082 3.25 0.00 0.03 4.87 0.02 to 0.04 increasing 1

Environment Canterbury Technical Report 59 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-40: TP, TN, chl-a concentrations and turbidity in Lake Lyndon from 2005-2019

Figure 4-41: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Lyndon, 2005-2019. Red line is the LWRP objective/outcome

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Lyndon was high in 2010 and 2013, with a probable improvement in native condition between 2010 and 2013, but no change between 2013 and 2018 (Table 4-29).

60 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-29: Lake Lyndon, LakeSPI overview, 1984-2013 1984 2010 2013 2018 % Change Change indicated? LakeSPI 57 61 66 64 -2 No Native Condition 68 64 78 73 -5 No Invasive Condition 52 38 44 40 -4 No Condition High High High High NA No Invasive species Elodea Elodea Elodea Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 506 nm, i.e. green (Figure 4-42).The FU index was mostly 6 (blue-green), but up to 15 (green-brown) on occasion.

Figure 4-42: Landsat derived water colour data for Lake Lyndon, 2013-2018 (n = 30): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The lake water level was reported to be low or very low in March 2005, March 2012, March 2013 and May 2015, and as average in January 2010. Large variations in lake level were observed. These frequently generate public complaints and enquiries as to why water levels fluctuate so wildly.

An investigation in summer 2018/2019 showed that the lake strongly stratified between December and February, with very low dissolved oxygen and increased ammonia concentration near the bottom.

Summary Lake Lyndon was oligotrophic all years except 2009 until 2014, but has become consistently mesotrophic or on the mesotrophic threshold since 2015. Total nitrogen, turbidity and phytoplankton biomass all increased significantly between 2007 and 2019. Increases in TLI seem to correlate most strongly with increases in chl-a and to a lesser degree TN. Average ‘summer’ algal biomass more than doubled between 2015 and 2019.

With the land tenure changes to the Conservation estate, land use nutrient sources are unlikely to be significant causes. Possible causes for the increased TN and algal biomass include increased tourism and pressure on facilities on lake shore (e.g. picnic area, toilet bloc with septic tank, the lakeside lodge and freedom camping). There have been significant Zone scoping and proposals to isolate, restrict or secure these activities further from the lake shore. These appear to be the most relevant activities to address and reverse these lake water quality issues. We should also, however, note that there were also significant biological invasions that may alter lake processes, including the invasive nuisance algae Lindavia intermedia (that can cause ‘Lake Snow’), and invasion of the large zooplankton water flea Daphnia pulex that can potentially change phytoplankton communities. Phytoplankton and nutrient

Environment Canterbury Technical Report 61 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

dynamics in Lake Lyndon are also likely affected by the depth, stability and duration of the summer stratification, which is driven by local climate factors such as air temperatures and wind direction and speed. Because the lake strongly stratifies and there is evidence for nutrient release or recycling during anoxic episodes, changes in local climate (e.g. increasing summer air temperature 2014-2019, Figure A3-10) may be a factor in changes in TLI. If warmer summers are in fact a contributing factor to increasing phytoplankton biomass in Lake Lyndon, since warmer air temperatures are predicted with climate change, minimising nutrient inputs from anthropogenic sources becomes an even higher priority as any additional nutrients available will likely amplify any adverse effects of climate change on lake water quality.

4.3.6 Lake Georgina

Figure 4-43: Sampling location in Lake Georgina – aerial photograph (EcanMaps)

Lake Georgina (Figure 4-43) is a small (0.17 km2), shallow (10m maximum depth) kettlehole lake, which has been described as productive and brownish in colour. Main land use of the catchment has been extensive tussock grasslands and low producing pastoral farming (47%, Appendix 2), although there is potential for these tussock grasslands to be fertilized or improved by the current landowners. The lake intermittently discharges to Scamander Stream, which flows into Lake Coleridge. Freshwater crayfish (Paranephrops zealandicus) are found in the lake (Cromarty & Scott, 1995) and the lake is an accessible and popular trout fishery. It now has a constructed toilet block to help manage the potential effects of the high recreational (fishing) activity.

Nutrients, Phytoplankton biomass, Trophic Level Index and Turbidity Lake Georgina was graded eutrophic in the years 2007-08, 2012-14 and 2016-17 (Table 4-30), with relatively high concentrations of TN and elevated concentrations of TP (Table 4-30). Concentrations of TN, TP, chl-a and turbidity all increased between 2007 and 2019. Due to the influence of some high events, trends were probable for chl-a and turbidity, but not statistically significant (Figure 4-44, Table 4-31).

TLI values were highly variable ranging from oligotrophic in 2005, but alternating between mesotrophic and eutrophic the other 15 years (Table 3-30). A particularly high value in 2014 graded Lake Georgina just below supertrophic status. This high variability and these high levels of productivity were possibly related in part to highly varying lake water levels, and particularly low lake water levels. Whenever we record low lake levels, the lake is usually stained a deep yellow brown colour with either algal blooms, humic substances, or both. Large storm events or high intensity rain events may also cause high localised input of sediment to the lake, although the surface catchment area of the lake is relatively small and it is without any contributing stream or scree catchments that could be expected to yield high flood

62 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

sediment loads. However, such a storm event in April 2014 may have caused high turbidity and possibly fuelled a phytoplankton bloom in April 2014. But turbidity and TP were also high in February 2013, when rainfall was low, as well as prior to the storm in 2014,

Therefore, the cause of the high and increasing nutrient status and trophic condition of Lake Georgina is concerning, but the primary cause of this enriched status is unclear.

Table 4-30: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Georgina from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.41 OLIGO Yes 240 4 0.5 0.7 A A A A 2006 3.25 MESO Yes 460 5 1.0 1.5 B A A A 2007 4.64 EUTRO No 1850 18.5 3.8 8.5 D B B A 2008 4.25 EUTRO No 780 14 6.6 8.9 C B C A 2009 3.59 MESO Yes 450 11 2.6 4.1 B B B A 2010 3.54 MESO Yes 390 11 1.5 4.1 B B A A 2011 3.65 MESO Yes 395 11 1.9 4.6 B B A A 2012 3.35 MESO Yes 460 9 1.4 2.0 B A A A 2013 4.04 EUTRO No 420 17 1.5 5.1 B B A A 2014 4.94 EUTRO No 620 28 8.9 69.0 C C C D 2015 3.85 MESO Yes 550 18 1.7 2.5 C B A A 2016 4.27 EUTRO No 700 17 5.9 10.6 C B C B 2017 4.03 EUTRO No 730 14 1.5 8.0 C B A A 2018 4.11 EUTRO No 490 13 5.8 11.6 B B C B 2019 3.99 MESO Yes 550 10 4.2 9.4 C A B A 5 year 4.05 EUTRO No 604 14.4 3.8 11.6 C B B B MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-31: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Georgina

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 36 1092 1.06 0.29 0.28 2.12 -0.10 to 0.72 increasing 0.85 TN 63 33 1107 0.96 0.34 6.12 1.18 -3.92 to 15.09 increasing 0.84 chl-a 63 61 1125.67 1.79 0.07 0.16 6.4 0.00 to 0.33 increasing 0.96 Turbidity 63 60 1112.67 1.77 0.08 0.03 3.03 0.00 to 0.07 increasing 0.96

Environment Canterbury Technical Report 63 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-44: TP, TN, chl-a concentrations and turbidity in Lake Georgina from 2005-2019

Figure 4-45: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Georgina, 2005-2019. Red line is the LWRP objective/outcome

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Georgina was moderate in 2018 (Table 4-32). A large number of macrophytes were lost as a result of a large storm and a substantial algal bloom in the summer of 2013/2014 (Sutherland & Burton, 2015). An improvement in native condition between 2010 and 2015 suggests that the native macrophyte community was able to recover more quickly from

64 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

these events than the invasive Elodea (Sutherland & Burton, 2015). Therefore, the LakeSPI index increased between 2010 and 2015, with a marked decrease in Invasive Condition. Together, these trends indicate an overall improvement of condition of the aquatic macrophyte community between 2010 and 2015 (Sutherland & Burton, 2015), which has largely been maintained until the 2018 survey. However, between 2015 and 2018 invasive condition increased again.

Table 4-32: Lake Georgina, LakeSPI overview, 1984-2013 1984 2010 2015 2018 % Change Change indicated? LakeSPI 36 28 42 39 -3 No Native Condition 38 28 43 46 3 No Invasive Condition 70 77 52 59 7 Possible Condition Moderate Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea Elodea NA No Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 548 nm, i.e. green (Figure 4-46), which FU index indicating frequent occurrence of green-brownish colour, possibly due to high DOM with runoff or sediment resuspension.

Figure 4-46: Landsat derived water colour data for Lake Georgina, 2013-2018 (n = 40): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The lake water level was observed to be particularly low from January to April 2006, in April 2008 and in May 2015. Because of a suspected link between water quality and lake level, this lake would benefit from continuous recording of lake levels to help explain the observed patterns.

Summary Lake Georgina is a productive lake, with increasing trophic state and regular TLI results above LWRP objectives. High fluctuations in TLI appear to be mostly related to lake water level, with low lake levels correlating with highly coloured water, high TLI values and high nutrient concentrations. The causes of this observation may be a result of many mechanisms, and deserved more scrutiny. Possibly mechanisms of low water levels impacting water quality include nutrients being concentrated due to longer residence times, increased sediment resuspension as well as resulting in warmer water temperatures and higher phytoplankton growth.

Environment Canterbury Technical Report 65 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

While increasing trends in nutrients and phytoplankton biomass since 2007 are not statistically significant, deterioration of water quality is probable (probability of >0.8), and TLI objectives were exceeded more often than not in the past 5 years.

4.3.7 Lake Ida Lake Ida (Figure 4-47) is a small, shallow lake, near Lake Coleridge, perched at a higher elevation than the adjacent Lake Selfe. It is in a south easterly aspect in a shaded valley and so gets little direct sunlight. The lake has a surface area of 0.1 km2 and a maximum depth of approximately 9 m. A landslip in recent decades has divided the lake into two parts. At the Eastern end a part of the lake was historically bunded off to make a learner ice-skating rink to complement more advanced ice skating on the main body of the lake. While popular as an ice skating venue some decades ago, it is not currently promoted or accessible for ice skating. Due to its steep banks, the lake only has minor fringing wetlands. 30% of its catchment is low-producing grassland (Appendix 2). The lake has been described in the past as having “an interesting and unusual” phytoplankton composition (Cromarty & Scott, 1995). It also has a productive trout population.

Figure 4-47: Sampling location in Lake Ida – aerial photograph (EcanMaps)

Nutrients, Phytoplankton biomass, Trophic Level Index and Turbidity Lake Ida was graded oligotrophic until 2013, but since 2014 it is mesotrophic or on the threshold between oligotrophic and mesotrophic (2016 and 2019) (Table 4-33). High chl-a, and TN values recorded in February 2017 caused an unusually high TLI score in 2017. TN, TP and chl-a concentrations were low in 2005 to 2013, but TN and chl-a were elevated in most years since 2013 (Table 4-33). Turbidity was low in most years, with an unusually high reading in February 2017, associated with high chl-a. TN, TP, chl-a and turbidity all increased significantly between 2007 and 2019 (Figure 4-48, Table 4-34). TLI results reflected the increase in chl-a, TN and TP between 2005 and 2019 (Figure 4-49).

66 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-33: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Ida from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.31 OLIGO Yes 110 3 1.2 1.6 A A A A 2006 1.97 MICRO Yes 108 2 0.8 1.4 A A A A 2007 2.37 OLIGO Yes 110 3 1.6 4.6 A A A A 2008 2.43 OLIGO Yes 120 3 1.5 3.0 A A A A 2009 2.59 OLIGO Yes 110 6 1.4 2.7 A A A A 2010 2.78 OLIGO Yes 180 2.5 2.0 2.6 B A A A 2011 2.76 OLIGO Yes 145 7 1.6 2.3 A A A A 2012 2.45 OLIGO Yes 160 5 1.1 1.3 A A A A 2013 2.54 OLIGO Yes 142 8 0.5 1.6 A A A A 2014 3.19 MESO No 181 6 2.4 3.7 B A B A 2015 3.11 MESO No 178 6 3.3 4.5 B A B A 2016 2.97 OLIGO Yes 195 5 2.5 3.9 B A B A 2017 3.81 MESO No 181 6 4.4 32.0 B A B C 2018 3.09 MESO No 230 5 1.7 3.0 B A A A 2019 2.94 OLIGO Yes 149 4 3.8 4.9 A A B A 5 year 3.18 MESO No 187 5.2 3.1 32.0 B A B C MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-34: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Ida

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 69 1079.67 2.07 0.04 0.2 4 0.00 to 0.37 increasing 0.9 TN 63 165 1103 4.94 0 7.34 4.8 5.04 to 9.98 increasing 1 chl-a 63 120 1128 3.54 0 0.16 8.89 0.08 to 0.23 increasing 1 Turbidity 62 126 1056 3.85 0.00 0.03 6.29 0.02 to 0.05 increasing 1

Environment Canterbury Technical Report 67 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-48: TP, TN, chl-a concentrations and turbidity in Lake Ida from 2005-2019

Figure 4-49: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Ida, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI The ecological condition of Lake Ida was excellent in 2018 (Table 4-35). Results indicate a stable ecological condition between 2010 and 2018 (de Winton, 2018), despite a reduction in invasive condition.

68 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-35: Lake Ida, LakeSPI overview 2010 2018 % Change Change indicated? LakeSPI 73 83 10 Probable Native Condition 75 75 0 No Invasive Condition 25 7 -18 Yes Condition High Excellent NA Yes Invasive species Elodea Elodea NA No Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 517 nm, i.e. green (Figure 4-50). It varied little and indicated a stable green colour condition.

Figure 4-50: Landsat derived water colour data for Lake Ida, 2013-2018 (n = 28): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Water quality in Lake Ida appears to be deteriorating. Phytoplankton biomass, nutrient concentrations, and turbidity in Lake Ida all increased significantly from 2007 to 2019. Lake Ida was oligotrophic until 2013, and mesotrophic in 4 of the last 6 years of monitoring (2014, 2015 and 2017, 2018) and the other two years right on the boundary between oligotrophic and mesotrophic. The abnormally high TLI in 2017 was caused by unusually high concentrations of chl-a, TN and turbidity probably due to an algal bloom in February 2017. These trends are unexpected for a lake that is in an isolated position with little potential for significant land use effects. The causes of this are not immediately obvious, but may possibly lie in a changing climatic state, possibly due to lesser ice cover, changes in magnitude and timing of inflows, and increased potential for persisting lake stratification in this sheltered valley position. Nutrient inputs from its historical use as a very popular winter ice skating venue may be increasing be made available as increased internal nutrient cycling from the bed sediments under a changing limnological regime as climate patterns change.

Environment Canterbury Technical Report 69 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.8 Lake Selfe

Figure 4-51: Sampling location in Lake Selfe – aerial photograph (EcanMaps)

Lake Selfe (Figure 4-51) is a small (0.65 km2), but deep (30 metres) lake upstream of Lake Henrietta, and downstream of Lakes Ida and Evelyn. It differs from the other lakes in this series being particularly deep and without extensive shallow marginal wetlands. The lake is a Wildlife Refuge and is considered the most important of the ‘Lake Coleridge lakes’ as grebe habitat (Cromarty & Scott, 1995) and is a highly fished trout fishery. Steep banks limit the extent of fringing wetlands. About 40% of the catchment is low producing grassland (Appendix 2).

Nutrients, Phytoplankton biomass and Trophic Level Index Lake Selfe was oligotrophic in all years until 2015 but mesotrophic in 2015, 2017 and 2018 (Table 4-36, Figure 4-53), with low levels of chl-a and nutrients in most years (Table 4-36). 2017 was an unusually productive year, with high chl-a recorded in several months, resulting in an unusually high TLI. However, there was no significant trend in nutrient concentrations, TP or TN, between 2007 and 2019 (Table 4-37). However, phytoplankton biomass increased significantly between 2007 and 2019 (Table 4-37), mostly due to increased concentrations from 2014-2017 (Figure 4-52).

Turbidity There was a step change in turbidity in 2011, which may possibly be associated with the Canterbury earthquakes, but at low level (i.e. turbidity is still low compared to many other lakes). Turbidity increased significantly between 2007 and 2019 (Table 4-37).

70 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-36: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Selfe from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.03 OLIGO Yes 130 3 0.5 0.8 A A A A 2006 2.34 OLIGO Yes 140 2.5 1.2 1.5 A A A A 2007 2.28 OLIGO Yes 140 3.5 0.9 1.1 A A A A 2008 2.78 OLIGO Yes 150 5 2.9 3.7 A A B A 2009 2.90 OLIGO Yes 150 6 2.0 5.7 A A A A 2010 2.94 OLIGO Yes 180 7 1.4 3.7 B A A A 2011 2.82 OLIGO Yes 160 7 1.5 2.7 A A A A 2012 2.81 OLIGO Yes 180 6 1.4 2.1 B A A A 2013 2.68 OLIGO Yes 163 8 1.2 1.6 B A A A 2014 2.60 OLIGO Yes 154 2 1.7 4.9 A A A A 2015 3.04 MESO No 162 6 2.1 7.2 B A B A 2016 2.62 OLIGO Yes 146 2 3.0 3.2 A A B A 2017 3.52 MESO No 162 7 11.0 18.0 B A C B 2018 3.28 MESO No 152 6 3.9 9.1 A A B A 2019 2.77 OLIGO Yes 148 4 2.9 6.1 A A B A 5 year 3.05 MESO No 154 5 4.6 18.0 A A B B MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-37: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Selfe

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 15 1065 0.43 0.67 0 0 -0.08 to 0.20 0.50 TN 62 17 1036.33 0.5 0.62 0.25 0.16 -1.00 to 1.75 increasing 0.66 chl-a 63 137 1126.33 4.05 0 0.15 8.78 0.10 to 0.23 increasing 1 Turbidity 63 163 1117.67 4.85 0.00 0.03 6.42 0.02 to 0.05 increasing 1

Environment Canterbury Technical Report 71 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-52: TP, TN, chl-a concentrations and turbidity in Lake Selfe from 2005-2019

Figure 4-53: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Selfe, 2005-2019. Red line is the LWRP objective/outcome

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Selfe was high in 2010 and 2014 (Table 4-38), with a possible improvement in native plant condition.

72 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-38: Lake Selfe, LakeSPI overview, 2010-2014 2010 2014 % Change Change indicated? LakeSPI 52 57 5 No Native Condition 53 61 8 Possible Invasive Condition 48 44 -4 No Condition High High NA No Invasive species Elodea Elodea Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 499 nm, i.e. blue-green (Figure 4-54). It was relatively stable, indicating little change in colour hue.

Figure 4-54: Landsat derived water colour data for Lake Selfe, 2013-2018 (n = 55): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Selfe was oligotrophic until 2015, but has had mesotrophic conditions in 3 of the last 5 years of monitoring. Overall, phytoplankton biomass and turbidity have increased since 2009. It is not immediately apparent why Lake Selfe has recently recorded higher phytoplankton biomass (chl-a) as this is not accompanied by higher nutrient concentrations. This may be a result of a changing composition of the phytoplankton community, or changes in stratification patterns (affecting e.g. light or dissolved nutrient availability) and deserves further study.

Environment Canterbury Technical Report 73 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.9 Lake Coleridge

Figure 4-55: Sampling location in Lake Coleridge (SQ31045) – aerial photograph (EcanMaps)

Lake Coleridge (Figure 4-55) is a large, deep lake with a surface area of 37 km2 and a maximum depth of over 200 m. Since 1914 the lake has been used for hydro-power generation, and inflows were increased by diversions of the Harper and the Wilberforce Rivers in 1921 and 1977, respectively (Biggs et al., 1986, Schallenberg et al., 2001). At the time there were concerns that Wilberforce waters, which carry at times glacial meltwaters rich in fine sediments would impact on visual clarity in the lake (Biggs et al., 1986). These concerns appear justified as there was a “a significant decrease in visual clarity of the lake as measured by Secchi disk depth (which fell from 13.4 to 8.6 m) has occurred since the Oakden Canal was commissioned” (Biggs et al., 1990).

There was considerable monitoring of the clarity and ecological state of Lake Coleridge required by the hydro-electric operators and conducted by NIWA in the 1990s (e.g. Hawes & Schwarz, 1997; Schwarz & Hicks, 1995; Downes, 1995). Changes to the consents, in association with the 2013 amendments to the 1988 Rakaia Water Conservation Order, have allowed additional diversion of water to Lake Coleridge and further degrees of level regulation and water storage. It is unclear what effects this new regime may be having on the light climate, limnology, or ecology of the lake as no comparable studies to the NIWA reports has been undertaken over the past 15 years (or during the duration of this study), with the exception of a memorandum on water colour by Ryder Consulting in 2014/2015, which indicated a further decrease in Secchi Depth (visual clarity) to an average of 6.7 m.

Predominant land use in the catchment is extensive pastoral farming, but areas near the lake have recently increasingly been developed as higher producing grasslands (Figure 4-19). Bach and caravan camping on the eastern end of the lake has been closed down over the past decade.

Fish present in the lake include two species of Bully, Koaro (Galaxias brevipinnis), and Long-finned Eel (Anguilla dieffenbachii) as well as introduced Brown Trout, Rainbow Trout and Quinnat Salmon. The lake also had abundant koura (freshwater crayfish) and kakahi (freshwater mussels). Lake Coleridge is valued for its scenic beauty, natural character and its recreational opportunities. The lake is likely to

74 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

have been a source of water birds to Māori travelling between the coasts and seasonal hunting parties, and is referred to in Māori mythology.

Lake Coleridge stratifies thermally in summer, with a complex and deep thermocline that results from strong adiabatic winds (Schallenberg et al., 2001, Figure 4-56). The lake usually does not completely mix until July and remains isothermal until September or October (Schallenberg et al., 2001). The lake was previously regarded as often particularly clear, with plant growth recorded to very deep levels.

Nutrients Lake Coleridge had very low nutrient concentrations from 2005 to 2019. Total nitrogen decreased significantly between 2007 and 2019 (Table 4-40). For total phosphorus a trend was not determined due to large number of data points below detection limit, and frequent changes of the detection limit for the lab analysis.

Phytoplankton biomass Unlike other large, deep microtrophic lakes of the South Island (see e.g. Duthie & Stout, 1986), Lake Coleridge does not have a spring peak of phytoplankton biomass when surveyed in 1993-1994, but highest chl-a concentrations in summer (Schallenberg et al., 2001). Schallenberg et al. (2001) assumed that the lack of a spring peak of phytoplankton in Lake Coleridge may be due to a combination of deep mixing in spring (due to high prevailing NW winds) and frequent high turbidity due to spring floods. These two factors both cause light limitation of phytoplankton growth. Our data also shows frequent elevated turbidity in December. There was no trend for chl-a between 2007 and 2019 (Table 4-40).

Because nutrient concentrations are very low, phytoplankton in the mixed layer are likely to be nutrient limited over the summer stratification. The gradual breakdown or deepening of the thermocline from April to May seemed to fuel the autumn peak of phytoplankton observed in 1993/94 (Schallenberg et al., 2001). Schallenberg et al. (2001) calculated that in May/June nutrient supply from the meta- and hypolimnion was much greater than nutrient loads from the river. Thus, physical lake processes as well as internal nutrient cycling seem to drive phytoplankton dynamics in Lake Coleridge.

Trophic Level Index Lake Coleridge had very low nutrient concentrations and was classified as microtrophic in all years, except 2009. In 2009 the lake was assigned an oligotrophic status due to relatively high chl-a concentrations in March 2009, and high TN in April 2009 (Table 4-39).

Turbidity There was an increasing trend in turbidity between 2007 and 2019 (Table 4-40). We observed frequent high turbidity in December coinciding with spring floods. The introduction of Lindavia intermedia, and associated lake snow is a potential cause for increased turbidity, but lake snow usually contains abundant chlorophyll a and chlorophyll a did not seem to be correlated to turbidity. Other possible explanations for increasing turbidity include higher sediment loads due to larger amount of water diverted or larger spring floods, or increased rainfall. There appears to be no obvious relationship of in- lake turbidity with summer rainfall, total annual rainfall or number of windy days at Snowdon (Figure A3-7, Figure A3-11, Figure A3-14).

Environment Canterbury Technical Report 75 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-56: Depth distribution of temperature (top) and chl-a (bottom) in Lake Coleridge, 1993- 1994 (reproduced from Schallenberg et al., 2001) Table 4-39: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Coleridge from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 1.01 MICRO Yes 49 2 0.1 0.3 A A A A 2006 1.32 MICRO Yes 46 1.5 0.4 0.8 A A A A 2007 0.95 MICRO Yes 29 1.5 0.5 0.6 A A A A 2008 1.85 MICRO Yes 40 3 0.8 1.8 A A A A 2009 2.26 OLIGO No 55 4 1.6 2.9 A A A A 2010 1.83 MICRO Yes 60 2.5 0.3 1.5 A A A A 2011 1.57 MICRO Yes 50 3.75 0.4 0.5 A A A A 2012 1.36 MICRO Yes 40 2.5 0.5 0.8 A A A A 2013 1.15 MICRO Yes 30 2 0.1 0.6 A A A A 2014 1.13 MICRO Yes 41 2 0.4 0.5 A A A A 2015 1.30 MICRO Yes 35 2 0.5 0.7 A A A A 2016 1.06 MICRO Yes 31 2 0.4 0.6 A A A A 2017 1.29 MICRO Yes 27 2 0.6 0.7 A A A A 2018 1.60 MICRO Yes 30 2 0.5 4.0 A A A A 2019 1.73 MICRO Yes 39 2 0.9 1.2 A A A A 5 year 1.40 MICRO Yes 32 2 0.6 4.0 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

76 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-40: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Coleridge

ance

Probability Parameter Sample size Kendall statistic Vari Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 62 -89 1057.67 -2.71 0.01 -1.36 -3.64 -2.20 to -0.33 decreasing 1 chl-a 62 21 1040.33 0.62 0.54 0 0 -0.01 to 0.03 0.50 Turbidity 62 108 1059.33 3.29 0.00 0.03 5.02 0.01 to 0.04 increasing 1

Figure 4-57: TP, TN, chl-a concentrations and turbidity in Lake Coleridge from 2005-2019

Environment Canterbury Technical Report 77 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-58: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Coleridge, 2005-2019. Red line is the LWRP objective/outcome

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Coleridge continuous to be excellent (Table 4-41), but maximum depth extent of native aquatic plants seems to have reduced between the 1970/1980 and 2014. Biggs et al. (1990) reported a euphotic depth of ca 30m on average which is consistent with the maximum depth of aquatic plants recorded in LakeSPI studies in the 1984 and 2010. However, depth extend was lower in 2001 and 2014 suggesting a possible long-term decrease in visual clarity.

Table 4-41: Lake Coleridge, LakeSPI overview, 1978-2014 1978 1984 2001 2010 2014 % Change Change indicated? LakeSPI 84 91 85 89 91 2 No Native Condition 83 84 83 85 88 3 No Invasive Condition 13 0 11 4 4 0 No Condition Excellent Excellent Excellent Excellent Excellent NA No Invasive species Elodea None Elodea Elodea Elodea NA No Max depth (native) 35 36 25 32 26 Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 487 nm, i.e. blue (Figure 4-59). Compared to other Canterbury high-country lakes, there was very little variability in water colour in Lake Coleridge, and the lake appeared to be the “bluest” in colour.

78 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-59: Landsat derived water colour data for Lake Coleridge, 2013-2018 (n = 35): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations Ryder Consulting prepared a memorandum on a ‘baseline’ for water colour from August 2014 to August 2015. They reported an average Secchi depth of 6.7 m (5.0-9.7 m), turbidity of 0.89 NTU (0.42-1.42 NTU), and an average Munsell hue colour of 60 (52.5-65). Samples were taken from sites ‘downstream’ of the Environment Canterbury sampling site. Their study found no strong relationships between water colour and the sediment loading from the hydro canals or water colour and lake level. However, water colour was only established on 12 occasions.

Summary Lake Coleridge was microtrophic with very low nutrient and chl-a concentrations in all years except 2009. Relatively high chl-a between 2008-2010 caused a higher TLI in those years peaking in 2009 and increasing again in 2018 and 2019. In Lake Coleridge intra-annual fluctuations in TN and chl-a (Figure 4-57) (and thus TLI, Figure 4-58) were driven by samples taken in March (and April). Therefore these fluctuations could be a reflection of variability in timing and strength of the erosion (deepening) of the thermocline in autumn.

Turbidity in Lake Coleridge has increased significantly between 2007 and 2019, and there is evidence that visual clarity has reduced since the 1980s, although no systematic monitoring was carried out.

The lake’s aquatic plant community was in excellent ecological condition (measured as LakeSPI) in 2014. However, Lindavia intermedia has been reported as abundant in the phytoplankton, and lake snow has been present since 2012 and was reported at ‘nuisance’ levels (causing the ‘lake snow’ effect) in 2016. Depth extent of macrophyte beds have reduced since 1984. This reduction could be linked to changes in visual clarity. But despite recommendations by Bigg et al., 1986 no long-term records for visual clarity exist to our knowledge. It is therefore recommended that long-term trends in visual are made a priority.

For a lake with such high scenic beauty, natural character, recreational opportunities, and cultural value, it is surprising that the extensive research monitoring conducted in the 1990s has not been repeated since significant changes to the diversion and storage regime. Increasing degrees of regulation of water inflows and water levels can have influence a wide range of limnological processes in a large lake such as this. Relatively simple trophic level monitoring may not pick up systemic changes in lake processes at early stages.

Environment Canterbury Technical Report 79 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.10 Lake Evelyn

Figure 4-60: Sampling location in Lake Evelyn – aerial photograph (EcanMaps)

Lake Evelyn (Figure 4-60) is a small (0.17 km2), shallow (3.2 m) lake east of Lake Selfe, and one of the complex of lakes adjacent to Lake Coleridge. Land cover in the catchment includes low producing (33%, Appendix 2) and depleted grassland as well as native vegetation (LCDB v 4.1). The lake is a Wildlife Refuge. Thick reed-beds of Raupo and other wetland vegetation are present around most of the lake edge, forming a large area of fringing wetlands. As with the other lakes in this complex the lake can support large numbers of water birds, and a small trout fishery.

Nutrients, Phytoplankton biomass and Trophic Level Index Lake Evelyn was oligotrophic and 2014 but mesotrophic in 2015 and 2019, (Table 4-42), with low concentrations of TP, but higher TN (Table 4-42). One large chl-a reading in 2019 caused a higher than usual TLI (Table 4-42). The large area of fringing wetlands may strongly influence this lake. Catchment modelling suggests that nutrient uptake by macrophytes is an important factor in nutrient cycling in this shallow, macrophyte dominated lake (Kelly et al., 2014). Turbidity was generally low, below 2 NTU.

Table 4-42: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Evelyn from 2014- 2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2014 2.79 OLIGO Yes 173 6 1.1 1.6 B A A A 2015 3.14 MESO No 208 8 2.5 3.1 B A B A 2019 3.87 MESO No 191 4 0.6 37.0 B A A C MED = seasonal median concentration (Dec-May)

80 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake ecological condition: LakeSPI The condition of the aquatic macrophyte community of Lake Evelyn has been deteriorating from high to moderate between 2010 and 2014 (Table 4-41), and appears to be due to an increase in invasive plant cover at the expense of native condition. “Dense beds of elodea now form closed canopies, reaching up to 1.5 m in height, throughout the lake.” (Sutherland & Edwards, 2014).

Table 4-43: Lake Evelyn, LakeSPI overview, 2010-2014 2010 2014 % Change Change indicated? LakeSPI 56 44 -12 Probable Native Condition 82 67 -15 Yes Invasive Condition 52 64 12 Probable Condition High Moderate NA Yes Invasive species present Elodea Elodea Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 555, i.e. green-orange (Figure 4-61). Comparatively high FU numbers which were frequently in the green-brown range may reflect the shallow, macrophyte-rich nature of the lake, and possibly DOM inputs from the surrounding wetlands.

Figure 4-61: Landsat derived water colour data for Lake Evelyn, 2013-2018 (n = 56): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Evelyn was only monitored for three years, and appeared to be on the threshold between oligotrophic and mesotrophic condition 2014/2015, but well into the mesotrophic band by 2019. This 2019 result was strongly influenced by one high chl-a sample. High uptake of nutrients by macrophytes may result in a lower TLI than was predicted based on the catchment nutrient load (Kelly et al., 2014). Invasive macrophytes (Elodea) are becoming more prominent in the lake, causing a decline in LakeSPI score and probably driving other changes in the values and limnology of Lake Evelyn. It is not obvious what is causing such a change in the invasive macrophytes of the lake.

Environment Canterbury Technical Report 81 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.3.11 Lake Catherine

Figure 4-62: Sampling locations in Lake Catherine – aerial photograph (EcanMaps)

Lake Catherine (Figure 4-62) (also known as Lake Monck) is a small, shallow lake (6 m depth, 0.18 km2 surface area) and the northern most lake in this complex of lakes adjacent to Lake Coleridge. Its permanent outflow drains to the Ryton River and into Lake Coleridge. The lake is a Wildlife Refuge, with small fringing wetlands and willow trees and a small hut adjacent to the lake. The lake has a productive trout fishery, and was renowned for its clear blue waters and high landscape values.

The catchment has undergone some significant pasture development on the upper (outlet) end of the lake, with cultivation of tussock lands converted to rye grass at the Northern end of the lake and 18% of catchment classed as highly producing grassland in 2014 (Appendix 2).

Nutrients, Phytoplankton biomass and Trophic Level Index The lake was oligotrophic in 2014 and 2015 with low levels of TP and chl-a, but elevated TN which has increased from 2014 to 2019. Maximum chl-a has also increased over 2014 to 2019 (Table 4-44).

Table 4-44: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Catherine from 2014-2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2014 2.16 OLIGO Yes 120 2 0.6 2.4 A A A A 2015 2.57 OLIGO Yes 164 4 1.0 2.9 B A A A 2019 2.54 OLIGO Yes 181 2 0.8 4.0 B A A A MED = seasonal median concentration (Dec-May)

Water colour The median of the dominant wavelength between 2013 and 2018 was 508 nm, i.e. green (Figure 4-63). FU scores varied between 4 and 11, i.e. from blue to green.

82 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-63: Landsat derived water colour data for Lake Catherine, 2013-2018 (n = 28): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations In November 1978, Environment Canterbury staff noted that Lake Catherine was particularly clear compared to other small lakes in the area.

Summary Lake Catherine was monitored only between 2014 and 2015 and in 2019, and was in oligotrophic condition in these years meeting its LWRP objectives. But increases in both TN and maximum chl-a over this period and coinciding with of significant pasture development, is concerning for this particularly clear and attractive lake.

4.3.12 Lake Henrietta

Figure 4-64: Sampling locations in Lake Henrietta – aerial photograph (EcanMaps)

Lake Henrietta (Figure 4-64) is a small, shallow lake (5 m deep, 0.04 km2 surface area) downstream of Lake Selfe. The lake is a remnant of a larger lake and wetland complex. A large wetland / swampy area is located at the southern end of the main lake. Lake Henrietta is fringed by thick raupō reed-beds on northern end of lake. Both brown and rainbow trout are present. The lake is occasionally fished for trout but access around the margins of the lake is limited by the extensive wetland vegetation.

Environment Canterbury Technical Report 83 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Nutrients, Phytoplankton biomass, Trophic Level Index Lake Henrietta was graded eutrophic in 2014 and 2015 (Table 4-45), with high levels of nitrogen and moderate levels of chl-a, but graded mesotrophic in 2019 (Table 4-45). The lake did not meet the TLI objectives in the LWRP in all three years of monitoring.

Table 4-45: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Henrietta from 2014-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2014 4.20 EUTRO No 360 13 4.4 21.0 C B B B 2015 4.78 EUTRO No 590 22 9.0 21.0 C C C B 2019 3.64 MESO No 290 8 5.3 11.0 B A C B MED = seasonal median concentration (Dec-May)

Water colour The median of the dominant wavelength between 2013 and 2018 was 538 nm, i.e. green (Figure 4-65).

Figure 4-65: Landsat derived water colour data for Lake Henrietta, 2013-2018 (n = 56): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations The water level was particularly low in February 2015.

Summary Lake Henrietta was eutrophic when sampled in 2014 and 2015, and mesotrophic in 2019, exceeding the LWRP TLI target of oligotrophic. Nutrient concentrations were nearly twice as high as in those in the much deeper and larger Lake Selfe located a short distance upstream that discharges into Lake Henrietta. CLUES catchment modelling suggests that Lake Henrietta receives a much higher sediment and nutrient load than Lake Selfe (Kelly et al., 2014). The modelling also suggests groundwater as a possible important source of nitrogen to Lake Henrietta (Kelly et al., 2014). Lake Henrietta displays unusual results that appear to be dominated by extensive groundwater and wetland systems. It may not therefore be directly comparable to the other lakes in this Coleridge complex.

84 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.4 Ashburton Management Water Zone The lakes in the Ashburton Basin area are mostly small and relatively shallow. The exceptions to this are Lakes Heron, Camp and Clearwater which have deeper basins over 12 metres deep. The Ashburton Basin lakes mostly drain to the South Branch of the Ashburton River, although Lake Heron drains to the Rakaia River, and Lake Denny Drains to the Rangitata River.

Several of the lakes, including Lake Denny and the Māori Lakes, are in catchments predominantly used for pastoral farming over improved pastures. Substantial areas near Lake Heron were converted from low to high producing grassland between 1996 and 2001 or 2008 and 2012 (LCDB 4). Land cover in the basin is illustrated in Figure 4-66, Figure 4-67 and Appendix 2. Lakes Emma, Emily and Clearwater are in catchments with high a high proportion of public conservation land (Figure 4-68), but Lake Emily and Clearwater also have significant proportions of pastoral agriculture in their catchments.

The Ashburton Basin lakes are valuable habitats for waterfowl, especially Crested Grebe and New Zealand Scaup (Cromarty & Scott 1995). They are also popular for fishing and duck hunting. Lake Camp is primarily managed as a recreational lake, allowing power boats, and is a significant water ski resource. Ō Tū Wharekai, which includes the Ashburton lakes and upper Rangitata River, is also one of the three sites across the country that make up the national Department of Conservation Arawai Kakariki wetland restoration programme.

“The Ō Tū Wharekai / Ashburton Lakes area holds immense cultural significance to Ngāi Tahu Whānui, being valued as an important mahinga kai and travelling area, both in the past and today” (Te Rūnanga o Arowhenua et al., 2010). A number of lakes were identified as significant sites as well as priority areas for management: Kirihonuhonu (Lake Emma), Ō Tū Roto (Lake Heron), Ō Tū Wharekai (Māori Lakes), Ōtautari (Lake Camp), Te Puna a Taka (Lake Clearwater). The overall cultural health of the Ō Tū Wharekai / Ashburton Lakes area was assessed to be moderate, due to “historical loss of native flora and fauna and subsequent grazing and stock pressure on both the landscape and waterways of the area” (Te Rūnanga o Arowhenua et al., 2010).

Lake water quality and the condition of the aquatic macrophyte community reflect these pressures. Many of the Ashburton lakes have high concentrations of total nitrogen and phytoplankton biomass (Figure 5-2, Figure 5-3). All but one of the Ashburton lakes failed to reach their LWRP TLI objectives between 2015 and 2019 (based on the 5-year average, Table 5-1). Lake Denny is the lake with highest TN, TP, chl-a and TLI of all Canterbury high-country lakes sampled (Figures 5-1 to 5-4). Three lakes had increasing trends in either TN and/or chl-a between 2007 and 2019 (Table 5-8). Concentrations of TN increased significantly in the Māori Front lake between 2007 and 2019, and with high probability in Lake Heron (Figure 5-13). Phytoplankton biomass also increased significantly in Lake Heron and Lake Camp (Figure 5-12), with the largest increase between 2016 and 2019.

Most lakes are only in moderate ecological condition as measured by the LakeSPI index, and Lake Denny has lost most of its macrophytes community (Table 5-4). Concerns about the vulnerability of the Ashburton Lakes to further degradation and possible macrophyte collapse in some lakes were also raised by Kelly et al. (2014): A number of shallow lakes may be at risk of ‘flipping’ from a macrophyte- rich to a phytoplankton dominated, turbid state (Kelly et al., 2014), as observed for Lake Denny. Once a lake has ‘flipped’ macrophyte communities are difficult to reinstate, so efforts should be directed towards preventing loss of macrophytes, including the reduction of nutrient loads. While there have been concerns about water birds contributing to nutrient loads, estimated bird contributions were less than 6% of TN load, and less than 15% of TP load (with the exception of Lake Emily at 26%) (Kelly et al., 2014).

To prevent further degradation and to increase the likelihood that LWRP lake water quality objectives are met nutrient inputs to all lakes should be minimized and no further increases permitted. To protect water quality in high-country lakes, these lakes are situated in “sensitive lake zones” under the LWRP, which is a tool to ensure the areas surrounding these lakes are flagged as nutrient sensitive and requiring specific management. Currently this special management consists of identifying 2009-2013 baseline loads and stopping exceedance of baseline loads into the future. However, many lakes were already degrading in water quality between 2005 and 2013 and the data presented here illustrates that the ‘status quo’ management appears not to safeguard long-term ecosystem health or “halt” degradation of water quality. Reductions in nutrient loading should be considered especially for ‘at risk” lakes where

Environment Canterbury Technical Report 85 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

either water quality objectives are frequently exceeded by a large margin (e.g. Lake Denny), or where water quality is likely to be deteriorating (e.g. Lake Heron).

Figure 4-66: Land Cover Classification (LCDB 4) in the Ashburton Lakes basin (Lakes Heron, Emily and Māori Lakes). Green lines are catchment boundaries

86 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-67: Land Cover Classification (LCDB 4) in the Ashburton Lakes basin (Lakes Clearwater, Camp, Emma and Denny). Green lines are catchment boundaries

Environment Canterbury Technical Report 87 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-68: Public conservation land in the Ashburton Lakes Basin. Green lines are catchment boundaries

88 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.4.1 Lake Emily

Figure 4-69: Sampling location in Lake Emily – aerial photograph (EcanMaps)

Lake Emily (Figure 4-69) is a small, shallow lake (0.19 km2, 2.3 m depth) with a large wetland margin located in a catchment that is predominantly public conservation land (Figure 4-68).The lake is linked to a 50 ha swamp to the west and northwest which collects water from numerous streams and seeps. Drainage of the wetland is via Jacobs Stream to the Māori Lakes (Cromarty & Scott, 1995). The lake has high angler usage.

Nutrients, Phytoplankton biomass and Trophic Level Index Lake Emily went through a period of eutrophic conditions between 2012 and 2016, but returned to a mesotrophic status in 2017 and 2018 (Table 4-46). TLI peaked in 2013 and 2019 (Figure 4-71).

Lake Emily had high median chl-a between 2008 and 2014, but low median concentrations of chl-a between 2015 and 2017 (Table 4-46). Concentrations of TN were high in most years, and TP was elevated (Table 4-46). TP was highest between 2012 and 2015 and in 2019 (Figure 4-70).

There were no significant trends in TN, TP or chl-a between 2008 and 2019 (Table 4-47).

Environment Canterbury Technical Report 89 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-46: Trophic Level Index and attribute states (NPS-FM 2014) for from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2008 3.77 MESO Yes 450 15 2.5 2.8 B B B A 2009 3.98 MESO Yes 455 16.5 4.1 5.4 B B B A 2012 4.15 EUTRO No 410 17 2.4 10.8 B B B B 2013 4.56 EUTRO No 450 42 5.9 20.0 B C C B 2014 4.17 EUTRO No 330 20 6.0 17.0 B B C B 2015 4.19 EUTRO No 410 20 2.0 21.0 B B A B 2016 4.20 EUTRO No 440 20 2.0 19.0 B B A B 2017 3.94 MESO Yes 480 16 1.4 7.0 B B A A 2018 3.99 MESO Yes 390 23 3.4 6.0 B C B A 2019 4.83 EUTRO No 430 26 5.5 50.0 B C C C 5 year 4.23 EUTRO No 430 21 2.9 50.0 B C B C MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-47: Seasonal Mann-Kendall Trend Test Results, 2008-2019, Lake Emily

ance

Probability Parameter Sample size Kendall statistic Vari Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 46 37 429.67 1.74 0.08 0.73 3.65 0.00 to 1.27 increasing 0.96 TN 46 10 435.33 0.43 0.67 2.26 0.52 -5.10 to 9.81 increasing 0.69 chl-a 46 4 449.33 0.14 0.89 0.02 0.88 -0.13 to 0.29 increasing 0.53 Turbidity 46 -14 451.33 -0.61 0.54 -0.04 -2.38 -0.08 to 0.04 decreasing 0.74

Figure 4-70: TP, TN, chl-a concentrations and turbidity in Lake Emily from 2008-2019

90 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-71: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Emily, 2008-2019. Red line is the LWRP objective/limit Turbidity Turbidity levels in the lake appear to be linked to phytoplankton biomass (Figure 4-70), i.e. high chl-a readings coincided with high turbidity. There was no trend in turbidity between 2008 and 2019 (Figure 5-10).

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Emily was moderate in 2007 and 2012, with a possible decline in native condition (Table 4-48). Elodea was present in the entire basin (de Winton et al., 2013).

Table 4-48: Lake Emily, LakeSPI overview, 2007-2017 2007 2012 2017 % Change Change indicated? LakeSPI 29 28 28 0 No Native Condition 27 32 25 -7 Possible Invasive Condition 76 78 74 -4 No Condition Moderate Moderate Moderate Moderate No Invasive species Elodea Elodea Elodea Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 554nm, i.e. green (Figure 4-72). The FU index indicates highly variable water colour, from blue to green-brown, indicating the frequent occurrence of high algal biomass and sediment resuspension.

Environment Canterbury Technical Report 91 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-72: Landsat derived water colour data for Lake Emily, 2013-2018 (n = 77): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations  Lake level was low in April 2008.  At times, large numbers of waterfowl were observed on the lake.

Summary Lake Emily was mesotrophic or eutrophic in the sampling period. High numbers of water fowl reside in the lake and surrounding wetlands. Because of the high number of water fowl and the relatively small catchment area, it was modelled that bird contributions to total nutrient loads could be relatively high, with 6% of total TN and 26% of total TP load being from bird sources (Kelly et al., 2014). The TLI objective for Lake Emily is mesotrophic recognising that the lake was likely naturally more productive and utilised extensively by water birds. However, Lake Emily exceeded its TLI target in a number of years being periodically eutrophic, mostly due to high TN.

4.4.2 Māori Lake (East/Front) - Ō Tū Wharekai (East)

Figure 4-73: Sampling location in the Māori Lakes – aerial photograph (EcanMaps)

92 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

The Māori Lakes (East and West or Front and Back, Figure 4-73) are two small, shallow lakes within a lake-wetland complex (ca. 100 ha swamp and 30 ha open water). Their catchment is large, and dominated by highly producing exotic grassland (45% or more, Appendix 2, LRIS 2012, v4.0), but the lakes are surrounded by large wetland margins. Māori Lake East covers an area of 0.09 km2 and is 1.2 to 1.3 m deep (de Winton et al., 2013). Both Māori Lakes - Ō Tū Wharekai - are significant sites for Ngāi Tahu (Te Rūnanga o Arowhenua et al., 2010). Water fowl and fish (including trout and eels) are abundant in Māori Lake East. The Māori Lakes have been given Wildlife Refuge status (this prohibits shooting of indigenous species and the use of motorboats) as well as Nature Reserve status (this protects the lake bed and a narrow marginal area, but does not control land use in the catchments) (Cromarty & Scott, 1995).

Nutrients TN concentrations were consistently high (eutrophic), but TP concentrations were generally low despite occasional large peaks (Table 4-49, Figure 4-74). There were no significant trends in total phosphorus, but total nitrogen increased significantly between 2008 and 2019 (Table 4-50).

Phytoplankton biomass The lake had intermittent large peaks in phytoplankton biomass, some coinciding with peaks in TP. This resulted in large fluctuations in water quality and TLI (Figure 4-74, Figure 4-75). There were no trends in chl-a between 2008 and 2019 (Table 4-50).

Trophic Level Index The lake experienced large fluctuations in TLI (Figure 4-75), due to intermittent large peaks in TP and phytoplankton biomass (Figure 4-74). Possible reasons for these fluctuations are discussed in the summary section below.

Turbidity The lake is generally relatively clear, with periods of high turbidity associated with high chl-a and nutrients.

Table 4-49: Trophic Level Index and attribute states (NPS-FM 2014) for the Māori Lake (East) from 2008-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2008 4.47 EUTRO No 480 33 6.7 12.3 B C C B 2009 2.94 OLIGO Yes 295 7.5 0.9 1.4 A A A A 2012 4.28 EUTRO No 370 10 1.7 50.0 B A A C 2013 3.24 MESO Yes 530 11 0.2 1.7 C B A A 2014 5.45 SUPER No 740 9 1.0 61.0 C A A D 2015 2.69 OLIGO Yes 410 8 0.3 0.5 B A A A 2016 3.02 MESO Yes 500 4 1.0 2.2 B A A A 2017 4.74 EUTRO No 510 11 2.9 34.0 C B B C 2018 4.13 EUTRO No 770 12 0.5 21.0 C B A B 2019 4.70 EUTRO No 630 6 1.8 137.0 C A A D 5 year 3.86 MESO Yes 564 8.2 1.3 137.0 C A A D MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Environment Canterbury Technical Report 93 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-50: Seasonal Mann-Kendall Trend Test Results, 2008-2019, Front (East) Māori Lake

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 46 -21 421 -0.97 0.33 -0.4 -4.48 -1.14 to 0.00 decreasing 0.81 TN 46 79 438.33 3.73 0 32.19 6.19 20.05 to 45.09 increasing 1 chl-a 46 -5 450.33 -0.19 0.85 -0.03 -2.79 -0.10 to 0.11 decreasing 0.57 Turbidity 46 11 450.33 0.47 0.64 0.02 1.38 -0.08 to 0.12 increasing 0.69

Note: Data points of 4000 µg/L TN and 210 µg/L chl-a on 24/04/2014 were excluded from with graph to improve readability.

Figure 4-74: TP, TN, chl-a concentrations and turbidity in Māori Lake (East), 2008-2017

94 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-75: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in the East Māori Lake, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI As vegetation cover was below the 10% threshold, no LakeSPI assessments could be made in 2007 or 2012 (de Winton, 2008; de Winton et al., 2013). LakeSPI improved between 2012 and 2017 (Table 4-51) with larger parts of the lake now vegetated, however, “the low stature of these plants (≤0.2 m) might suggest browsing and uprooting by waterfowl or wave action in this shallow lake, which has loose flocculent sediments” (de Winton et al., 2013).

Table 4-51: Māori Lake (East), LakeSPI overview, 2007-2017 2007 2012 2017 Change indicated? LakeSPI 47 Yes Native Condition 50 Yes Invasive Condition 49 Yes Condition Non-Veg Non-Veg Moderate Yes Invasive species Elodea Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 574 nm, i.e. green-orange (Figure 4-76), and an FU index frequently between 13-16, indicating a frequent brown colouration. The lake is shallow and wind-exposed, likely resulting in frequent sediment-resuspension events.

Environment Canterbury Technical Report 95 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-76: Landsat derived water colour data for Māori Lake (East), 2013-2018 (n = 69): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Cultural health Cultural health of Ō Tū Wharekai East / Māori Lake (East) was assessed as moderate by Te Rūnanga o Arowhenua (2010), despite being the most abundant tuna (eel) fishery of the Ashburton Lakes. However, the cultural assessment report also notes the extensive siltation of the lakebed, high E. coli results and agricultural anti-biotic resistant E. coli present at the outlet. Willow control and measures to control siltation and bacterial contamination, e.g. by developing better buffers along incoming water ways, were recommended to improve the health of the lake. Also recommended were the “complete and ongoing removal of exotic fish from the Māori Lakes and work towards making the lake complex a native fish only area” (Te Rūnanga o Arowhenua et al., 2010).

Other observations  In March 2012 and Feb 2013 lake level was reported as low.  Abundant water fowl and fish (including trout) frequently observed.

Summary Water quality, as reflected by the TLI, was extremely variable in this lake spanning the oligotrophic to supertrophic range. At times the lake was dominated by macroalgae attached to the lake sediment. When macroalgae are abundant and dominate primary production, current assessment methods may not accurately reflect primary production in the lake as macroalgae are not captured by either LakeSPI surveys or open water-column sampling of planktonic chl-a. The dominance of macroalgae in Māori Lake East is potentially linked to the very short hydraulic residence time of ca. 4 days (David Kelly, pers. comm.), which may be too short to allow phytoplankton populations to grow. However, stream diversions may have altered sediment and water influx and residence times. In addition to the macroalgae which cover the lake bed, the lake also seemed to be experiencing blooms of planktonic algae. For instance, in April 2014 chl-a concentrations of 210 µg/L were recorded, along with very high TN and TP, pushing the lake into a supertrophic state in 2014. These phytoplankton blooms may be linked to reduced flows and prolonged residence times in dry periods. Peaks of turbidity and total phosphorus concentrations coincided with high chl-a, suggesting phytoplankton blooms or possibly high sediment re-suspension during low flows as cause of high turbidity. The median TN concentration (2015-2019) in Māori Lake East were the highest among the high-country lakes sampled, suggesting a large external source of nitrogen to the lake. The wetlands around the lake are likely to be more effective in retaining phosphorus than nitrogen inputs from surrounding pastoral farming.

96 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.4.3 Māori Lake (West/Back) - Ō Tū Wharekai (West) Māori Lake (West) is the larger of the two Māori lakes with an area of 0.1 km2. It is 1.7-1.8 m deep and large bird numbers have been observed on the lake. The lake is surrounded by extensive raupō wetlands.

Nutrients, Phytoplankton biomass and Trophic Level Index TN and TP were elevated or high in Māori Lake (West) (Table 4-52), with highest concentrations in 2017 (Figure 4-77).

In 2017 the lake had a large algal bloom (80 µg/L chl-a) in January and a smaller bloom in March (40 µg/L chl-a), both well above typical chl-a levels (Table 4-52, Figure 4-77). TN and TP concentrations associated with these blooms were high, resulting in C and D attribute states in 2017 (Table 4-52).

There were no significant trends in nutrients or chl-a between 2008 and 2019 (Table 4-53).

Between 2008 and 2016 the lake’s TLI fluctuated around the mesotrophic and eutrophic boundary (Table 4-52).A large algal bloom that occurred in 2017 (Figure 4-77) moved the lake into a supertrophic state (Figure 4-78). The West (or Back) Māori Lake returned to a mesotrophic state in 2018 and 2019 (Table 4-52).

Turbidity Turbidity increased significantly between 2008 and 2019 (Table 4-53). Occurrences of high turbidity were often associated with high or elevated chl-a concentrations, which is also showed a (probable) increase.

Table 4-52: Trophic Level Index and attribute states (NPS-FM 2014) for from 2005-2019, Māori Lake West

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2008 3.76 MESO Yes 460 15 2.9 3.2 B B B A 2009 3.78 MESO Yes 420 13 2.7 3.3 B B B A 2012 3.56 MESO Yes 350 15 0.8 3.1 B B A A 2013 3.96 MESO Yes 330 17 1.7 7.3 B B A A 2014 4.07 EUTRO No 330 14 2.4 19.8 B B B B 2015 3.87 MESO Yes 400 13 2.1 6.0 B B B A 2016 4.02 EUTRO No 400 19 4.3 12.0 B B B B 2017 5.20 SUPER No 760 22 9.0 80.0 C C C D 2018 3.94 MESO Yes 410 13 2.1 9.0 B B B A 2019 3.77 MESO Yes 360 10 3.1 10.0 B A B A 5 year 4.16 EUTRO No 466 15.4 4.1 80.0 B B B D MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-53: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Māori Lake West

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 46 -20 435.33 -0.91 0.36 -0.29 -2.1 -0.78 to 0.33 decreasing 0.8 TN 46 5 429.67 0.19 0.85 0 0 -8.60 to 10.06 0.50 chl-a 46 34 446.67 1.56 0.12 0.11 4.24 0.00 to 0.32 increasing 0.94 Turbidity 46 54 447.33 2.51 0.01 0.03 5.07 0.01 to 0.09 increasing 1

Environment Canterbury Technical Report 97 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-77: TP, TN, chl-a concentrations and turbidity in Māori Lake (West), 2008-2017

Figure 4-78: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Māori Lake (West), 2008-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community was moderate, with a probable increase in native condition and a possible increase in LakeSPI index between 2012 and 2017 (Table 4-54). Lake vegetation was dominated by the exotic weed Elodea, with some native species also present (de Winton et al., 2013). Between 2007 and 2012 there was a decrease in native condition, but the lake seems to have partially recovered between 2012 and 2017.

98 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-54: Māori Lake (West), LakeSPI overview, 2007-2017 2007 2012 2017 % Change Change indicated? LakeSPI 39 29 37 8 Possible Native Condition 60 31 43 12 Probable Invasive Condition 69 78 72 -6 Possible Condition Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea NA No Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 552nm (Figure 4-79), giving the lake a green to brown appearance.

Figure 4-79: Landsat derived water colour data for Māori Lake (West), 2013-2018 (n = 58): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Cultural health Lake cultural health was assessed as ‘good’ by Te Rūnanga o Arowhenua (2010). However, the lake was reported to have very low eel abundance and diversity (Te Rūnanga o Arowhenua et al., 2010).

Other observations In March 2012 the lake level was reported as low. Abundant water fowl were frequently observed.

Summary Between 2008 and 2016 the lake declined from being mesotrophic to slightly eutrophic, but returned to a mesotrophic state in 2018 and 2019. In 2017, concentrations of phytoplankton biomass and nutrients were exceptionally high. Nutrient sources include land development for intensive pastoral farming and the large number of waterfowl observed on the lake. However, the very large catchment for the lake (see Figure 4-67) means that water fowl contributions are relatively small compared with predicted catchment sources (Kelly et al., 2014). If the high concentrations of phytoplankton and nutrients observed in 2017 were to reoccur, Māori Lake West could become at risk of ‘flipping’ into a turbid, phytoplankton dominated state and losing its macrophyte community.

Environment Canterbury Technical Report 99 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.4.4 Lake Denny

Figure 4-80: Sampling location in Lake Denny – aerial photograph (EcanMaps)

Lake Denny (Figure 4-80) is a small, shallow lake (2.1 m maximum depth, de Winton et al. (2013)) situated in a catchment of predominantly highly and low producing exotic grassland (LRIS 2012, v 4.0). The lake drains to the Rangitata River.

Nutrients, Phytoplankton biomass and Trophic Level Index The lake had regular phytoplankton blooms in February or March (Figure 4-81), and very high concentrations of both phosphorus and nitrogen (Table 4-55). All TLI parameters are regularly below the NPS-FM national bottom line for protection of ecological health (Table 4-55). Lake Denny’s trophic state did not meet the LWRP TLI requirement in any of the monitoring years (Table 4-55).

Turbidity The lake experienced frequent high turbidity events, often associated with high nutrients and chl-a. In March 2018 in-stream works in the inflow stream were observed by Environment Canterbury staff that send large sediment plumes into the lake, resulting in a very brown lake (Figure 4-84) with extremely high turbidity and TP in the lake (Figure 4-81).

Table 4-55: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Denny from 2013- 2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2013 6.21 HYPER No 1900 106 9.0 144.0 D D C D 2014 3.80 MESO No 400 30 1.4 2.7 B C A A 2015 5.69 SUPER No 730 86 16.0 45.0 C D D C 2016 6.51 HYPER No 2200 146 55.0 140.0 D D D D 2017 5.26 SUPER No 770 73 12.0 24.0 C D C B 2018 6.22 HYPER No 780 88 3.8 56.0 C D B C 2019 3.88 MESO No 270 26 3.9 6.2 A C B A 5 year 5.51 SUPER No 950 83.8 18.1 140.0 D D D D MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

100 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-81: TP, TN, chl-a concentrations and turbidity in Lake Denny, 2013-2019

Figure 4-82: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Denny, 2013-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Denny was moderate in 2007 and 2012 prior to the time water quality monitoring was initiated (Table 4-56). The macrophyte community was dominated by the invasive weed Elodea, and patches of native milfoil were more developed in 2012 than 2007 (de Winton et al., 2013). However, the lake has undergone a collapse of its macrophyte community after 2012 (i.e., flipping to a turbid state) and was classed as ‘non-vegetated’ in 2017.

Environment Canterbury Technical Report 101 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-56: Lake Denny, LakeSPI overview, 2007-2017 2007 2012 2017 % Change Change indicated? LakeSPI 26 30 13 -17 Yes Native Condition 18 32 9 -23 Yes Invasive Condition 74 71 37 -34 Yes Condition Moderate Moderate Non vegetated NA Yes Invasive species Elodea Elodea Elodea Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 571 nm (Figure 4-83). The green- brown colour reflects the high turbidity in the lake. Lake Denny is more similar in colour to coastal lakes than other small high-country lakes (Figure 5-11).

Figure 4-83: Landsat derived water colour data for Lake Denny 2013-2018 (n = 79): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations A lack of fencing of the inflowing stream has been reported, along with instances of cattle observed in the stream, a loss of adjacent wetlands and erosion at the lake edge.

102 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-84: Lake Denny (right) and Lake Emma (left) in March 2018

Summary Lake Denny is an extremely nutrient enriched lake in a farmed catchment. The lake frequently does not meet national bottom lines for water quality and failed to achieve LWRP objectives in any of the monitoring years. The lake recently “flipped” and has undergone a total loss of macrophyte community. This change in state implies that ‘clear water years’ such as 2014 may become unlikely to occur because of its shallow depth and susceptibility to sediment resuspension unless macrophytes can re-establish. Improvements in nearby land management may facilitate improvements in water quality, and it is encouraging that the lake returned to a mesotrophic state in 2019. However, in-lake measures may be required to return the lake to a clear state.

Environment Canterbury Technical Report 103 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.4.5 Lake Heron - Ō Tū Roto

Figure 4-85: Sampling location in Lake Heron – aerial photograph (EcanMaps)

Lake Heron (Figure 4-85) is the largest and deepest lake sampled in the Ashburton Lakes Basin, with a surface area of 6.95 km2 and a maximum depth of 36.2 m. It receives water from small streams and shingle fan seepages (Cromarty & Scott, 1995). The lake has three basins and of which we sampled the largest basin to the South. Catchment land cover is largely native vegetation or gravel/rock, plus fringing wetlands and exotic grassland (9% highly producing and 21% low producing, Appendix 2, LRIS 2012, v. 4.0, Cromarty & Scott, 1995). Substantial areas near the lake were converted from low to high producing grassland between 1996 and 2001 or 2008 and 2012 (LCDB 4, Figure 4-66). Parts of the catchment are a Nature Reserve (Figure 4-68). Lake Heron has been given Wildlife Refuge status and Nature Reserve status, banning motorised crafts and protecting native species. The lake is popular for trout fishing. Lake Heron is a significant site for Ngāi Tahu (Te Rūnanga o Arowhenua et al., 2010).

Nutrients, Phytoplankton biomass and Trophic Level Index The lake had consistently low concentrations of TP between 2005 and 2019, but TN was elevated in 2013, 2014 and 2018 (Table 4-57). Total nitrogen increased between 2007 and 2019 with a probability of 96% (Table 4-58).

Phytoplankton biomass was low between 2005 and 2015 but elevated from 2016 to 2019. Chl-a concentrations increased significantly between 2007 and 2019 (Table 4-58), with the most recent three years of monitoring indicating mesotrophic conditions (Figure 4-86).

Lake Heron was in an oligotrophic state over most years of monitoring between 2005 and 2016 (Table 4-57). However, TLI has increased beyond the LWRP target of oligotrophic in five monitoring years (2009, 2011, and most concerning from 2017 to 2019) indicating some degree of nutrient enrichment (Figure 4-87). Recent increases in TLI seem mostly driven by increases in phytoplankton biomass and in part by increasing total nitrogen.

Turbidity Turbidity was generally low, but increased significantly between 2007 and 2019 (Table 4-58).

104 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-57: Trophic Level Index and attribute states (NPS-FM 2014) for from 2005-2019

Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.22 OLIGO Yes 93 3 0.7 1.0 A A A A 2006 2.30 OLIGO Yes 104 2.5 1.5 2.7 A A A A 2007 2.66 OLIGO Yes 120 4.5 1.1 4.5 A A A A 2008 2.76 OLIGO Yes 160 6 1.3 2.0 A A A A 2009 3.03 MESO No 140 9 2.0 2.2 A A A A 2010 2.89 OLIGO Yes 140 6 1.4 4.8 A A A A 2011 3.11 MESO No 140 8.5 1.7 2.5 A A A A 2012 2.92 OLIGO Yes 160 8 1.2 4.3 A A A A 2013 2.78 OLIGO Yes 161 7 0.8 2.6 B A A A 2014 2.92 OLIGO Yes 161 6 1.5 2.8 B A A A 2015 2.64 OLIGO Yes 147 6 1.3 2.7 A A A A 2016 2.91 OLIGO Yes 147 6 3.8 7.5 A A B A 2017 3.19 MESO No 144 6 3.9 7.7 A A B A 2018 3.45 MESO No 200 6 7.7 14.0 B A C B 2019 3.54 MESO No 140 4 5.2 38.0 A A C C 5 year 3.15 MESO No 156 5.6 4.4 38.0 A A B C MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Figure 4-86: TP, TN, chl-a concentrations and turbidity in Lake Heron 2005-2019

Environment Canterbury Technical Report 105 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-58: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Heron

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction TP 63 12 1085.33 0.33 0.74 0 0 -0.17 to 0.25 0.50 TN 63 62 1104 1.84 0.07 2.04 1.41 0.12 to 4.72 increasing 0.96 chl-a 63 136 1125.33 4.02 0 0.2 11.33 0.10 to 0.32 increasing 1 Turbidity 63 99 1119.67 2.93 0.00 0.03 4.98 0.01 to 0.04 increasing 1

Figure 4-87: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Heron, 2005-2017. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Heron was moderate in 2017 (Table 4-59). The LakeSPI index appears to have been stable since 1982. On shallow rocks at the south-western shoreline Didymo (Didymosphenia geminata) was observed in 2017. Spatial variability of the submerged aquatic vegetation in Lake Heron was higher than usual, e.g. with much greater vegetation depth in the North- Eastern arm (1.5 to 3.3 m deeper), where spring inflows have been observed. Due to the large variability, using one summary indicator for the whole lake could mask any (future) changes.

Table 4-59: Lake Heron, LakeSPI overview, 1982-2017 1982 2007 2012 2017 % Change Change indicated? LakeSPI 42 42 46 45 -1 No Native Condition 45 45 47 49 2 No Invasive Condition 59 60 53 56 3 No Condition Moderate Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea Elodea Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

106 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Water colour The median of the dominant wavelength between 2013 and 2018 was 527 nm, i.e. green (Figure 4-88).

Figure 4-88: Landsat derived water colour data for Lake Heron, 2013-2018 (n = 77): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Cultural health Lake Heron was assigned ‘very good’ mahinga kai status, and ‘good’ overall cultural health (Statistics NZ, https://statisticsnz.shinyapps.io/cultural_health/). Ō Tū Roto / Lake Heron was also noted as critical habitat for long fin eel (Te Rūnanga o Arowhenua et al., 2010). However, the health of the outlet stream was described as ‘poor’, along with high concentrations of E. coli and agricultural anti-biotic resistant E. coli found at the outlet of Lake Heron.

Other observations Lake levels fluctuated over a range of approximately 0.5 m on an annual basis. The lowest lake levels occurred in autumn 2008 and late summer 2015. Levels were consistently highest in spring associated with spring snowmelt (Figure 4-89).

Figure 4-89: Lake Heron lake level variation from January 2007 to 2016

Summary Lake Heron was mostly oligotrophic between 2005 and 2016, but was mesotrophic and did not meet its LWRP objectives in the most recent three years. Turbidity and phytoplankton biomass have increased significantly between 2007 and 2019, with particularly large increases in chl-a over the last 4 years. Annual average chl-a increased from less than 2.5 µg/L prior to 2015 to more than 7.5 µg/L in 2018 and 2019 (Figure 4-87). Large areas near the lake were converted from low to high producing grassland between 1996 and 2001 or 2008 and 2012 (LCDB 4, Figure 4-66). Between 2007 and 2019 total nitrogen in the lake also increased with a high probability. The lake’s aquatic vegetation is in moderate ecological condition as assessed by LakeSPI, associated with the widespread presence of Elodea. The recent deterioration in water quality, particularly the increase of phytoplankton biomass, is worrying and indicates that current protection provisions for Lake Heron may be insufficient and the catchment nutrient

Environment Canterbury Technical Report 107 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

load may need to be reduced to meet plan outcomes and maintain the current ecological condition. Catchment nutrient limit setting should also consider the difference in impact of immediately bioavailable dissolved nutrients vs. less bioavailable particulate loads, as e.g. limiting total nitrogen without limiting its dissolved forms (nitrate and ammonia) may allow for a much larger phytoplankton response than could be expected based on the total load limit alone.

4.4.6 Lake Emma – Kirihonuhonu

Figure 4-90: Sampling location in Lake Emma – aerial photograph (EcanMaps)

Lake Emma (Figure 4-90) is a shallow (2.7 m), medium sized (1.67 km2) lake. The lake is a significant site for Ngāi Tahu and has been described as having diverse and intact wetland fringes (Te Rūnanga o Arowhenua et al., 2010) on the western side. Prior to the conversion of the surrounding area to conservation estate, there were reports of poor management practices, including livestock accessing lake marginal areas. The wetlands are now part of the Lake Emma Government Purpose Reserve. The lake underwent a macrophyte collapse in 2007 (Kelly et al., 2014; Schallenberg & Sorell, 2007), but has been recovering since.

Nutrients Lake Emma is enriched in TN and TP, with nutrient concentrations at times in excess of the NPS-FM national bottom line (Table 4-60). There was very high inter-annual variability in both TN and TP, but no statistically significant trends were detected between 2007 and 2019 (Table 4-61). TN, TP, chl-a concentrations and turbidity all rose to very high levels in 2008 and 2009 (Figure 4-91), the years following the macrophyte collapse. Since this time macrophytes have re-established and nutrient ranges remained mostly mesotropic between 2012 and 2016. As with other high-country lakes in the region, concentrations in 2017 were elevated into the eutrophic range, and continued to be high 2018 and 2019, which could pose risks to macrophyte communities in the lake.

Phytoplankton biomass There was large variability in phytoplankton biomass (chl-a) between years, with highest levels in 2008 and 2009 (Table 4-60, Figure 4-91) after the loss of the macrophytes. The largest algal bloom was observed in February 2009. Recent data indicate elevated phytoplankton biomass for Lake Emma between 2017 and 2019. Variability in phytoplankton biomass was very high, but there were no statistically significant trends in chl-a between 2007 and 2019 (Table 4-61).

Trophic Level Index Lake Emma had large fluctuations in TLI between 2005 and 2019 (Figure 4-92), reflecting large changes in phytoplankton biomass (chl-a) (Table 4-60), linked to the macrophyte collapse that occurred around 2007. After recovering from the macrophyte collapse 2012-2014, TLI has again increased between 2015 and 2019 (Figure 4-92). TLI scores were in excess of the LWRP objective of 3 in all years, but meet the objective of 4 set in earlier version of the LWRP in a number of years.

108 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Turbidity Turbidity rose to high levels in 2008 and 2009 (Figure 4-91). These peaks were associated with high TP, TP and chl-a, and may have been a result of sediment resuspension after the collapse of the macrophyte community in 2007. There was no statistically significant trend in turbidity between 2007 and 2019 (Table 4-61).

Table 4-60: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Emma from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 3.88 MESO No 320 12 5.4 10.0 B B C A 2006 3.93 MESO No 360 12 5.2 6.6 B B C A 2007 4.64 EUTRO No 575 29 8.1 15.1 C C C B 2008 6.20 HYPER No 1400 110 36.0 52.0 D D D C 2009 5.74 SUPER No 1300 80 22.3 37.0 D D D C 2010 4.31 EUTRO No 720 25 4.2 5.3 C C B A 2011 4.78 EUTRO No 625 35.5 6.4 20.0 C C C B 2012 3.45 MESO No 460 18 0.8 1.3 B B A A 2013 3.76 MESO No 430 21 1.4 2.0 B C A A 2014 3.75 MESO No 370 20 1.1 4.3 B B A A 2015 3.46 MESO No 400 13 0.8 3.8 B B A A 2016 4.53 EUTRO No 620 29 4.4 16.0 C C B B 2017 4.94 EUTRO No 880 31 13.0 18.0 D C D B 2018 4.28 EUTRO No 530 19 7.0 10.0 C B C A 2019 5.12 SUPER No 730 40 21.0 29.0 C C D C 5 year 4.46 EUTRO No 632 26.4 9.2 29.0 C C C C MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Figure 4-91: TP, TN, chl-a concentrations and turbidity in Lake Emma 2005-2019

Environment Canterbury Technical Report 109 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-61: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Emma

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 46 37 429.67 1.74 0.08 0.73 3.65 0.00 to 1.27 increasing 0.96 TN 46 10 435.33 0.43 0.67 2.26 0.52 -5.10 to 9.81 increasing 0.69 chl-a 46 4 449.33 0.14 0.89 0.02 0.88 -0.13 to 0.29 increasing 0.53 Turbidity 46 -14 451.33 -0.61 0.54 -0.04 -2.38 -0.08 to 0.04 decreasing 0.74

Figure 4-92: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Emma, 2005-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI The ecological condition of Lake Emma was considered moderate in 2007, 2012 and 2017 (Table 4-62). A collapse of the macrophyte community was reported in 2007 (Kelly et al., 2014, Schallenberg & Sorell 2007), but macrophyte beds had recovered by 2012. Elodea is the dominant weed and its beds have grown in size between 2007 and 2012, suggesting a possible increase in invasive condition. A storm event in 2015/16 was reported to have stirred up the lake sediments and to have dislodged macrophytes, but macrophytes were re-established by 2017.

110 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-62: Lake Emma, LakeSPI overview, 2007-2017 2007 2012 2017 % Change Change indicated? LakeSPI 37 32 35 3 No Native Condition 45 40 38 -2 No Invasive Condition 69 77 71 -6 Possible Condition Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea Ranunculus Potamogeton Ranunculus trichophyllus crispus trichophyllus Ranunculus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 567nm, i.e. green-brown (Figure 4-93). Possibly due to Lake Emma’s shallow nature the lake is relatively turbid, which is reflected in the lake’s colour. The colour of Lake Emma was less variable than that of other small high-county lakes, and it experience much fewer ‘blue colour’ events than other mesotrophic, small high-country lakes.

Figure 4-93: Landsat derived water colour data for Lake Emma, 2013-2018 (n = 75): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Cultural health Lake Emma had very low eel abundance and diversity, but was assigned an overall Takiwā rating of ‘good’ (Te Rūnanga o Arowhenua et al., 2010). The cultural assessment report commended the large, intact remnant wetland present, with very few large exotic species (Te Rūnanga o Arowhenua et al., 2010).

Summary Lake Emma has undergone alternate periods of mesotrophic conditions with extensive macrophytes, and highly enriched (eutrophic or more) conditions with little or no macrophytes. This pattern of ‘flipping’ between a clear macrophyte dominated state (e.g. 2012-2014) and a turbid, phytoplankton dominated state (e.g. 2008-2009, 2016-2017) is prevalent amongst some shallow lakes in New Zealand (Schallenberg & Sorell 2009). The reporting of a macrophyte collapse in 2007 and observations of a more recent macrophyte dislodgement in 2016 with subsequent higher chl-a concentrations in the lake supports this hypothesis. In particular, the weed species Elodea canadensis that dominates macrophyte communities in the lake can undergo growth and die-off cycles that facilitate these state changes (Schallenberg & Sorell, 2009). Because LakeSPI assessments are carried out at approximately 5-yearly

Environment Canterbury Technical Report 111 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

intervals, they are too infrequent to fully capture these cycles. Inputs of external nutrients from the catchment can facilitate more rapid growth and collapse of the Elodea beds. Therefore, nutrient management or weed control measures could be required to stabilise the lake in a mesotrophic, clear- water state.

Water level in Lake Emma was possibly impacted by a diversion of some flow of the inflowing stream to Lake Camp in recent years (DOC Geraldine, pers. comm.). Impacts of low water level on lake health include warmer summer temperatures, the ‘concentration’ of nutrients in the lake, effects on macrophytes due to low water level and elevated temperatures, and increased sediment resuspension. We recommend the installation of a continuous water level recorder at Lake Emma.

4.4.7 Lake Camp – Ōtautari

Figure 4-94: Sampling location in Lake Camp (bottom) and Lake Clearwater (top) – aerial photograph (EcanMaps)

Lake Camp (Figure 4-94) is a small (0.44 km2) lake of intermediate depth (13 m maximum depth). The lake is popular for swimming, boating and water skiing. Its swimming beach is monitored as part of the recreational lake quality monitoring programme. The lake is a significant site for Ngāi Tahu (Te Rūnanga o Arowhenua et al., 2010).

Nutrients, Phytoplankton biomass, Trophic Level Index The lake was generally low in TP, but had elevated levels of TN (Table 4-63). In Lake Camp TN increased significantly between 2005 and 2017 (see also Figure 4-95), but there were no significant trends in TN or TP between 2007 and 2019 (Table 4-64).

112 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-63: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Camp from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.57 OLIGO Yes 210 4 0.9 1.3 B A A A 2006 2.77 OLIGO Yes 235 3.5 1.4 2.4 B A A A 2007 3.18 MESO No 290 7 2.1 3.9 B A B A 2008 3.09 MESO No 350 6 1.5 2.5 B A A A 2009 3.41 MESO No 345 12.5 1.7 2.8 B B A A 2010 2.86 OLIGO Yes 310 2.5 1.0 1.6 B A A A 2011 3.19 MESO No 320 9.5 1.5 2.3 B A A A 2012 3.18 MESO No 320 8 2.0 2.5 B A A A 2013 3.38 MESO No 320 10 1.0 1.2 B A A A 2014 3.01 MESO No 340 8 1.1 1.7 B A A A 2015 3.23 MESO No 380 9 1.4 2.1 C A A A 2016 3.33 MESO No 330 8 1.9 2.7 B A A A 2017 3.22 MESO No 430 7 2.1 2.3 C A B A 2018 3.19 MESO No 300 5 2.3 3.4 B A B A 2019 3.22 MESO No 330 2 4.2 4.4 B A B A 5 year 3.24 MESO No 354 6.2 2.4 4.4 C A B A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

The lake was generally low in chl-a, but median chl-a concentration was in the B-band in 5 of the 12 years of monitoring, including the most recent 3 years (Table 4-63). Chl-a increased significantly between 2007 and 2019 (Table 4-64).

Lake Camp was oligotrophic in 2005, 2006 and 2010, but mesotrophic in all other years, including the last 9 years of monitoring (Table 4-63).This mesotrophic status was mostly related to the TN component of TLI, but on occasion, and since 2017, chl-a was also in the mesotrophic range. TLI increased between 2005 and 2009 (Figure 4-96), along with TN and TP (Figure 4-95).

Turbidity Turbidity in the lake was usually low (Figure 4-95), but there was an increasing trend in turbidity between 2007 and 2019 (Table 4-64, Figure 5-10).

Environment Canterbury Technical Report 113 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-95: TP, TN, chl-a concentrations and turbidity in Lake Camp 2005-2019

Table 4-64: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Camp

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 -6 1084.67 -0.15 0.88 0 0 -0.25 to 0.18 0.50 TN 63 49 1076.33 1.46 0.14 2.23 0.67 0.00 to 5.84 increasing 0.92 chl-a 63 67 1129 1.96 0.05 0.08 4.43 0.01 to 0.12 increasing 0.98 Turbidity 63 90 1104.67 2.68 0.01 0.03 4.31 0.01 to 0.04 increasing 1

114 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-96: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Camp, 2004-2019. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community in Lake Camp was rated high between 1982 and 2017 (Table 4-65), with a limited impact of invasive weeds and a diverse native vegetation (de Winton et al., 2013).

Table 4-65: Lake Camp, LakeSPI overview, 1982-2012 1982 2007 2012 2017 % Change Change indicated? LakeSPI 61 58 70 65 -5 No Native Condition 57 59 71 66 -5 No Invasive Condition 30 36 28 31 3 No Condition High High High High NA No Invasive species Elodea Elodea Elodea Elodea Ranunculus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 504 nm, i.e. blue-green (Figure 4-97), with a seasonal pattern apparent in some years.

Environment Canterbury Technical Report 115 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-97: Landsat derived water colour data for Lake Camp, 2013-2018 (n = 59): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Suitability for contact recreation was very good or good in all bathing seasons monitored since 2004 (Figure 4-98, Table 5-3).

Figure 4-98: E. coli in Lake Camp 2004-2019

Cultural health The outlet of Lake Camp was given a Takiwā score of ‘poor’ (Te Rūnanga o Arowhenua et al., 2010).

Other observations The water l level in the lake was reported as low in January to March 2006, and in February 2009, May 2012, and May 2015.

In the summer of 2015, a mass mortality event of Euhyridella sp populations (Kakahi or mussels) resulted from anoxic bottom water conditions.

Summary Lake Camp was mostly mesotrophic due to elevated total nitrogen concentrations, failing to meet its LWRP objectives. Intermittent high TP concentrations also contributed to higher than normal TLI values in some years. TLI fluctuations may have been be partly driven by stratification events, and an anoxia driven mortality event of Euhyridella (Kakahi) populations occurred in 2015. In years in which the lake stratified thermally, bottom waters may have gone anoxic, which may have resulted in a release of

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phosphorus from sediments. This release can in turn could have fuelled the increases in TP and phytoplankton biomass observed in some years. To better understand and manage the lake ecosystem the continuous monitoring of dissolved oxygen is recommended in summer. Due to the small volume of the lake and its hypolimnion, Lake Camp may be more susceptible to increases in organic loads from its catchment or phytoplankton production which drive anoxia in the lake bottom waters. Therefore, maintaining low phytoplankton biomass, and minimising sediment loading from the catchment are priorities for the lake to protect it from further anoxia events.

4.4.8 Lake Clearwater - Te Puna a Taka Lake Clearwater (Figure 4-99) covers an area of 1.97 km2. It has a maximum depth of 18 m within a deep central basin, but much of the lake’s area is shallow (ca. 3-4m). Lake Clearwater is a significant site for Ngāi Tahu (Te Rūnanga o Arowhenua et al., 2010). The lake is a Wildlife Refuge, with no motor boats permitted. However, it is popular for wind surfing. Part of the land upstream of the lake has been developed for pastoral use and some cropping, and a large bach community is situated near its shore between Lakes Camp and Clearwater. Lake Camp has a small outflow into Lake Clearwater.

Nutrients, Phytoplankton biomass, Trophic Level Index TN was consistently high, and within the eutrophic range since 2007. Concentrations of TP were also elevated in most years, typically in the mesotrophic range (Table 4-66). Temporal patterns in TN, TP, and chl-a have been similar, with increasing concentrations between 2005 and 2011, followed by a decline to 2014, and then increasing concentrations between 2015 and 2017 (Figure 4-99).

Phytoplankton biomass has been elevated in most years (Table 4-66). While there has been considerable variability in phytoplankton biomass over the monitoring period, there were no significant trends in nutrients or chl-a between 2007 and 2019 (Table 4-67).

Lake Clearwater was mesotrophic in most years, but occasionally broaching the eutrophic boundary in 2008, 2010-2011 and 2017 (Table 4-66). The most significant increases in TLI were between 2005 and 2010, and since this time TLI has fluctuated near the upper mesotrophic or eutrophic range (Figure 4-100). Objectives in the LWRP for Lake Clearwater have been exceeded in all years except 2005.

Table 4-66: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Clearwater from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.89 OLIGO Yes 240 5 1.0 1.5 B A A A 2006 3.38 MESO No 320 5 3.5 6.6 B A B A 2007 3.60 MESO No 440 8.5 3.2 3.9 C A B A 2008 4.16 EUTRO No 620 14 3.9 9.4 C B B A 2009 3.83 MESO No 545 12 3.4 5.1 C B B A 2010 4.27 EUTRO No 670 19 4.6 7.5 C B B A 2011 4.08 EUTRO No 645 17.5 3.9 4.7 C B B A 2012 3.89 MESO No 580 14 2.3 3.9 C B B A 2013 3.83 MESO No 440 14 0.9 9.0 C B A A 2014 3.10 MESO No 380 8 0.9 1.8 C A A A 2015 3.84 MESO No 480 13 2.7 5.2 C B B A 2016 3.90 MESO No 510 13 3.0 6.0 C B B A 2017 4.12 EUTRO No 580 15 4.1 6.0 C B B A 2018 3.62 MESO No 500 11 2.1 3.9 C B B A 2019 3.85 MESO No 490 12 4.2 6.4 C B B A 5 year 3.86 MESO No 512 12.8 3.2 6.4 C B B A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019 *Assuming the TLI objective for this ‘natural state’ waterbody is oligotrophic (TLI score of 3.0 or better)

Environment Canterbury Technical Report 117 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-99: TP, TN, chl-a concentrations and turbidity in Lake Clearwater from 2005-2019 Table 4-67: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Clearwater

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 29 1096.33 0.85 0.4 0.19 1.47 -0.17 to 0.47 increasing 0.79 TN 63 -17 1093.67 -0.48 0.63 -2.08 -0.42 -7.49 to 3.72 decreasing 0.74 chl-a 63 -14 1132 -0.39 0.7 -0.03 -0.91 -0.15 to 0.07 decreasing 0.64 Turbidity 63 6 1126 0.15 0.88 0 0 -0.02 to 0.02 0.50

Figure 4-100: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Clearwater, 2004-2019. Red line is the LWRP objective/limit

118 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Turbidity Turbidity was mostly low, and followed a similar pattern as phytoplankton biomass indicating phytoplankton may be a driver of turbidity in the lake. Although highly variable, there was no significant trend in turbidity between 2007 and 2019 (Table 4-67).

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Clearwater was moderate, with no change between 2007 and 2017 (Table 4-68). In 2012, vegetation was dominated by high cover charophyte meadows (de Winton et al., 2013).

Table 4-68: Lake Clearwater, LakeSPI overview, 1982-2017 1982 2007 2012 2017 % Change Change indicated? LakeSPI 42 47 48 49 1 No Native Condition 40 51 54 54 0 No Invasive Condition 45 51 48 49 1 No Condition Moderate Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea Elodea NA No Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 540 nm, i.e. green (Figure 4-101), with a clear seasonal pattern of greener appearance in the summer months.

Figure 4-101: Landsat derived water colour data for Lake Clearwater, 2013-2018 (n = 73): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Suitability for contact recreation was high in every bathing season monitored since 2004 and consistently met LWRP objectives (Figure 4-102, Table 5-3).

Cultural health Lake Clearwater had very low eel abundance and diversity, and was given a Takiwā score of ‘moderate’ (Te Rūnanga o Arowhenua et al., 2010).Overall health of the outlet stream was described as ‘poor’, with ‘very poor’ mahinga kai status (Statistics NZ, https://statisticsnz.shinyapps.io/cultural_health/).

Environment Canterbury Technical Report 119 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-102: E. coli in Lake Clearwater 2004-2019

Summary Lake Clearwater is enriched with nutrients, having been either mesotrophic or eutrophic since 2006. While historical monitoring data is limited, it would seem the lake has undergone a period of sustained nutrient enrichment between 2005-2011 and has subsequently failed achieve its LWRP objectives. Lake water quality was potentially affected by:  Land intensification upstream  Possible septic tank leakage from Clearwater Huts village (Wadworth-Watts 2013)  Water quality of Lake Camp  Groundwater inputs to the lake

A hydrological and nutrient load balance for the Lake Clearwater catchment indicated that nitrogen export from the lake exceeded estimated inputs, suggesting an additional unaccounted source of nitrogen into Lake Clearwater (Figure 4-103, Wadworth-Watts, 2013). This source was identified as being possibly via shallow groundwater. The study also suggested that nutrients were elevated downstream of farmland, and that nitrate in farmland subsurface runoff contributed >50% of total nitrogen yield from farmland (Wadworth-Watts, 2013).

The relative contributions from Lake Camp are likely to be low, because nutrient concentrations in Lake Camp are significantly lower than in Lake Clearwater (Figure 5-6, Figure 5-8). Modelling of potential nutrient contributions by water birds were shown to be less than 2% of total TN load and less than 4% of total TP load (Kelly et al., 2014). Losses of nutrients from the Clearwater huts septic fields are unknown. Both Lakes Camp and Clearwater tended to have very high TN:TP ratios, indicating that phosphorus limitation of phytoplankton growth likely prevails in these lakes. There is very close tracking of chl-a and TP (but also of chl-a and TN) concentrations over the monitoring record. As such, management of P inputs to Lake Clearwater is of highest priority, but to meet LWRP objectives TN inputs also need to be reduced.

120 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-103: Schematic of estimated loads in Lake Clearwater waterways (Reproduced from Wadworth-Watts, 2013)

Environment Canterbury Technical Report 121 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.5 Orari Temuka Opihi Pareora Water Management Zone

4.5.1 Lake Opuha Lake Opuha is a man-made on-river reservoir. It is primarily fed by the North and South Branches of the Opuha River and discharges back to the Opuha River through intermittent withdrawal of deep water or over a spill-way at high water level (Figure 4-104). The lake was first filled in 1997 but following dam failure was not fully filled until late 1998. When full, it has a surface area of ca 710 ha and a maximum depth of ca. 40m, although due to operation as an irrigation reservoir water levels can vary by up to 21 vertical metres. The flooding of fertile, previously irrigated pastures created a substantial pool of carbon and nutrients in the lake sediments that created a large oxygen demand as it decomposed (Gibbs & Measures 2017). This resulted in oxygen loss in the hypolimnion and associated release of reduction compounds (i.e. methane, H2S, ammonia) and phosphorus. Due to concerns about the quality of water in the lake and of the water drawn from the hypolimnion and discharged to the downstream Opuha and Opihi Rivers a review of dam consents required an aeration system to be designed, installed and operated. An aeration system was fully installed in the lake in October 2003 with consent conditions requiring operation to artificially de-stratify and oxygenate the lake when thresholds were reached. Thresholds have been reached almost every summer requiring extended periods of operation of the system, probably in perpetuity.

The Lake Opuha Dam resource consents also require a lake management plan and address aquatic macrophyte growth, sedimentation on lake margins, exotic/pest species and a number of other attributes. Because Lake Opuha is an artificial on-river reservoir that is managed primarily by a suite of resource consent conditions it requires specific management and is not expected to bracket closely with the natural high-country lakes in this report.

Figure 4-104: Sampling location in Lake Opuha, water quality in yellow and recreational water quality monitoring in orange– aerial photograph (EcanMaps) Nutrients, Phytoplankton biomass, Trophic Level Index Lake Opuha was mesotrophic is most years. Thus, it did not meet the general LWRP outcome of a TLI of 3 for on-river artificial lakes, but in most years meet the TLI outcome of 4 proposed for Lake Opuha under plan change 7 (Table 4-69). Both TN and median chlorophyll-a concentrations were in the NPS- FM C-band.

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Table 4-69: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Opuha from 2016- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2016 3.76 MESO Yes* 270 8 7.3 13.7 B A C B 2017 3.91 MESO Yes* 380 10 6.7 8.1 C A C A 2018 3.86 MESO Yes* 420 13 3.4 8 C B B A 2019 4.03 EUTRO No 460 12 7 11.4 C B C B 4 year 3.89 MESO Yes* 400 10 6.5 13.7 C A C B MED = seasonal median concentration (Dec-May) * TLI outcome of 4 proposed under plan change 7

Recreational water quality Suitability for contact recreation was ‘fair’ at Ewarts Corner Boat Ramp and ‘very good’ at the Recreation Reserve in the period from 2015-2019 (Figure 4-105, Table 5-3). When first formed the lake exhibited potentially toxic cyanobacteria blooms. The aeration system manages conditions to reduce the likelihood of such blooms occurring and affecting recreational values.

Figure 4-105: E. coli counts at Lake Opuha (Recreation Reserve – left, Ewart Corner – right), 2013-2019

Other observations The lake is artificially de-stratified and aerated over the summer months. There is a water quality monitoring station near the dam, measuring temperature, dissolved oxygen, conductivity and turbidity to enable the operation of the aeration system (Gibbs & Measures, 2017). The high level of water regulation has allowed high variations in water level, particularly low levels in late summer/autumn at the end of the irrigation season. These level variations will inevitably influence water quality characteristics and targets, particularly in dry years.

Summary Lake Opuha is a relatively recently formed on-river artificial lake (reservoir) that has met the water quality outcome of a mesotrophic state proposed under plan change 7 in most years between 2016 and 2019, despite high Total Nitrogen and chlorophyll-a concentrations. Targeted lake aeration is now largely successful in preventing the onset of stratification and wide-spread bottom anoxia, thus avoiding the associated production of toxic reduction compounds and sediment phosphorus release that occurred when the lake was first formed. Lake Opuha serves as a local example illustrating the high level of active water quality management of reservoirs that can be required, especially when formed over fertile soils.

Environment Canterbury Technical Report 123 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6 Upper Waitaki Water Management Zone Lakes in this semi-arid area experience warm, dry summers with strong North-West winds and frosts in winter. Their catchments were (and in many cases still are) dominated by either dry, short tussock grassland used for extensive grazing of sheep, or high-altitude alpine environments. In the last two decades parts of the low-lying flats have been developed for irrigated intensive farming, changing the character of the landscape. For instance, land around Kellands Pond has been developed into highly producing grassland between 1996 and 2001.

The Central Southern Lakes have important spiritual value to Māori people. Food gathering (mainly birds) is thought to have occurred on a seasonal basis, and artefacts were found between Lakes Alexandrina and Tekapo (Tau et al., 1990). Lakes Alexandrina and Lake McGregor have legal protection as Wildlife Refuges.

Lakes Pukaki, Tekapo and Ōhau have been developed as key storage reservoirs for the Waitaki hydro- electric power scheme. They also act as buffers for water and sediment loads from snowmelt and floods to the downstream catchments. These large lakes were mostly in a microtrophic state, meeting their LWRP objectives in the majority of years. Possibly due to their relatively short residence times compared to other large South Island lakes, the large Waitaki lakes seem to respond rapidly to changes in quantity and quality of inflows. While other factors may also be important in determining TLI, for Lakes Ōhau, Tekapo, Pukaki and Benmore periods of low summer rainfall often coincided with relatively low TLI, and years with high summer rainfall with relatively higher TLI. Atypically low TLI scores have previously been linked to very low rainfall, low river flows and the lack of large flood events in summer 2013-2014 (Clarke, 2015).

Phytoplankton biomass has been observed to peak in spring and summer in the large lakes (Tekapo, Pukaki, Ōhau, and Benmore) in the past, but biomass and periodicity of phytoplankton in these lakes appear “greatly influenced by seasonal patterns of turbidity from inflowing glacial silt” (Duthie & Stout, 1986). Turbidity in Lakes Benmore and Pukaki during the summer seems to be decreasing, along with total nitrogen, but increasing in Lake Ōhau. The reduction in turbidity may be related to the retreat of glaciers in the catchment or a decreasing trend in summer rainfall (December – April) in Tekapo Township from 2011 to 2016 (Data from NIWA). Phytoplankton biomass increased in Lake Benmore only. While an increase in phytoplankton biomass in Lake Benmore may be linked to increased clarity and light availability that come with decreased turbidity, increased nutrient availability due to upstream land development is likely a strong influence, and phytoplankton is likely responding to a combination of both.

The smaller lakes Alexandrina, McGregor and Middleton were meso- or oligotrophic, with no trends apparent from 2007 to 2019 (although only Lake Alexandrina was sampled continuously). While the large, nutrient-poor lakes generally met their TLI target of microtrophic, the smaller lakes tended to exceed their targets and were frequently in a mesotrophic state. Suitability for contract recreation in Lakes Alexandrina and Middleton improved from ‘poor’ to ‘good’ between the start of monitoring and the summer of 2016/2017, but deteriorated again in 2018 and/or 2019. Turbidity increased in two smaller lakes, Lake Alexandrina and Kellands Pond. Total nitrogen also increased significantly in Kellands Pond), where land has been developed for intensive agriculture.

Condition of the aquatic macrophyte community in Lake Benmore was only ‘moderate’ and the lake failed to meet its LWRP objectives for lake ecological condition due to the presence of the invasive oxygen weeds Lagarosiphon major and Elodea canadensis. While still meeting the LWRP objectives, the ecological condition of Lakes Alexandrina and Middleton appear to be deteriorating, driven by an increase in invasive impact. By contrast, the ecological condition of Lakes Tekapo and Ōhau is improving and is now ‘excellent’.

In contrast to other water management zones, objectives for TLI scores for this zone were set as part of plan change 5 (PC5) and were based on median concentrations, not trophic bands as in the other zones, e.g. TLI objective for Lake Alexandrina is 3.1 (not 3), for Lake Tekapo 1.7 (not 2). The intent of this approach was to reflect the “maintain or improve” narrative in the NPS-FM. In addition, TN, TP and chl-a limits were set based on NPS-FM(2014) bands not trophic (TLI) bands. For the microtrophic lakes (e.g. Lake Tekapo) these limits were set to be at the bottom of the A-Band, allowing much higher concentrations than the thresholds between the microtrophic and oligotrophic bands for TN, TP and

124 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

chl-a. This discrepancy allows for situations were limits for all TLI components are met, but the TLI objective is not met. It also means that managing Lake Tekapo, Pukaki, Ōhau, Benmore and Aviemore to TN and TP limits set in PC5 is not necessarily sufficient to meet the TLI objective set in the LWRP, and that maximum catchment nutrient loads should instead be set based on TN and TP loads corresponding to the TLI limit.

4.6.1 Lake McGregor - Whakarukomoana or Otetoto

Figure 4-106: Sampling locations in Lake McGregor – aerial photograph (EcanMaps)

Lake McGregor (Figure 4-106) is located a short distance downstream from Lake Alexandrina. The lake is 25 m deep and has a surface area of 0.37 km2. It separated from Lake Tekapo by a narrow barrier and can be affected by water level fluctuations of the larger lake. Lake McGregor is a Wildlife Refuge, and a significant salmon spawning site (inflowing stream), but has a dense band of exotic willow trees along its shoreline in places (Cromarty & Scott, 1995).

Nutrients, Phytoplankton biomass, Trophic Level Index Only limited data exist for Lake McGregor: The lake was on the boundary between mesotrophic and oligotrophic in 2012 and 2013. It had elevated concentrations of TP, but low chl-a concentrations in both years (Table 4-70). Both TP and TN increased over summer from December to April, suggesting internal loading as a potential source (data not shown). Table 4-70: Trophic Level Index and attribute states (NPS-FM 2014) for Lake McGregor from 2012-2013 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2012 3.27 MESO No 260 12 2.0 2.7 A B A A 2013 2.99 OLIGO Yes 250 12 1.1 1.5 A B A A MED = seasonal median concentration (Dec-May)

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake McGregor was moderate from 1982 to 2017 (Table 4-71).

Water colour The median of the dominant wavelength between 2013 and 2018 was 499 nm, i.e. blue-green (Figure 4-107).

Environment Canterbury Technical Report 125 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-71: Lake McGregor, LakeSPI overview, 1982-2017 1982 2012 2017 % Change Change indicated? LakeSPI 47 48 48 0 No Native Condition 59 63 63 0 No Invasive Condition 62 64 64 0 No Condition Moderate Moderate Moderate NA No Invasive species Elodea Elodea Elodea NA No Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Figure 4-107: Landsat derived water colour data for Lake McGregor, 2013-2018 (n = 69): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Cultural health assessment The outlet of Lake McGregor was assessed to be of moderate cultural health (Figure 4-108). Concerns raised were the presence of willow trees and low eel population.

Figure 4-108: Cultural health assessment of Lake McGregor (Tipa and Associates & Williams, 2015)

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Summary Since Lake McGregor receives most of its water from the outflows of Lake Alexandria, water quality in Lake McGregor reflects the water quality in the larger lake. However, TP concentrations were higher in Lake McGregor than in Lake Alexandrina, indicating an additional source of phosphorus, potentially internal loading. Although only two years of monitoring data were available for Lake McGregor, based on the data available the TLI fluctuates around the mesotrophic threshold.

4.6.2 Lake Alexandrina – Takamoana or Te Kaupururu

Figure 4-109: Sampling locations in Lakes Alexandrina (left) and Tekapo (right) – aerial photograph (EcanMaps). The small lake in between is Lake McGregor

Lake Alexandrina (Figure 4-109) is a medium-sized lake to the west of Lake Tekapo (Figure 2-2). The lake has two deep basins and covers an area of 6.46 km². While its maximum depth is 30 m, most of the lake is shallower and the mean depth is 13.6 m. Compared to other small or medium-sized high- country lakes, Lake Alexandrina has a relatively long residence time (ca. 4 years). Less than one third of water inflows are from permanent streams, other water sources are rainfall, overland runoff and groundwater inputs (Ward & Stewart, 1989). The lake drains to Lake McGregor, which is located ca. 500 m downstream. Lake Alexandrina is a Wildlife Refuge, and partially lies within a Scenic Reserve. The wetland fringing the lake to the North is dominated by Carex secta and Raupo (Typha orientalis), but elsewhere willow trees form a dense band around the lake (Cromarty & Scott, 1995). Four species of native fish (long finned eels, kōaro, and common and upland bullies) and three introduced species (Brown Trout, Rainbow Trout, and Quinnat/Chinook Salmon) are present in the lake. Large populations of New Zealand Scaup and Great Crested Grebes have been observed on the lake (Cromarty & Scott, 1995).

While some land on the Northern end of the lake has been developed for pastoral farming, the main land use in the catchment is recreation. There are two long-established house and bach communities on the lake, as well as camping and fishing areas. However, large parts of the lake shore were previously accessible to livestock and sheep (Cromarty & Scott, 1995). The lake is also highly valued for trout fishing.

The lake is exposed to strong North-Westerly winds, causing high wind-induced mixing that prevents the lake surface from freezing in winter. While the lake was reported to not have a stable stratification throughout the summer of 1978/79, Lake Alexandrina can stratify (intermittently) over the summer

Environment Canterbury Technical Report 127 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

(Hayes, 1980). In fact, new monitoring data from 2018-2019 shows a long, stable summer stratification, with hypolimnion oxygen depletion lasting several months (Environment Canterbury, unpublished data).

In the early 1980s the lake experienced regular algal blooms, including a bloom of potentially toxic Anabaena sp. (Taranaki Catchment Commission, 1987). However, no algal scums have been observed by or reported to Environment Canterbury in recent years (Clarke, 2015). Management efforts in the past focused on reducing the phosphorus load to the lake (Ward-Smith et al., 1985).

Nutrients Total phosphorus (TP) and phytoplankton (chl-a) attributes were low, whereas total nitrogen (TN) levels were elevated (Table 4-73). Both TP and TN were lower in 2005-2017 than in 1979-1984 (Table 4-72) when the lake experienced large blooms of cyanobacteria. There was no significant trend in TP concentrations between 2007 and 2019, but TN decreased significantly (Table 4-77).

Table 4-72: Mean nutrient concentrations in Lake Alexandrina, 1978-2017 Year 1978-79 1979-84 1992-95 2005-2017 TP 9 12 16 8 TN 190* 268 201 218 *TON Sources: Taranaki Catchment Commission 1987, Burns & Rutherford 1998, Hayes 1980

Table 4-73: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Alexandrina from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 2.72 OLIGO Yes 170 7 0.8 3.8 B A A A 2006 2.69 OLIGO Yes 145 4.5 1.9 2.2 A A A A 2007 3.15 MESO NO 265 7 2.2 3.1 B A B A 2008 3.12 MESO NO 240 7 2.2 4.4 B A B A 2009 3.32 MESO NO 230 9 1.9 5.3 B A A A 2010 3.09 MESO Yes 230 10 1.7 3.0 B A A A 2011 2.94 OLIGO Yes 220 9.5 1.1 1.7 B A A A 2012 3.00 MESO Yes 240 10 1.0 1.8 B A A A 2013 2.96 OLIGO Yes 210 9 1.1 1.4 B A A A 2014 3.15 MESO NO 210 8 2.2 4.3 B A B A 2015 3.14 MESO NO 230 7 1.6 3.9 B A A A 2016 2.90 OLIGO Yes 210 7 1.6 2.1 B A A A 2017 3.33 MESO NO 210 12 2.5 3.1 B B B A 2018 2.85 OLIGO Yes 220 5 1.5 3.7 B A A A 2019 2.89 OLIGO Yes 210 6 1.6 2.2 B A A A 5 year 3.02 MESO Yes 216 7.4 1.8 3.9 B A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Phytoplankton biomass Phytoplankton biomass was generally low (Table 4-73) but exceeded the LWRP objectives in 5 years. The main phytoplankton peak appeared to occur in late autumn (April/May), suggesting that stratification breakdown may supply nutrients in autumn (Figure 4-110). An autumn/winter peak of phytoplankton was also recorded in 1992-1995 (Burns & Rutherford, 1998). Chl-a levels in 2005-2019 were lower than those previously reported when algal blooms occurred in the early 1980s (2-18 mg/L, Taranaki Catchment Commission 1987) and in 1992-1995 (1-11 µg/L, Burns & Rutherford, 1998). Phytoplankton biomass was comparable to concentrations recorded in the summer of 1978/79, of 0.3 to 3.8 mg/L

128 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

(Hayes, 1980). There was no statistically significant trend in phytoplankton biomass between 2007 and 2019 (Table 4-74).

Figure 4-110: TP, TN, chl-a concentrations and turbidity in Lake Alexandrina from 2005-2019

Table 4-74: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Alexandrina

ample

Probability Parameter S size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 63 3 1074.33 0.06 0.95 0 0 -0.17 to 0.22 0.50 TN 63 -73 1050.33 -2.22 0.03 -1.66 -0.76 -3.32 to 0.00 decreasing 0.91 chl-a 63 31 1121.67 0.9 0.37 0.03 1.77 -0.02 to 0.07 increasing 0.83 Turbidity 63 102 1076 3.08 0.00 0.01 3.31 0.01 to 0.02 increasing 1

Trophic Level Index Between 2005 and 2017 the lake fluctuated between a TLI of 2.7 and 3.3, near the boundary between oligotrophic and mesotrophic states. The lake therefore met the LWRP objective of 3.1 in 10 of the 16 years of monitoring (Table 4-73).

Turbidity Turbidity was generally low, but increased significantly between 2007 and 2019 (Figure 4-110, Table 4-74).

Environment Canterbury Technical Report 129 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-111: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Alexandrina, 2005-2017. Red line is the LWRP objective/limit

Lake ecological condition: LakeSPI Despite a continuous decrease in LakeSPI index from 1982 to 2015 due to an increasing impact from invasive macrophytes, the lake still maintains a high ecological condition (Table 4-75).

Table 4-75: Lake Alexandrina, LakeSPI overview, 1982-2017 1982 2001 2009 2015 2017 %Change Change (2009- indicated? 2017) LakeSPI 74 66 57 54 51 -6 Possible Native 70 67 57 63 59 2 No Condition Invasive 19 31 39 50 56 17 Yes Condition Condition High High High High High No Invasive Elodea Elodea Elodea Elodea Elodea species Ranunculus Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 493 nm, i.e blue-green (Figure 4-112). Isolated periods of a high FU index scores indicate a dark green to yellow-brown lake colour at times.

130 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-112: Landsat derived water colour data for Lake Alexandrina, 2013-2018 (n = 63): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Suitability for contact recreation was fair or poor in the 2012/13 to 2014/2015 bathing seasons, but improved to good in the 2015/16 and 2016/2017 bathing seasons (Figure 4-113). Exceedance of the recreational E. coli action level concentrations (MfE 2003) occurred again in the 2018/2019 season, resulting in an overall ‘fair’ rating from 2015-2019 (Table 5-3).

Figure 4-113: E. coli counts at Lake Alexandrina at bottom huts, 2013-2019

Cultural health assessment Overall cultural health for the outlet stream of Lake Alexandrina was rated as moderate (Figure 4-114). The perceived prioritization of sports fish over native species was a main concern (Tipa and Associates & Williams, 2015). This assessment likely relates to the development of trout spawning habitat in this stream by Fish and Game and the broader trout fishing community.

Environment Canterbury Technical Report 131 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-114: Cultural health assessment of the outlet of Lake Alexandrina (Tipa and Associates & Williams, 2015)

Summary Lake Alexandrina’s TLI fluctuated around the mesotrophic threshold, with the lake not meeting the freshwater objective outlined in the LWRP in six of sixteen monitoring years between 2005 and 2019. In contrast to previous decades (1980s), no large algal blooms were observed between 2005 and 2019. Recreational water quality was poor in 2012-2014, good or very good in 2015-2017, but deteriorated again in 2018-2019, resulting in an overall rating from 2015-2019 of ‘fair’. Given the lake’s history of blooms of nitrogen-fixing cyanobacteria, any future increase in TP would be of concern. Lake stratification patterns are likely to affect bottom water nutrient cycling, which could source phosphorus to the lake from legacy sediments. The prevalence of autumn phytoplankton biomass peaks suggests increases in nutrient availability around the breakdown of stratification in autumn, and recently collected data confirms this assumption (Environment Canterbury, unpublished data). Nutrient concentrations, especially TP in the hypolimnion, and dissolved oxygen levels near the lake bottom, should be monitored closely in the future to prevent increases to levels where algal blooms may be triggered again.

132 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6.3 Lake Tekapo/Takapō Lake Tekapo (Figure 4-109) is a large (96.59 km2), 120 m deep lake near the Tekapo township and is part of the Waitaki Power Scheme operated by Genesis Energy Ltd. The lake has two small islands. The outlet of the lake is regulated with flows diverted through Tekapo A Power station and into the Tekapo Canal that flows to Lake Pukaki. Flows though the natural outflow (Tekapo River) are occasionally released and provide for recreational white-water activities and water level management. The banks of the lake are generally steep, bare, and prone to erosion due to frequent lake level changes, except for the shallow bays in the north, southeast and the west near Lake McGregor. Due to the high input of glacial meltwater and glacial rock flour, the lake has a distinctive milky-blue colour.

Nutrients, Phytoplankton biomass and Trophic Level Index Lake Tekapo was very low in nutrients in all years (Table 4-76). TN decreased significantly between 2005 and 2019 (Table 4-77), mostly driven by a few higher values in 2007-2008 (Table 4-77). Due to the high number of data points below the limit of analytical detection, no trends were analysed for TP.

In large glacial lakes such as Lake Tekapo, spikes in TP concentrations are often linked to the input of suspended inorganic sediment sourced from floods or glacial melt. Turbidity and TP was relatively high in 2010/2011 and 2013, coinciding with high rainfall in January 2010 and 2013 (Appendix 3.2). While glacial flour tends to be enriched in mineralised phosphorus, this phosphorus is not usually readily bioavailable to phytoplankton (Filippelli and Souch, 1999). The relatively high concentrations of chl-a in the period from 2009-2011 may be due to slight increases in inputs of bioavailable nutrients contained in the flow flows.

Lake Tekapo was very low in phytoplankton biomass in all years, with a peak in 2009 that mirrored increases in TP (Table 4-76, Figure 4-115). There were no significant changes in chl-a between 2007 and 2019 (Table 4-77).

Lake Tekapo was microtrophic or ultramicrotrophic (i.e. very low in nutrients and chl-a) between 2005 and 2019, with the exception of one year (2010) where it was oligotrophic (Figure 4-116). Observed variability in TLI (Figure 4-116) predominantly mirrored changes in TP concentrations (Figure 4-115).

Table 4-76: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Tekapo from 2005- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2005 1.31 MICRO Yes 55 2 0.2 0.3 A A A A 2006 1.45 MICRO Yes 48 2 0.6 0.6 A A A A 2007 1.41 MICRO Yes 60 2 0.4 0.4 A A A A 2008 1.15 MICRO Yes 30 1 0.7 1.0 A A A A 2009 1.79 MICRO Yes 35 6 0.7 1.3 A A A A 2010 2.17 OLIGO No 55 8 0.9 1.1 A A A A 2011 1.95 MICRO Yes 45 9.5 0.5 0.7 A A A A 2012 1.54 MICRO Yes 40 2.5 0.8 1.2 A A A A 2013 1.43 MICRO Yes 31 6 0.2 0.6 A A A A 2014 1.25 MICRO Yes 37 2 0.4 0.8 A A A A 2015 1.22 MICRO Yes 39 2 0.4 0.5 A A A A 2016 1.32 MICRO Yes 32 2 0.4 0.6 A A A A 2017 1.05 MICRO Yes 25 2 0.3 0.5 A A A A 2018 0.86 ULTRA Yes 27 2 0.4 0.4 A A A A 2019 1.26 MICRO Yes 38 2 0.4 0.8 A A A A 5 year 1.14 MICRO Yes 32 2 0.4 0.8 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Environment Canterbury Technical Report 133 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-115: TP, TN, chl-a concentrations and turbidity in Lake Tekapo from 2005-2019

Table 4-77: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Tekapo

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 62 -70 1038.67 -2.14 0.03 -0.86 -2.34 -1.62 to -0.13 decreasing 0.99 chl-a 63 -59 1106.33 -1.74 0.08 -0.02 -5 -0.03 to 0.00 decreasing 0.95 Turbidity 63 18 1130 0.51 0.61 0.02 1.54 -0.03 to 0.09 increasing 0.7

134 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-116: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Tekapo, 2005-2017. Red line is the LWRP objective/limit Turbidity Turbidity in Lake Tekapo is higher than in most large high-country lakes, except Lake Pukaki, due to high input of fine glacial silt. Periods of higher turbidity occurred infrequently and were associated with floods. There was no trend in turbidity between 2007 and 2019 (Table 4-77).

Lake ecological condition: LakeSPI Macrophyte growth in Lake Tekapo is limited by relatively low visual clarity resulting from glacial flour inputs (deWinton & Burton, 2017), and lake level fluctuations. The condition of the aquatic macrophyte community of Lake Tekapo improved from moderate in 2012 to excellent in 2017 (Table 4-78), due to increased depth of plant growth resulting from increasing visual clarity (deWinton & Burton, 2017).

Table 4-78: Lake Tekapo, LakeSPI overview 2012-2017 2012 2017 %Change Change indicated? LakeSPI 50 88 38 Yes Native Condition 34 79 45 Yes Invasive Condition 4 0 -4 No Condition Moderate Excellent NA Yes Invasive species Elodea None NA Yes Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 490, i.e. blue, with very little variability in water colour (Figure 4-117).

Environment Canterbury Technical Report 135 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-117: Landsat derived water colour data for Lake Tekapo, 2013-2018 (n = 36): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Recreational water quality at Lake Tekapo Beach was graded as ‘very good’ between 2015 and 2019. E. coli counts were generally low with few exceedances of the recreational water quality guideline thresholds (Figure 4-118).

Figure 4-118: E. coli counts at Lake Tekapo beach, 2002-2019

Other observations Lake levels were generally lowest in late winter / early spring. The lake level was higher than usual in 2009-2011 (Figure 4-116).

136 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-119: Lake level of Lake Tekapo, 2004-2017. Data: NIWA/Genesis Energy

Summary Lake Tekapo is a microtrophic lake with very low nutrients concentrations, meeting the LWRP objective in all but one year. The condition of the aquatic macrophyte community (LakeSPI) was excellent in 2017 despite large lake level variations. Variation in TP concentrations and phytoplankton biomass appear to be linked to inorganic sediment inputs from the catchment, mostly as glacial flour. Phytoplankton growth in the lake is probably highly limited by bioavailable phosphorus and nitrogen, as well as by light.

Environment Canterbury Technical Report 137 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6.4 Lake Pukaki

Figure 4-120: Sampling location in Lake Pukaki – aerial photograph (EcanMaps)

Lake Pukaki (Figure 4-120) is 70 m deep, and covers an area of 172.7 km2. The lake is now part of the Waitaki hydroelectric scheme with water from Lake Pukaki flowing to Lake Ruataniwha via a canal. While Lake Pukaki also receives water from Lake Tekapo, the main inflow is the Tasman River. The lake has a large operating range of 232 to 218 metres above mean sea level, and often has a milky colour due to the input of glacial flour via the Tasman River.

Nutrients Lake Pukaki generally had very low concentrations of TN and TP (Table 4-79). While TN decreased significantly between 2007 and 2019 (Table 4-80), no trends could be analysed for TP due to the high number of data points below the detection limit. TP was unusually high between 2009 and 2011, and low in 2014 and 2016. High TP was associated with high turbidity, and is probably linked to the volume of inflow from glacial meltwater or flood events (Figure 4-121).

Phytoplankton biomass Chl-a was very low throughout the sampling period (Figure 4-121), and there were no significant trends in chl-a between 2007 and 2019 (Table 4-80). High turbidity in Lake Pukaki is likely to limit chl-a biomass due to restricted light penetration.

Trophic Level Index Lake Pukaki was microtrophic or ultra-microtrophic in all years except 2010 and 2011 (Table 4-79). TLI was higher between 2009 and 2011, due to high TP, and low in 2014 and 2016 (Figure 4-122).

138 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Turbidity Due to high inputs of glacial flour, the lake was generally more turbid than the other large, deep lakes monitored. Turbidity decreased significantly between 2007 and 2019 (Table 4-80). The change in turbidity is likely linked to a change in timing and magnitude of glacial melt and precipitation. According to the National Institute for Water and Atmospheric Research (NIWA) the Hooker, Mueller and Tasman Glaciers upstream of Lake Pukaki are retreating. With the increasing size of terminal lakes such as the in Tasman and Hooker Lake a larger area for retaining sediment may also have become available.

Table 4-79: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Pukaki from 2006- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2006 1.41 MICRO Yes 46 2 0.6 1.0 A A A A 2007 1.74 MICRO No 49 7.5 0.2 0.4 A A A A 2008 1.72 MICRO No 40 7 0.5 0.5 A A A A 2009 1.94 MICRO No 35 8.5 0.5 1.7 A A A A 2010 2.00 MICRO No 50 15 0.2 0.7 A B A A 2011 2.00 MICRO No 40 21 0.3 0.4 A C A A 2012 1.59 MICRO Yes 40 7 0.4 0.6 A A A A 2013 1.32 MICRO Yes 22 7 0.3 0.6 A A A A 2014 0.99 ULTRA Yes 23 2 0.2 1.1 A A A A 2015 1.84 MICRO No 35 2 0.3 0.7 A A A A 2016 1.00 ULTRA Yes 31 2 0.4 0.6 A A A A 2017 1.78 MICRO No 31 4 0.4 0.5 A A A A 2018 1.25 MICRO Yes 28 2 0.4 0.9 A A A A 2019 1.41 MICRO Yes 41 2 0.6 0.6 A A A A 5 year 1.45 MICRO Yes 33 2.4 0.4 0.9 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Figure 4-121: TP, TN, chl-a concentrations and turbidity in Lake Pukaki from 2006-2019

Environment Canterbury Technical Report 139 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-80: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Pukaki

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 63 -73 1081 -2.19 0.03 -1 -2.86 -1.67 to 0.00 decreasing 0.98 chl-a 63 16 1070.67 0.46 0.65 0 0 0.00 to 0.01 0.5 Turbidity 63 -87 1131 -2.56 0.01 -0.32 -4.63 -0.63 to -0.09 decreasing 0.99

Figure 4-122: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Pukaki, 2005-2019. Red line is the LWRP objective/limit

Water colour The median of the dominant wavelength between 2013 and 2018 was 492nm, i.e. blue (Figure 4-123), with very little variability.

Figure 4-123: Landsat derived water colour data for Lake Pukaki, 2013-2018 (n = 29): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

140 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Other observations Because of the large lake level operating range and high turbidity, no macrophytes are established in Lake Pukaki.

White Koaro (Galaxias brevipinnis) have been reported in Lake Pukaki, where the fish can hide from trout in its milky waters.

Minimum lake levels usually occurred during winter or spring (Figure 4-124). Average lake level was highest between 2009 and 2011, the years with highest TP and turbidity.

534

532

530

528

526 Stage in m in Stage 524

522

520 Jan-04Jan-05Jan-06Jan-07Jan-08Jan-09Jan-10Jan-11Jan-12Jan-13Jan-14Jan-15Jan-16Jan-17

Figure 4-124: Lake level variation of Lake Pukaki, 2004-2017. Data NIWA/Meridian Energy

Summary Lake Pukaki is a highly turbid, mostly microtrophic lake with very low algal biomass. Trophic level conditions in Lake Pukaki met the LWRP objective of 1.7 in all but six monitoring years but were microtrophic or better in all years. TN and turbidity both decreased significantly between 2007 and 2019. High turbidity in Lake Pukaki is likely to limit chl-a biomass due to restricted light penetration, but no effect of increased clarity of phytoplankton biomass have been observed yet.

Environment Canterbury Technical Report 141 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6.5 Lake Middleton

Figure 4-125: Sampling location in Lakes Middleton – aerial photograph (EcanMaps)

Lake Middleton (Figure 4-125) is a small, shallow lake (4 m maximum depth and a surface area of 0.23 km2), next to Lake Ōhau. Inflows are from a small stream and wetland, and the lake does not have a surface outflow. There is a well utilised campsite on the lake shore, and the lake is used for swimming and boating activities, partly because of the relatively clear and warm water compared to the larger glacial lakes in the area.

Nutrients, Phytoplankton biomass, Trophic Level Index Lake Middleton was mesotrophic in 2012 & 2013 and 2017 & 2018 (Table 4-81). The lake had elevated concentrations of TN in all years, and elevated TP in 2013 (Table 4-81). Phytoplankton biomass was consistently in the oligotrophic range.

Table 4-81: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Middleton from 2012-2018 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2012 3.52 MESO Yes 320 9 1.5 6.7 B A A A 2013 3.60 MESO Yes 340 14 0.6 6.8 B B A A 2017 3.39 MESO Yes 350 9 1.5 3.5 B A A A 2018 3.40 MESO Yes 330 9 1.7 2.0 B A A A MED = seasonal median concentration (Dec-May)

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Middleton was high in 2012 and but moderate in 2017 due to the increase in invasive condition (Elodea canadensis beds) between the two monitoring events (Table 4-82).

142 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-82: Lake Middleton, LakeSPI overview, 2012-2017 2012 2017 % Change Change indicated? LakeSPI 57 50 -7 Possible Native Condition 84 77 -7 Possible Invasive Condition 56 68 12 Probable Condition High Moderate NA Yes Invasive species Elodea Elodea NA No Data from https://lakespi.niwa.co.nz/

Recreational water quality Suitability for contact recreation fluctuated, with high concentrations of E. coli recorded in 2008, 2010, 2011 and 2019 (Figure 4-126). The overall grade between 2015 and 2019 was ‘fair’ (Table 5-3).

Figure 4-126: E. coli counts at Lake Middleton, 2013-2019

Water colour The median of the dominant wavelength between 2013 and 2018 was 502nm, i.e. blue-green, with a district seasonal pattern in most years (Figure 4-127).

Figure 4-127: Landsat derived water colour data for Lake Middleton, 2013-2018 (n = 43): Dominant wavelength (lambda dom) and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Summary Lake Middleton was mesotrophic when sampled in 2012, 2013, 2017 and 2018, and met the LWRP TLI objective of 3.6 in all years. The increase in exotic weeds has caused a shift from a high to moderate ecological state between 2012 and 2017. Because of the very limited historic data for this lake, it is difficult to assess if recent TLI scores deviate from a natural reference state. Given the elevated nitrogen concentrations, management of phosphorus and nitrogen inputs is a priority if the TLI objective is to be achieved long term.

Environment Canterbury Technical Report 143 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6.6 Lake Ōhau

Figure 4-128: Sampling location in Lake Ōhau (SQ32909) – aerial photograph (EcanMaps)

Lake Ōhau (Figure 4-128) is located at the head of the Waitaki River hydro-electric scheme. The lake receives water from the Hopkins and Dobson Rivers, which contain a small amount of glacial meltwater. Therefore, Lake Ōhau’s water contains less glacial flour than Lakes Pukaki and Tekapo. Lake level is controlled by a concrete weir. The lake is 129 m deep and covers an area of 59.27 km2. The catchment is largely mountainous and covered by native vegetation, with some grazing of river beds and grassland development in the lower reaches.

Nutrients Lake Ōhau was very low in nutrients (Table 4-83), with the exception of a number of elevated TP measurements between 2009 and 2013. There were no significant trends in TN between 2007 and 2019 (Table 4-84). No trends analysis was carried out for TP due to the high number of data points below the limit of analytical detection. The relatively high TP concentrations between 2009 and 2013 were only partially linked to high turbidity but occurred after high rainfall (App 3.2) and high flows in nearby rivers in December 2008, December 2010, January 2010 and January 2013 (Environment Canterbury, unpublished data).

Phytoplankton biomass Phytoplankton biomass was low (Table 4-83), and there were no statistically significant trends in chl-a between 2007 and 2019 (Table 4-84).

Trophic Level Index Lake Ōhau was microtrophic in most years, but was assigned an oligotrophic status in 2010 and 2013 (Table 4-83). TLI varied substantially between years (Figure 4-130), with higher TLI scores associated with elevated concentrations of TP (Figure 4-129).

144 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-83: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Ōhau from 2006- 2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2006 1.44 MICRO Yes 47 1.5 0.9 1.1 A A A A 2007 1.70 MICRO Yes 84 2 0.7 1.1 A A A A 2008 1.06 MICRO Yes 30 2 0.4 0.6 A A A A 2009 1.90 MICRO No 40 5.5 1.0 1.5 A A A A 2010 2.08 OLIGO No 50 6 0.6 0.9 A A A A 2011 1.96 MICRO No 35 11 0.5 0.9 A B A A 2012 1.46 MICRO Yes 40 2.5 0.5 0.7 A A A A 2013 2.07 OLIGO No 29 10 1.3 1.7 A A A A 2014 0.87 ULTRA Yes 40 2 0.2 0.3 A A A A 2015 1.12 MICRO Yes 31 2 0.3 1.1 A A A A 2016 1.42 MICRO Yes 29 2 0.9 1.1 A A A A 2017 1.38 MICRO Yes 37 2 0.6 1.8 A A A A 2018 1.31 MICRO Yes 24 2 0.6 0.8 A A A A 2019 1.44 MICRO Yes 50 2 0.7 1.0 A A A A 5 year 1.34 MICRO Yes 34 2 0.6 1.8 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Figure 4-129: TP, TN, chl-a concentrations and turbidity in Lake Ōhau from 2006-2019

Environment Canterbury Technical Report 145 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-84: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Ōhau

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 63 -71 1094.33 -2.12 0.03 -0.82 -2.22 -1.95 to -0.12 decreasing 0.98 chl-a 63 4 1110 0.09 0.93 0 0 -0.02 to 0.03 0.50 Turbidity 63 69 1133 2.02 0.04 0.05 4.43 0.01 to 0.12 increasing 0.98

Figure 4-130: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual means in Lake Ōhau, 2006-2017. Red line is the LWRP objective/limit

Turbidity Lake Ōhau is the least turbid of the three large, natural lakes we sampled in South Canterbury. Lake Ōhau receives less glacial melt water than Lake Tekapo and Lake Pukaki. In contrast to Lake Pukaki, turbidity significantly increased between 2007 and 2019 (Table 4-84), possibly linked to changes in timing and magnitude of precipitation or increased glacial meltwaters associated with the retreat of glaciers in the catchment.

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Ōhau was high from in 1982 to 2012, and excellent in 2015 and 2017 (Table 4-85). The relatively stable lake level and higher clarity has likely contributed to the development of high macrophyte cover compared to Lakes Tekapo and Pukaki.

146 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-85: Lake Ōhau, LakeSPI overview, 1982-2017 1982 1989 2001 2009 2015 2017 % Change Change indicated? LakeSPI 68 69 60 75 81 84 3 No Native 56 67 57 62 68 84 16 Probable Condition Invasive 14 25 30 4 0 13 13 Probable Condition Condition High High High High Excellent Excellent NA No Invasive Elodea Elodea Elodea Elodea None Elodea NA species Ranunculus Ranunculus present trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 491nm, i.e. blue, with little variability (Figure 4-131).

Figure 4-131: Landsat derived water colour data for Lake Ōhau, 2013-2018 (n = 21): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Other observations Lake level did not fluctuate to the same extend as in the large hydropower lakes (Lakes Pukaki and Tekapo). Highest levels were recorded in January 2011 and 2013 (Figure 4-132).

Environment Canterbury Technical Report 147 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-132: Lake level of Lake Ōhau, 2005-2017. Data: NIWA/Meridian Energy

Summary Lake Ōhau is a relatively clear, (mostly) microtrophic lake with macrophytes communities in excellent ecological condition. Apart from an increase in turbidity there were no significant trends in any of the water quality parameters sampled.

4.6.7 Lake Benmore

Figure 4-133: Sampling locations in Lake Benmore: near dam (bottom), Haldon Arm (top) and Ahuriri Arm (left) – aerial photograph (EcanMaps)

148 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake Benmore (Figure 4-133) is a man-made hydro-reservoir with a maximum depth of 91 m and a surface area of 75.85 km2. The lake is important for recreation (including boating, swimming and campsites) and it is one of the most fished water bodies in Canterbury.

The lake was sampled at three sites for water quality (Haldon Arm, Ahuriri Arm and near the dam) and LakeSPI surveys were carried out at two sites (Haldon Arm and Ahuriri Arm).

In the Haldon Arm, the maximum extent of the epilimnion in summer is usually 30 m. In the shallower Ahuriri Arm the water column is frequently mixed to the bottom near the delta, but can be stratified at the sampling site (Spigel et al., 2015). Water from the Haldon Arm can underflow into the Ahuriri Arm, creating or stabilising a temporary stratification in the shallower Ahuriri Arm (Pickrill & Irwin, 1986; Spigel et al., 2015; Norton et al., 2017).

Nutrients The Ahuriri Arm had comparatively high concentrations of chl-a and nutrients, resulting in a higher TLI score than the Haldon Arm (Table 4-87, Table 4-88, Table 4-86). The Ahuriri Arm is not only shallower, but also receives a smaller and more nutrient enriched inflow and has a longer residence time than the Haldon Arm (Spigel et al., 2015; Clarke, 2015). Due to these characteristics the Ahuriri Arm is assumed to be more sensitive to changes in nutrient loads especially since the Haldon Arm receives “significantly larger glacial derived inflows …. that are largely unaffected by catchment development” (Spigel et al., 2015).

There were no trends in TN at any of the sites in Lake Benmore (Table 4-89, Table 4-90, Table 4-91). The Ahuriri Arm and Dam site have only been monitored since 2009, thus trends are reported from 2009-2019 for these two sites but 2007-2019 for the Haldon Arm. Analysis of dissolved inorganic nitrogen concentrations in the lower Ahuriri River indicates higher plant available nitrogen loads are entering the lake over time (Figure 4-140). No trends analysis was carried out for TP due to the high number of data points below the limit of analytical detection.

Phytoplankton biomass Phytoplankton biomass was in the microtrophic or oligotrophic range for all three sites until 2018 (Table 4-87, Table 4-88, Table 4-86). However, elevated median chl-a concentrations were measured in the Ahuriri Arm in 2019 (Table 4-86, Figure 4-134) and chl-a increased at all three sites arm between 2007 and 2019 (Table 4-89, Table 4-90, Table 4-91). This trend was statistically significant for the near-dam site and the Haldon Arm, and probable for the Ahuriri Arm. It is possible this pattern is related to increased water clarity, and particularly for the Ahuriri Arm, increasing nutrient (DIN) inputs over time.

Trophic Level Index TLI was highest in the Ahuriri Arm, where it was also most variable (Figure 4-135). TLI increased in the Haldon Arm between 2006 and 2010, but decreased from 2010 to 2016, to increase again from 2016 to 2019 (Figure 4-135). The high TLI in the Haldon Arm in 2010, 2011 and 2013 was due to high TP, but the high TLI in 2019 seems to have been driven by relatively high chl-a at all sites (see Figure 4-134).

Environment Canterbury Technical Report 149 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-86: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Benmore (Ahuriri Arm) from 2010-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2010 2.55 OLIGO Yes 90 11 1.2 1.5 A B A A 2011 3.03 OLIGO No 120 11 1.0 6.0 A B A A 2012 2.49 OLIGO Yes 90 8 1.0 2.0 A A A A 2013 2.80 OLIGO Yes 102 12 1.2 2.4 A B A A 2014 2.05 OLIGO Yes 69 4 0.7 1.7 A A A A 2015 2.58 OLIGO Yes 73 4 1.7 2.0 A A A A 2016 2.11 OLIGO Yes 64 4 1.2 2.2 A A A A 2017 2.31 OLIGO Yes 70 5 1.3 1.9 A A A A 2018 2.29 OLIGO Yes 93 4 1.2 2.3 A A A A 2019 2.94 OLIGO No 90 5 3.8 9.8 A A B A 5 year 2.45 OLIGO Yes 78 4.4 1.8 9.8 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-87: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Benmore (Near Dam) from 2010-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2010 2.21 OLIGO Yes 70 7 0.8 1.4 A A A A 2011 1.96 MICRO Yes 45 8 0.5 0.8 A A A A 2012 1.92 MICRO Yes 40 4 0.8 1.9 A A A A 2013 1.71 MICRO Yes 36 4 0.8 1.4 A A A A 2014 1.60 MICRO Yes 40 4 0.5 1.1 A A A A 2015 1.82 MICRO Yes 45 4 0.8 1.2 A A A A 2016 1.72 MICRO Yes 38 4 0.8 1.0 A A A A 2017 1.77 MICRO Yes 35 4 0.8 1.5 A A A A 2018 1.89 MICRO Yes 43 4 0.7 1.6 A A A A 2019 2.19 OLIGO Yes 53 4 1.3 2.5 A A A A 5 year 1.88 MICRO Yes 43 4 0.9 2.5 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

150 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-88: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Benmore (Haldon Arm) from 2006-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2006 1.30 MICRO Yes 45 1 0.7 1.0 A A A A 2007 1.57 MICRO Yes 36 4 0.6 0.7 A A A A 2008 1.63 MICRO Yes 40 3 0.8 0.9 A A A A 2009 1.68 MICRO Yes 45 4 0.5 0.8 A A A A 2010 2.14 OLIGO Yes 60 9 0.7 1.2 A A A A 2011 2.05 OLIGO Yes 50 10 0.6 0.8 A B A A 2012 1.73 MICRO Yes 40 4 0.6 0.7 A A A A 2013 2.03 OLIGO Yes 39 9 0.6 1.6 A A A A 2014 1.66 MICRO Yes 40 4 0.3 1.6 A A A A 2015 1.90 MICRO Yes 45 4 0.8 1.7 A A A A 2016 1.69 MICRO Yes 32 4 0.8 1.0 A A A A 2017 1.72 MICRO Yes 34 4 0.8 1.1 A A A A 2018 1.73 MICRO Yes 41 4 0.7 1.5 A A A A 2019 2.12 OLIGO Yes 46 4 1.4 2.3 A A A A 5 year 1.83 MICRO Yes 40 4 0.9 2.3 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

Table 4-89: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (Ahuriri Arm)

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 48 -23 455.67 -1.03 0.3 -1.31 -1.57 -3.03 to 0.69 decreasing 0.84 chl-a 48 35 454.33 1.6 0.11 0.08 6.64 0.00 to 0.18 increasing 0.94 Turbidity 47 -60 441.33 -2.81 0.00 -0.15 -19.57 -0.26 to -0.05 decreasing 1

Table 4-90: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (near dam)

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 47 -4 441.33 -0.14 0.89 -0.25 -0.6 -1.15 to 1.00 decreasing 0.56 chl-a 47 42 426.67 1.98 0.05 0.03 4.18 0.00 to 0.08 increasing 0.96 Turbidity 46 -82 430.67 -3.9 0.00 -0.16 -24.01 -0.23 to -0.10 decreasing 1

Environment Canterbury Technical Report 151 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-91: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Benmore (Haldon Arm)

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 61 -27 1000.33 -0.82 0.41 -0.6 -1.5 -1.58 to 0.29 decreasing 0.84 chl-a 63 113 1106.33 3.37 0 0.04 5.69 0.01 to 0.06 increasing 1 Turbidity 62 -91 1092.33 -2.72 0.01 -0.07 -8.26 -0.15 to -0.02 decreasing 1

Figure 4-134: TP, TN, chl-a concentrations and turbidity in Lake Benmore from 2010-2019 (Ahuriri Arm at top and near dam at bottom). Note different scales

152 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Fig 4-132 continued: TP, TN, chl-a concentrations and turbidity in Lake Benmore (Haldon Arm) from 2006-2019

Turbidity Turbidity was generally below 5 NTU, but increased in response to flood events. Turbidity decreased significantly between 2007 and 2019 at all three sites (Table 4-89, Table 4-90, Table 4-91), probably due to a lack of high turbidity events captured in our summer sampling programme between 2014 and 2019 (Figure 4-134). Turbidity seems to have become less variable over the summer months since 2014, possibly in response to changes in the magnitude and timing of glacial melt and/or spring/summer flood flows. Large flood flows in the Ahuriri River were absent between 2014 and 2018 compared to earlier years (Figure 4-136), which are likely linked to lower and less variable turbidity in the Ahuriri Arm 2014-2018. The Haldon Arm receives water from Lake Pukaki, in which turbidity is also decreasing, Lake Ōhau (where turbidity is increasing) and Lake Tekapo (no change in turbidity) (see Table 5-8 for comparison of turbidity trends).

Environment Canterbury Technical Report 153 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-135: Inter-annual variation TLI of Lake Benmore, 2006-2019

Figure 4-136: Flow in the Ahuriri River at South Diadem from 2005-2019 (in m3/s)

Lake ecological condition: LakeSPI The condition of the aquatic macrophyte community of Lake Benmore was moderate in 2018 in the Ahuriri Arm but high in the Haldon Arm (Table 4-92). Improvement in the Haldon Arm between 2012 and 2018 was due to “a significant extension in depth limits for native plant communities, which increased measures of native presence and quality, and decreased proportional measures of the predominant invasive weed Elodea (Elodea canadensis)” (de Winton, 2018). The increased depth limits for macrophytes in the lake is possibly related to increased water clarity. While there was some improvement in the Ahuriri Arm, this change was not statistically significant. In the Ahuriri Arm and the neck of the lake the invasive weed Lagarosiphon major is actively managed (sprayed and removed).

154 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-92: Lake Benmore, LakeSPI overview (data from Sutherland et al., 2012) Ahuriri Arm, Ahuriri Arm, 2018 Haldon Arm, Haldon Arm, 2018 2012 2012 LakeSPI 34 49 37 72 Native Condition 37 55 37 80 Invasive Condition 69 53 60 36 Condition Moderate Moderate Moderate High Invasive species Elodea Elodea canadensis Elodea Elodea canadensis canadensis Lagarosiphon major canadensis Ranunculus Lagarosiphon Ranunculus Ranunculus trichophyllus major trichophyllus trichophyllus Ranunculus trichophyllus

Water colour The median of the dominant wavelength between 2013 and 2018 was 494 nm, i.e. blue-green (Figure 4-144). Compared to some small high-country lakes, there was limited variability in water colour except for three outliers in the yellow/orange spectrum in autumn/winter 2017, possibly linked to high sediment inputs.

Figure 4-137: Landsat derived water colour data for Lake Benmore, 2013-2018 (n = 36): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Both beaches monitored at Lake Benmore for E. coli were graded as ‘good’ between 2015 and 2019 (Table 5-3), and generally had low E. coli counts over the past 5 years (Figure 4-138).

Environment Canterbury Technical Report 155 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-138: E. coli counts in Lake Benmore at Pumpkin Bay (top) and Sailors Cutting (bottom), 2002-2019

Other observations The lake level was relatively stable compared to Lake Pukaki and Tekapo (Figure 4-139).

Figure 4-139: Lake level variation of Lake Benmore, 2004-2017. Data NIWA/Meridian Energy

156 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-140: Trend in Dissolved Inorganic Nitrogen (DIN – mg/L) over time in the Ahuriri River at Ben Omar

Summary While the Haldon Arm and the site near the dam were generally microtrophic, the Ahuriri Arm was oligotrophic from 2010 to 2019. All sites met the LWRP objectives in all years, except the Ahuriri Arm in 2011 and 2019. Before 2015, the TLI in the Haldon Arm seemed driven largely by TP concentrations associated with flood flows. However, recent (2019) high TLI’s were due to high phytoplankton biomass at all sites.

The shallow Ahuriri Arm is considered to be more vulnerable to the effects of catchment development than the deeper parts of the lake. While TN did not increase in any of the sites in Lake Benmore between 2007 and 2019, TN appeared to increase in the Ahuriri Arm from 2016 to 2019. Both DIN and TN also increased significantly in the Ahuriri River between 2013 and 2019 (Figure 4-140), likely resulting in increased nutrient availability to phytoplankton in the lake. Considering that phytoplankton biomass increased at all three sites between 2007 and 2019 and reached concentrations indicative of mesotrophic conditions in the Ahuriri Arm in 2019, the effect of current and future nutrient inputs into the lake needs to be closely monitored, and nutrient loads (particularly bioavailable dissolved nutrients) to the lake minimised.

Between 2007 and 2019, turbidity decreased significantly in Lake Benmore at all three monitoring sites. If phytoplankton growth was light limited in Lake Benmore in summer, the increase in chl-a from 2007- 2019 could be linked to less turbid (clearer) water and higher light availability. At the same time intensification in land use upstream of Lake Benmore occurred, and both DIN and TN increased significantly in the Ahuriri River 2013-2019 (Figure 4-140). It is likely we are seeing interacting effects on phytoplankton biomass by clearer waters due to changes in glacial melt and flow patterns (caused by climatic change and changes in water use) and increased nutrient availability (mostly due to land use changes), which may be amplifying each other. Therefore, it is paramount that nutrient limits in the catchments are met, and that catchment nutrient limit setting is based on the TLI limits in Table 15B(d) of the LWRP (which are more conservative than the median TN and TP limits in the same table). Furthermore, ongoing compliance with current plan TLI objectives may require a revision of the nutrient input limits, given they are based on a model utilising fixed turbidity conditions which do not account for potential future increased clarity in the lake.

In terms of aquatic plant communities, both the Haldon and Ahuriri Arm were in moderate ecological condition in 2012, but have improved in the most recent survey, probably associated with Lagarosiphon control in the Ahuriri Arm, and increased water clarity in the Haldon Arm.

Environment Canterbury Technical Report 157 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

4.6.8 Lake Aviemore

Figure 4-141: Sampling location in Lake Aviemore – aerial photograph (EcanMaps)

Lake Aviemore (Figure 4-141) is a man-made hydro-reservoir on the Waitaki hydroelectric scheme, downstream of Lake Benmore. The dam was completed in 1968 and the lake has a surface area of 28.11 km2.

Nutrients, Phytoplankton biomass and Trophic Level Index The lake had consistently low concentrations of TP, TN, and chl-a (Table 4-93), with no trends in chl-a or TN (Table 4-94). Lake Aviemore was microtrophic or oligotrophic in all years (Table 4-93). Similar to Lake Benmore, TN and chl-a seem to have been increasing between 2016 and 2019. This is not surprising given the trends observed in upstream Lake Benmore. TLI fluctuated between 1.4 and 2.3, exceeding the LWRP objective of 2 in 2010, 2011 and 2013 (Figure 4-143).

Turbidity Turbidity decreased significantly between 2010 and 2019 (Table 4-94).

Table 4-93: Trophic Level Index and attribute states (NPS-FM 2014) for Lake Aviemore from 2005-2019 Trophic Level Index Numeric Attribute State (in µg/L) Attribute State LWRP Chl-a - Chl-a - Chl-a - Chl-a - Year TLI Grade TN TP TN TP met? MED MAX MED MAX 2010 2.24 OLIGO No 60 2.5 1.2 2.4 A A A A 2011 2.02 OLIGO No 50 10.5 0.5 0.6 A B A A 2012 1.85 MICRO Yes 50 5 0.8 1.2 A A A A 2013 2.33 OLIGO No 44 7 1.1 2.3 A A A A 2014 1.44 MICRO Yes 42 2 1.0 1.5 A A A A 2015 1.93 MICRO Yes 40 2 0.9 1.3 A A A A 2016 1.61 MICRO Yes 37 2 0.8 0.9 A A A A 2017 1.74 MICRO Yes 37 4 0.8 1.1 A A A A 2018 1.95 MICRO Yes 55 5 0.7 1.2 A A A A 2019 1.97 MICRO Yes 61 2 1.6 2.7 A A A A 5 year 1.84 MICRO Yes 46 3 1.0 2.7 A A A A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2015-2019

158 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-142: TP, TN, chl-a concentrations and turbidity in Lake Aviemore from 2010-2019

Table 4-94: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Lake Aviemore

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TN 48 -9 454.33 -0.38 0.71 -0.25 -0.51 -1.50 to 1.01 decreasing 0.63 chl-a 48 25 445.67 1.14 0.26 0.02 2.79 0.00 to 0.09 increasing 0.88 Turbidity 47 -88 441.33 -4.14 0.00 -0.13 -20.09 -0.20 to -0.07 decreasing 1

Figure 4-143: Trophic Level Index, Total Nitrogen, Total Phosphorus and chlorophyll a annual mean results in Lake Aviemore, 2010-2017. Red line is the LWRP objective/limit

Environment Canterbury Technical Report 159 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Lake Aviemore has been high since 2012, with a possible improvement in native condition and lessening of the Invasive Condition between 2012 and 2015 (Table 4-71).

Table 4-95: Lake Aviemore, LakeSPI overview, 2012-2017 2012 2015 2017 % Change Change indicated? LakeSPI 54 62 64 2 No Native Condition 59 68 70 2 No Invasive Condition 51 42 42 0 No Condition High High High NA No Invasive species Elodea Elodea Elodea canadensis canadensis canadensis Ranunculus Ranunculus Ranunculus trichophyllus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Water colour The median of the dominant wavelength between 2013 and 2018 was 498 nm, i.e. blue-green (Figure 4-144). Compared to some small high-country lakes, there was limited variability in water colour except for a number of outliers in the yellow/orange spectrum, possibly linked to high sediment inputs. One of these outliers coincided with high turbidity readings in late January 2017.

Figure 4-144: Landsat derived water colour data for Lake Aviemore, 2013-2018 (n = 78): Dominant wavelength (lambda dom), purity and Forel-Ule (FU) colour index. The ‘horseshoe plot’ shows the distribution of dominant wavelength with the cross representing the white point and the black outline the pure spectral colours. Source: Moritz Lehmann, University of Waikato

Recreational water quality Suitability for contact recreation grades (2015-2019) were good for Waitangi and the Te Akatarawa Camp, but fair for Lake Aviemore at Loch Laird (Table 5-3). The Loch Laird site had more frequent and higher exceedances of the microbial recreational water quality guidelines (Figure 4-145).

160 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 4-145: E. coli counts in Lake Aviemore at Campground (top), Waitangi (middle) and Loch Laird (bottom), 2002-2019

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Cultural health The cultural heath of Lake Aviemore was given an overall grade of ‘moderate’. An assessment of the cultural health of the outlet was also ‘moderate’, but mahinga kai status was ‘poor’ (Statistics New Zealand, https://statisticsnz.shinyapps.io/cultural_health/).

Other observations The lake water level was generally stable (Figure 4-146).

Figure 4-146: Lake level variation in Lake Aviemore, 2005-2017. Data NIWA/Meridian Energy

Summary Lake Aviemore was microtrophic or oligotrophic in all monitoring years, meeting its LWRP objective of 2.0 in most years. Turbidity decreased significantly from 2010 to 2019. Ecological condition of the macrophyte community was high, but cultural health was graded as ‘moderate’.

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4.6.9 Kellands Pond

Figure 4-147: Sampling locations in Kellands Pond (shore and mid-lake sites – waterbody on the left, Wairepo Arm is waterbody on the right) – aerial photograph (EcanMaps)

Kellands Pond (Figure 4-147) is a small (0.19 km2), shallow (6.5 m) man-made pond, near the Wairepo Arm of Lake Ruataniwha, upstream of Lake Benmore. The lake is largely fed by groundwater, and a culvert drains the lake to the Wairepo Arm. However, water can also enter the lake though the culvert, depending on level fluctuations in the Ōhau Canal and Lake Ruataniwha (Clarke, 2015). The lake is used for swimming and fishing (Clarke, 2015). In 2003 large changes occurred in the catchment, when “the land on the western side of SH8 around Kellands Pond was developed as an intensive irrigated dairy operation” (Clarke, 2015). In 2015, about 3850 cows grazed in the catchment. Nitrate and nitrite nitrogen (NNN) were reported to have increased about 45% per year between 2003 and October 2013 (Clarke, 2015). Kellands Pond was sampled quarterly from shore since 2004, and by helicopter at a mid-lake site in 2012 and 2013.

Nutrients, Phytoplankton biomass, Trophic Level Index The lake was mesotrophic in most years (Table 4-96), with a TLI above the LWRP objective of 3.2 and with elevated TN concentrations (Table 4-97). No statistical trend was observed in TP, whereas TN increased significantly between 2007 and 2019 (Figure 4-148, Table 4-98). There were insufficient data for trend analyses of chl-a (only measured since 2012). Total nitrogen levels peaked in 2014/2015 (Figure 4-148).

Turbidity Turbidity increased significantly between 2007 and 2019 (Table 4-98). There are two possible explanations for this change. It may be that phytoplankton growth in the pond has increased resulting in a more turbid water column, or it may be that changes in canal operation have resulted in an increase in the amount of water from Lake Ruataniwha moving from the canal into Kellands Pond by way of the Wairepo Arm.

Environment Canterbury Technical Report 163 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 4-96: Overview of Kellands Pond TLI status from 2012 to 2017 Hydrol. TLI Grade TLI Grade Meets LWRP objective? Year mid lake mid lake shore shore 2012 3.04 OLIGO Yes 2013 3.17 MESO 3.39 MESO Yes 2014 - - 3.21 MESO No 2015 - - 3.52 MESO No 2016 - - 3.59 MESO No 2017 - - 3.47 MESO No 2018 - - 3.53 MESO No 2019 - - 3.49 MESO No 2015-19 - - 3.52 MESO No

Table 4-97: Attribute states for Kellands Pond in 2012 and 2013 (according to NPS-FM 2014)

Numeric Attribute State (in µg/L) Attribute State Chl-a - Chl-a - Chl-a - Chl-a - Year TN-MED TP-MED TN TP MAX MED MAX MED 2012 1.9 1.3 280 8.0 A A B A 2013 2.7 1.2 330 8.0 A A B A MED = seasonal median concentration (Dec-May); Last line is 5 year average from 2013-2017

Figure 4-148: TP, TN, chl-a concentrations and turbidity in Kellands Pond (shore) from 2004-2017 Table 4-98: Seasonal Mann-Kendall Trend Test Results, 2007-2019, Kellands Pond, shore

Probability Parameter Sample size Kendall statistic Variance Z P Median Sen slope (annual) Percent annual change 90% confidence limits for slope Trend direction

TP 52 -12 1038 -0.34 0.73 0 0 -0.13 to 0.06 0.50 TN 52 179 1091.67 5.39 0 39.81 12.25 31.16 to 48.68 increasing 1 Turbidity 52 72 1067.33 2.17 0.03 0.02 4.27 0.00 to 0.04 increasing 0.98

164 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Lake ecological condition: LakeSPI Condition of the aquatic macrophyte community of Kellands Pond was moderate in 1982 and 2012, but improved to high in 2017 with a probable reduction in invasive condition and an improvement in native condition (Table 4-71), due to a “greater occupancy of the vegetation by native plants and extensions in the depths of native plants” (de Winton & Burton, 2017).

Table 4-99: Kellands Pond, LakeSPI overview, 2012-2015 2012 2015 2017 % Change Change indicated? LakeSPI 38 44 56 12 Probable Native Condition 38 51 60 9 Possible Invasive Condition 68 57 43 -14 Probable Condition Moderate Moderate High NA Yes Invasive species Elodea Elodea Elodea canadensis canadensis canadensis Ranunculus Ranunculus trichophyllus trichophyllus Data from https://lakespi.niwa.co.nz/

Other observations Introduced freshwater jellyfish were reported to bloom in the summer 2017, and were also noted in previous dive surveys of Kellands Pond (de Winton & Burton, 2017).

Summary Kellands Ponds was mesotrophic and did not achieved the LWRP objective of 3.2 since 2014. Both total nitrogen and turbidity have increased significantly between 2007 and 2019. The increasing TN concentrations in Kellands Pond can likely be attributed to adjacent land use intensification in the catchment. Ongoing management of phosphorus and nitrogen inputs, especially the reduction of bioavailable, dissolved nitrogen loads, is essential to reduce the risk of regular phytoplankton blooms.

Environment Canterbury Technical Report 165 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

5 Regional summary of state and trends 5.1 Regional state of lake water quality 5.1.1 Comparison to LWRP objectives While our large, deep high-country lakes meet their LWRP water quality objectives in most years, more than half of our small to medium sized lakes have nutrient concentrations or algal biomass that are above the objectives set in the Canterbury Land and Water Regional Plan. Most of these lakes are situated in catchments with more than a third of land area pastoral agriculture, often with improved pastures or cropping (Table 5-2). Just under half the lakes (43% percent) met Trophic Level Index (TLI) objectives and 42% met aquatic plant ecological condition (LakeSPI) objectives, and only 23 percent of lakes (7 lakes) met Land and Water Regional Plan objectives both for the TLI and LakeSPI (Table 5-1). Few lakes were sampled for E. coli. Those that were sampled mostly met the objective for microbial water quality. More details are provided in sections 5.1.1. to 5.1.10. Table 5-1: Do lakes meet LWRP objectives? TLI – TLI LakeSPI Zone Lake SFRG 5 y Ø Limit Condition Hurunui - Waiau Katrine 2.64 3 Mod No data Hurunui - Waiau Sumner 1.79 2 High No data Hurunui - Waiau Taylor 2.51 3 High No data Hurunui - Waiau Sheppard 3.41 3 Mod No data Hurunui - Waiau Marion 3.77 3 No data No data Hurunui - Waiau Mason 2.16 3 High No data Selwyn - Waihora Sarah 3.17 3 Mod No data Selwyn - Waihora Grasmere 3.37 3 Mod No data Selwyn - Waihora Pearson 3.84 3 High No data Selwyn - Waihora Hawdon 3.44 3 Excellent No data Selwyn - Waihora Lyndon 3.09 3 High No data Selwyn - Waihora Georgina 4.05 4 Mod No data Selwyn - Waihora Ida 3.18 3 High No data Selwyn - Waihora Selfe 3.05 3 High No data Selwyn - Waihora Coleridge 1.40 2 Excellent No data Selwyn - Waihora Evelyn 3.50 3 Mod No data Selwyn - Waihora Catherine 3.14 3 No data No data Selwyn - Waihora Henrietta 4.21 3 No data No data Ashburton Emily 4.23 4 Mod No data Ashburton Māori Front/East 3.86 4 Mod No data Ashburton Māori Back/West 4.16 4 Mod No data Ashburton Denny 5.51 3 Mod No data Ashburton Heron 3.15 3 Mod No data Ashburton Emma 4.46 3 Mod No data Ashburton Camp 3.24 3 High Very good Ashburton Clearwater 3.86 3 Mod Very good Upper Waitaki Alexandrina 3.02 3.1 High Fair Upper Waitaki Middleton 3.52 3.6 High Fair Upper Waitaki Tekapo 1.14 1.7 High Very Good Upper Waitaki Pukaki 1.45 1.7 No data No data Upper Waitaki Ōhau 1.34 1.7 Excellent No data Upper Waitaki Benmore -Haldon 2.45 2.7 Mod No data Upper Waitaki Benmore- Ahuriri 1.83 2.9 Mod Good Upper Waitaki Benmore - Dam 1.88 2.7 No data Good Upper Waitaki Aviemore 1.84 2 High Good/Fair* Upper Waitaki Kellands 3.52 3.2 Mod No data OTOP Opuha 3.89 4 NA Very good/fair* NS = not sampled. Green = objective met. Red = objective not met. Results in italics are based on less than 5 years of data. *Aviemore, microbial: Loch Laird: Fair, Campground & Waitangi: Good. Lake Opuha, microbial: Ewarts Corner Boat Ramp: Fair, Recreation Reserve: very good. Note: TLI Limits are TLI objectives in LWRP. These objectives may be based on ‘annual averages’, whereas the 5 year average and current TLIs are calculated from seasonal data.

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Trophic Level Index Several lakes in the monitoring dataset are only minimally influenced by anthropogenic factors and so can be considered more indicative of changes in natural conditions for the region. Among these, Lakes Lyndon and Ida fluctuated around the threshold of oligotrophic and mesotrophic (Figure 5-1). Lake Marion, which is entirely fed by a native forest catchment, had a higher trophic status than other ‘less- impacted’ lakes, possibly due to the small catchment size and long residence time enhancing in-lake nutrient recycling and possibly larger loading of organic material from a forested catchment. The large, deep lakes monitored all remained predominantly in microtrophic condition and within the target LWRP range (Figure 5-1), although some were intermittently in an oligotrophic condition around 2009/2010. This included Lake Sumner, a little impacted large, deep lake. 2009/2010 was a wet year following a number of drier years. Lake Coleridge is also micro/oligotrophic but cannot be considered natural state because of the high degree of managed diversion and lake level management.

Most of the smaller to medium sized lakes were in a mesotrophic state (17 lakes) between 2015 and 2019, with 20 out of 26 of these lakes breaching their LWRP objectives for TLI (based on 5 year average scores) (Figure 5-1). Five lakes were classified as eutrophic or supertrophic (Figure 5-1). Supertrophic Lake Denny, mesotrophic Kellands Pond, and the Māori Lakes have significant pastoral load sources of nutrients. Lakes Georgina, Emily and Henrietta were also eutrophic, and more than half of their catchments are under improved pastoral land cover. Lakes Georgina and Emily have relatively intact wetlands, but large wildfowl populations and possibly high natural sediment loading, which is reflected in their higher TLI objective of 4.0. However, despite their TLI allowing for a higher degree of nutrient enrichment, these lakes exceeded their TLI objective at times in the past 5 years (Table 5-2), indicating high nutrient loading from the catchment.

Average TLIs scores between 2015 and 2019 were above the TLI objective set in the LWRP for 20 lakes (Table 5-1). With the exception of Lake Denny, which was highly impacted by land use intensification, and Lake Henrietta, lakes that failed to reach their TLI objectives usually were in the TLI band above their objective (Table 5-1). That is, lakes that failed to reach their objective of ‘oligotrophic’ were usually ‘mesotrophic’, and lakes failed to reach their objective of ‘mesotrophic’ were generally ‘eutrophic’. However, because the TLI scale is logarithmic, not linear, slipping a whole TLI band can represent a considerable degradation of water quality. Lakes that repeatedly fail to meet their objectives and fail by more than 15 % (Table 5-2) are considered at risk of long-term degradation.

Looking at the frequency and magnitude or ‘severity’ of departure from TLI objectives can help identify ‘problem lakes’ that are unlikely to easily return to their LWRP objectives in the future (Table 5-2): Lakes Denny, Lake Emma, Lake Pearson, Kellands Pond and Lake Clearwater fall into this category. Lake Emma was possibly affected by the diversion of the main inflow to Lake Camp (DOC Geraldine, pers. comm). Lake Clearwater is likely to receive large inputs of nitrogen (via groundwater), and Kellands Pond and Lake Denny reflect the significant intensification of land use in their catchment or management practices resulting in large sediment loads to the lake (Lake Denny). The Front Māori Lake, Lake Sarah and Lake Georgina are also identified as lakes of concern, but show some possible scope for improvement. Lake Pearson has experienced a step-change degradation in turbidity, nutrient and chl-a in 2013, possibly linked to a large flood and storm event, resulting in macrophyte communities being negatively impacted by bed disturbance and reduced light availability.

In the Upper Waimakariri catchment Lakes Grasmere and Hawdon also regularly failed to meet their LWRP objectives, having exceeded them every year for the last 5 years. Results for Lake Hawdon are affected by a single large bloom (or sampling error?) in April 2017, which briefly pushed the lake into the eutrophic band in 2016/2017. Lake Grasmere has a relatively low N to P ratio (Figure 5-9), which is unusual among our high-country lakes, except for highly enriched Lake Denny (high TP) and the ultra- oligotrophic large glacial-fed lakes (low TN). Compared to other polymictic Canterbury high-country lakes Lake Grasmere is relatively low in TN (Figure 5-7) but has relatively high TP (Figure 5-8). Sources of phosphorus to Lake Grasmere may include fertiliser application on adjacent pastoral land, large wild fowl populations, and/or fine sediment inputs with large rainfall events (In-lake DRP is elevated during to milky-green colour events (Environment Canterbury, unpublished obs.)).

Environment Canterbury Technical Report 167 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

There is limited information on the past trophic state of many of the high-country lakes. In the 1970s Lakes Mason (shallow basin), Evelyn, Ida, Marion, Sarah, Grasmere and Sheppard were classed as mesotrophic (based on chl-a, TN, TP, and bacterial production) (Spencer 1977). In the same report, Lakes Katrine, Catherine, Hawdon, Pearson, Taylor, Lyndon, Georgina, Selfe, Mason (deep basin), Sumner, and Coleridge were described as oligotrophic. Timms (1983) described Lakes Sarah and Grasmere as mesotrophic, and Lakes Lyndon and Hawdon as oligotrophic. Thus, we could presume that lakes such as Lakes Marion, Ida, and Sarah may have been fluctuating around the boundary between oligotrophic and mesotrophic since the 1970s.

LWRP objectives have been established to ensure lake water quality is maintained or improved from their broad state when the plan was developed. The LWRP was first notified in 2012, and since limited data was available in the plan drafting phase, most lakes were assigned ’category’ or ‘trophic band’ TLI objectives (e.g. oligotrophic for small to medium sized lakes) unless they were consistently above that band and there was a reasonable reason for them to be in the band above.

Since then the increasing data record has allowed new limits or objectives for individual lakes to be made under new plan changes, based on observed median concentrations, to better safeguard against a decline in water quality. Lakes for which more specific individual targets have been set based on their respective recent median concentrations (i.e. all the Upper Waikati Lakes monitored) may allow for a degree of past degradation, but also have a lower probability of consistently meeting their objectives all of the time.

For lakes failing planning objectives, is it important to also look at both the frequency, and magnitude of the exceedance of objectives (Table 5-2), as well as changes over time (trends, see section 4.2) to determine the significance or importance of the breaches.

Figure 5-1: Trophic Level Index of 35 high-country lakes in Canterbury sorted on a basis of TLI (average from 2015-2019), red, yellow and green lines indicate oligotrophic, mesotrophic and eutrophic thresholds

Notes for Fig 5-1: Lakes Marion, Mason, Sheppard, Evelyn, Catherine, Henrietta, and McGregor, as well as Kellands Pond have less than 5 years of data. The LWRP set an objective of TLI <3 for most high-country lakes. Lakes Emily, Georgina and the Māori Lakes have a higher objective of 4, whereas Lakes Pukaki, Ōhau, Coleridge, Sumner and Tekapo have a lower objective of 2.

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Canterbury Environment Table 5-2: Summary of frequency and magnitude (severity) of lake TLI score exceeding LWRP objectives Canterbury high Canterbury Lake Exceed No of No of Frequency Frequency 10y Reason for Severity (% Catchment Catchment in 2019? exceeds exceeds last 5y exceedance average cover low cover highly last 5 years 10 years exceedance if producing producing objective failed) grasslands grasslands Katrine No 0 0 normal normal 6% 16 7 Sumner No 0 3 normal frequent chla/TP 11% Taylor No 0 0 normal normal 23 11 - Coleridge No 0 0 normal normal 13% programme monitoring lakes country

Technical Report Technical Tekapo No 0 2 normal normal 16% Ōhau No 0 3 normal frequent TP 18% Benmore_Haldon No 0 0 normal normal Benmore_Dam No 0 0 normal normal Aviemore No 0 3 normal frequent chla/TP 10% MāoriBack No 2 3* frequent normal* TN 11% 3 50 Alexandrina No 2 3 frequent frequent chla 5% 79 4

Pukaki No 2 4 frequent frequent TP 9% Selfe No 3 3 frequent frequent 9% 40

Ida No 3 4 frequent frequent 10% 30 Georgina No 3 5 frequent regular TN 8% 47 Sarah Yes 3 6 frequent regular TN 13% 61 Benmore_Ahuriri Yes 1 2 normal normal 3% Opuha Yes 1 NA normal NA 1%

Lyndon Yes 3 3 frequent frequent TN/chla 5% 34 –

Heron Yes 3 4 frequent frequent chla 9% 21 9 2005 trends, and state Emily Yes 3 6* frequent frequent* TN 9% 82 MāoriFront Yes 3 6* frequent frequent* 16% 33 45 Pearson Yes 5 6 regular regular 18% 26 5 Hawdon Yes 5 7 regular regular TN 12% 48 Camp Yes 5 9 regular regular TN 7% 86 Grasmere Yes 5 10 regular regular TP/chla 10% 33 6 Kellands_shore Yes 5 NA regular NA TN 8% 10 86 Emma Yes 5 10 regular regular 46% 58 -

Clearwater Yes 5 10 regular regular TN 27% 37 2019 Denny Yes 5 NA regular NA TN 79% 38 32

Catchment cover from Kelly et al. (2014) for selected lakes, not determined if blank 169 * Only 8 years of data available

NA: Not enough data available

Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Phytoplankton biomass Between 2015 and 2019, 12 lakes had average chl-a concentrations above the mesotrophic threshold (2 µg/L), and 8 lakes above the eutrophic threshold (5 µg/L) (Figure 5-2). This means that the algal biomass in Lakes Henrietta, Emma, Denny, Emily, Opuha and Georgina and the Māori Lakes is indicative of a highly enriched (i.e. eutrophic) ecosystem. The algal biomass is the most “visible” attribute generally indicating an increasingly “green” colour and reduced water clarity. Over the 5 years from 2015 to 2019 ,half of the lakes (15 of 30 lakes) had phytoplankton biomass in excess of their corresponding LWRP average chlorophyll limit (based on the 5 year mean, not annual means). The lakes with highest chl-a levels were either in the Ashburton Lakes area or the Selwyn-Waihora Zone.

Large intermittent algal blooms occurred in a number of lakes, including some lakes which had low average concentrations (Table 5-5). Although blooms can be related to natural factors such as floods or landslides, they can provide an early warning of nutrient conditions worsening, particularly for lakes which have low nutrient status targets in the LWRP. The Māori Lakes, Lake Georgina, and Lake Denny had blooms that exceeded the 60 µg/L national bottom line for maximum chl-a concentrations, indicating concerning conspicuous or nuisance bloom conditions (Figure 5-5).

Figure 5-2: 5 year mean chl-a concentrations in 35 high-country lakes in Canterbury (2015- 2019), lines indicate the oligotrophic (green), mesotrophic (yellow) and eutrophic (threshold)

Nutrients Between 2013 and 2017, 12 lakes had average TN concentrations indicative of eutrophic conditions (>340 µg/L). 11 had TN concentrations indicative of mesotrophic conditions (>160 µg/L) (Figure 5-3, Figure 5-6). The majority of smaller lakes show elevated nitrogen scores compared to other water quality parameters (chl-a and TP) (Table 5-5). Lakes Denny, Georgina, Emma, Clearwater and Māori Lake East had the highest average TN concentrations. Over the 5 years from 2015 to 2019, about two thirds of the monitored lakes (19 of 30 lakes) had TN concentrations in excess of their corresponding LWRP mean objective TN limit (based on the 5 year mean, not annual means) (Figure 5-3). (Note that the lakes in the Upper Waitaki have plans limits that may differ from trophic level thresholds, i.e. TN may be in the mesotrophic range but still meet their plan limit of being in the NPS-FM (2014) ‘A band’).

In the same period, mean TP concentrations were highest in Lakes Denny, Emily, Emma and Henrietta. Eleven lakes had average TP concentrations above the mesotrophic threshold (9 µg/L), and 3 lakes above the eutrophic threshold (20 µg/L) (Figure 5-4). Over the 5 years from 2015 to 2019, 8 of 30 lakes had TP concentrations in excess of their corresponding LWRP mean TP band limit (based on the 5 year mean, not annual means) (Figure 5-4).

These results indicate that many of the Canterbury high-country lakes have elevated nutrient concentrations (either Nitrogen or Phosphorus) beyond the appropriate trophic bands and plan objectives. This indicates that many lakes are accumulating nutrient loads in excess of those acceptable to achieve in-lake nutrient limits set in the LWRP and that some are exhibiting adverse effects of these nutrient loads. This is one reason why the LWRP identified most of these lakes within “sensitive lakes

170 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

zones” requiring close scrutiny and management of land use and nutrient loads generated from land use surrounding these lakes.

Figure 5-3: Mean total nitrogen concentrations in 35 high-country lakes in Canterbury (2015- 2019), lines indicate the oligotrophic (green), mesotrophic (yellow) and eutrophic (threshold)

Figure 5-4: Mean total phosphorus concentrations in 35 high-country lakes in Canterbury (2015-2019), lines indicate the oligotrophic (green), mesotrophic (yellow) and eutrophic (threshold)

Condition of the aquatic macrophyte community (LakeSPI) Macrophytes are a key functional habitat component of lake ecosystems, in particular in shallow lakes. In addition to providing habitat and food for birds, invertebrates and fish, they also remove nutrients from the water column and sediment, and cap the lake bed preventing resuspension of sediment and associated nutrients off the lake bed during windy periods, and during stratification events. By reducing sediment resuspension and reducing phytoplankton response to nutrient loading they can increase water clarity and improve overall water quality and lake condition.

About half the lakes were in only moderate ecological condition, the other half received high or excellent LakeSPI scores. More sites showed an improving than deteriorating trend, often due to increases in native species condition (Table 5-4). De Winton & Burton (2017) attributed improvement in Lakes Tekapo and Ōhau to increased water clarity possibly due to periods of a more settled climate and decreased glacial flour inputs. Deterioration in Lake Denny is likely linked to significant land use intensification changes (de Winton & Burton, 2017).

Environment Canterbury Technical Report 171 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Sixteen of the 33 surveyed sites meet the LakeSPI (ecological condition) requirements set in the LWRP (Table 5-4). Most lakes that failed to reach a ‘high’ ecological status did so because of the impacts of high growths of exotic species (i.e. high invasive condition score), mostly attributable to the dominance of Elodea canadensis which is widely distributed in the region’s lakes. Lakes presently meeting their LWRP ecological condition objectives are lakes with little or no exotic species presence or dominance. This emphasizes the importance of biosecurity measures for preventing new incursions occurring in lakes presently achieving high ecological condition status.

While benthic algae are not included in LakeSPI surveys, a survey of benthic mats in 35 lakes was conducted in 2010 and a summary of benthic mats/macroalgae can be found in Appendix 4 (from Smith et al., 2011).

Recreational water quality The high-country lakes were not routinely sampled for microbial indicators, but a number of lakes were monitored as part of our regional contact recreational water quality monitoring program during the bathing season. Seven out of 13 sites monitored for E. coli over the summer months were given a Microbiological Assessment Categories (MAC) grade ‘A”, two grade ’B” and four sites were grade ‘C’ (Table 5-3), based on a 5-year average from 2015-2019. Lakes Middleton, Alexandria and Aviemore at Loch Laird and failed to comply with the LWRP objective of a ‘good’ or better Suitability for Recreation Grade (SFRG).

Table 5-3: Summary of recreational water quality grades, 2015-2019 E. coli 95th Provisional Site MAC percentile SFRG Lake Camp at beach 16 A Very Good Lake Clearwater west of huts 57 A Very Good Lake Alexandrina at bottom huts 526 C Fair Lake Ruataniwha at camping ground 122 A Very Good Lake Tekapo Beach 117 A Very Good Lake Aviemore at Te Akatarawa Camp 193 B Good Lake Aviemore at Waitangi 134 B Good Lake Aviemore at Loch Laird 514 C Fair Lake Benmore at Pumpkin Bay 75 A Good Lake Benmore at Sailors Cutting 66 A Good Lake Middleton at north end of lake 347 C Fair Lake Opuha at Ewarts Corner Boatramp 419 C Fair Lake Opuha at Recreation Reserve 120 A Very Good MAC = Microbiological Assessment Categories SFRC = Suitability for Recreation Grade

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Report Technical Canterbury Environment Table 5-4: LakeSPI Regional Overview

LakeSPI index Trend (last two Driver of change (if known) high Canterbury Lake Condition Objective Zone (latest) surveys) Denny 0 Non-Veg High Deteriorating Macrophyte cover lost Ashburton Emily 28 Moderate High Stable Ashburton Grasmere 32 Moderate High Deteriorating Water clarity, macrophyte depth range reduced Selwyn - Waihora Sheppard 34 Moderate High Stable Hurunui - Waiau Emma 35 Moderate High Stable Ashburton -

Māori West 37 Moderate High Stable Ashburton programme monitoring lakes country Georgina 42 Moderate High Improving Increase in native condition Selwyn - Waihora Evelyn 44 Moderate High Deteriorating Increase in invasive condition Selwyn - Waihora Heron 45 Moderate High Stable Ashburton Māori East 47 Moderate High Improving Re-vegetated since 2007 Ashburton Katrine 47 Moderate High Stable Hurunui - Waiau McGregor 48 Moderate High Stable Upper Waitaki Clearwater 49 Moderate High Stable Ashburton Benmore-Ahuriri 49 Moderate High Stable Upper Waitaki Middleton 50 Moderate High Deteriorating Increase in invasive condition Upper Waitaki Alexandrina 51 High High Deteriorating Increase in invasive condition Upper Waitaki

Sarah 51 High High Stable Selwyn - Waihora Pearson 55 High High Deteriorating Water clarity, macrophyte depth range reduced Selwyn - Waihora Taylor 56 High High Stable Hurunui - Waiau Kellands 56 High NA Improving Increasing native & decreasing invasive Upper Waitaki Selfe 57 High High Improving Increase in native condition Selwyn - Waihora Sumner 61 High Excellent Stable Hurunui - Waiau –

Mason 63 High High Stable Hurunui - Waiau 2005 trends, and state Lyndon 64 High High Stable Selwyn - Waihora Aviemore 64 High High Stable Upper Waitaki Camp 65 High High Stable Ashburton Benmore-Haldon 72 High High Improving Increasing native & decreasing invasive Upper Waitaki Ida 83 High High Stable Selwyn - Waihora Ōhau 84 Excellent Excellent Improving Increasing native & decreasing invasive Upper Waitaki Tekapo 88 Excellent Excellent Improving Increased water clarity Upper Waitaki Coleridge 91 Excellent Excellent Stable Selwyn - Waihora Hawdon 96 Excellent High Stable Selwyn - Waihora - NA – Fit for purpose. 2019 Legend for trend in Lake SPI (i.e. percentage change in LakeSPI index) Lakes highlighted in grey do not meet the LWRP objective.

0-5% No change 173 5-10% Improvement possible Deterioration possible

10-15% Improvement probable Deterioration probable >15% Improvement indicated Deterioration indicated

Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

5.1.2 NPS-FM 2014 Attribute State More than half of the monitored high-country lakes were in the ‘A’ band for nutrients and/or chl-a, indicating these water bodies were generally “healthy and resilient” (Figure 5-5, Figure 5-6, Figure 5-8, Figure 5-7, Table 5-5). For algal biomass (chl-a median) 16 lakes were in the ‘A’ category. 12 lakes received an attribute grade of ‘B’, 4 lakes had an attribute grade of ‘C’, and no lakes had an attribute grade of ‘D’ (below the national bottom line) (Table 5-5). Scores for the maximum algal biomass (chl-a max) placed four lakes in the “D’ band, and 5 in the ‘C’ band due to large algal blooms, in particular in the Ashburton Lakes Area.

No lakes were below the national bottom line for TP, three lakes scored ‘C’, 7 ‘B’ and 22 ‘A’. More lakes were attributed a high attribute grade for TN than for any of the other parameters, with 12 ‘A’s, 12 ‘B’s, 8 ‘C’s, and no ‘D’. Thus, in relation to the NPS-FM attribute states, our high country lakes score poorly for nitrogen compared to phosphorus and chlorophyll a. Nitrogen sources include pastoral land use (often transported via groundwater), organic nitrogen associated with decomposing catchment vegetation or wetlands, or in some specific cases waterfowl populations or human settlements.

Figure 5-5: Range and median of chl-a concentrations (µg/L) in 35 high-country lakes in Canterbury (2015-2019), lines indicate the NPS band thresholds (green B, yellow C, red = national bottom line). Purple line is national bottom line for chl-a maximum

174 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 5-5: Attribute states for 5-year average from 2015 to 2019 (National Policy Statement Freshwater, 2014) Chl-a - Chl-a - Zone Lake TN TP MED MAX Hurunui - Waiau Katrine A A B A Hurunui - Waiau Sumner A A A A Hurunui - Waiau Taylor A A A A Hurunui - Waiau Marion B B B B Selwyn - Waihora Sarah B A A A Selwyn - Waihora Grasmere A B B A Selwyn - Waihora Pearson B B C A Selwyn - Waihora Hawdon C A A B Selwyn - Waihora Lyndon B A B B Selwyn - Waihora Georgina C B B D Selwyn - Waihora Ida B A B C Selwyn - Waihora Selfe A A B B Selwyn - Waihora Coleridge A A A A Selwyn - Waihora Evelyn B A A C Selwyn - Waihora Catherine B A A A Selwyn - Waihora Henrietta C B C B Ashburton Emily B C B C Ashburton Māori Front/East C A A D Ashburton Māori Back/West B B B D Ashburton Denny C C B D Ashburton Heron A A B C Ashburton Emma C C C C Ashburton Camp B A A A Ashburton Clearwater C B B A Upper Waitaki Alexandrina B A A A Upper Waitaki Tekapo A A A A Upper Waitaki Pukaki A A A A Upper Waitaki Ōhau A A A A Upper Waitaki Benmore-Haldon A A A A Upper Waitaki Benmore-Ahuriri A A A A Upper Waitaki Benmore-Dam A A A A Upper Waitaki Aviemore A A A A Upper Waitaki Kellands Pond shore B A A B OTOP Opuha C A C B Results in italics are based on less than 5 years of data. MED = seasonal median concentration (Dec-May). Note: Our data does not represent annual medians as suggested by NPS, but seasonal medians.

Environment Canterbury Technical Report 175 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 5-6: Range and median of total nitrogen concentrations (µg/L) in seasonally stratified high-country lakes in Canterbury (2015-2019), lines indicate the NPS band thresholds (green B, yellow C, red = national bottom line)

Figure 5-7: Range and median of total nitrogen concentrations (µg/L) in polymictic high- country lakes in Canterbury (2015-2019), lines indicate the NPS band thresholds (green B, yellow C, red = national bottom line)

176 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 5-8: Range and median of total phosphorus concentrations (µg/L) in 35 high-country lakes in Canterbury (2015-2019), lines indicate the NPS band thresholds (green B, yellow C, red = national bottom line)

5.1.3 Nutrient limitation There is considerable research and debate on the use and interpretation of nutrient concentration ratios (i.e. ratios of N and P) to determine the nutrient limitation status of lakes (e.g. Abell et al., 2010; Bergström et al., 2010). The TN:TP ratio is often used as to indicate nutrient limitation status because it incorporates both dissolved and bound nutrient fractions, and is more stable (Kelly et al., 2014). TN:TP ratios near 7 (Redfield ratio by mass), indicate that supply of N and P are roughly balanced in relation to the demands of plant and algae growth. Departures from these thresholds could suggest that primary productivity in the systems is increasingly limited by either N (< 7) or P (> 7), with single nutrient limitation more likely to occur when the ratios are >14 (P-limited) or < 3.5 (N-limited). Other ratios, such as DIN:TP have been shown to be more accurate (Bergström et al., 2010), but DIN was not sampled for the high- country lakes because it is generally below detectable limits. Abell et al. (2010) reported on N vs. P limitation in New Zealand lakes using thresholds of >15 (P limited) and <7 (N limited). In contrast, the previous report on the state of Canterbury’s high-country lakes thresholds were set to >30 (P limited) and <15 (N limited) (Meredith & Wilks, 2007). Other studies have set the threshold of high probability of N limitation to <9 (Guildford and Hecky, 2000), <14 (Downing and McCauley, 1992) and <19 (Bergström et al., 2010).

Median TN:TP ratios were higher than 15 in most lakes (Figure 5-9), suggesting that nitrogen is less likely to be limiting phytoplankton growth than phosphorus. Only the large, microtrophic glacial-fed lakes, and Lakes Denny and Grasmere frequently had low TN:TP ratios. Low TN:TP ratios in Lakes Denny and Grasmere are due to high TP concentrations, possibly linked to inputs of sediment or application of phosphorus fertilisers in the catchment. High TN:TP ratios in Kellands Pond, Catherine and Hawdon are likely due to high TN concentrations. The large, microtrophic lakes are less enriched in TN than the smaller lakes, reflecting the different dominant pathways of nutrients to the lakes (e.g. groundwater and small rivers vs. big glacial-fed rivers) and the different nutrient composition in inflows (e.g. organic sediment vs. glacial flour). Historical New Zealand studies on lake nutrient limitation prior to widespread agricultural intensification found nitrogen was frequently the nutrient limiting phytoplankton growth (White, 1983).

While there remain several ways in which nutrient status can potentially be inferred by nutrient ratios, based on the data available a large proportion of Canterbury’s high-country lakes currently appear likely to be more P rather than N limited in most months between December and May. Possible exceptions to

Environment Canterbury Technical Report 177 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

this include Lakes Denny, Grasmere, and the large glacial-fed lakes, which show some evidence of co- limitation or even N limitation over some periods. However, our data only includes the months from December to May and many lakes may be co-limited by nitrogen and phosphorus for at least parts of the year. Trophic state and lake condition are driven by the availability of both common nutrients.

It must also be acknowledged that this current finding may not indicate the natural status, but under a condition where nitrogen concentrations/loads have increased due to land use development and leading to an increasing likelihood of nitrogen excess.

Phosphorus, once deposited in the lake sediment, can be released and form an in-lake source under certain circumstances (e.g. low oxygen conditions, sediment resuspension), thus has the potential to become an ongoing source and long-term legacy. Thus, a management approach focused on P alone is precarious as long-term internal P contributions (i.e. proportion of P recycled from the sediment) are difficult to predict. Furthermore, while nitrogen loads can be relatively poor predictors of phytoplankton biomass in many lakes (Kelly et al., 2014), the condition of the aquatic macrophyte community is often more strongly influenced by nitrogen than phosphorus (Kelly et al., 2014, Moss et al., 2013). Thus, for the vast majority of lakes maintenance or improvement of lake trophic state relies on the successful management of both phosphorus and nitrogen inputs.

Figure 5-9: Range and median of TN:TP ratio for 35 high-country lake sites in Canterbury (2013-2017). The blue line indicates the threshold of P limitation (Abell et al., 2010), the green line is the threshold of N limitation (Meredith & Wilks, 2007)

5.1.4 Turbidity Floods resulted in intermittent high turbidity levels in many lakes that are fed by discrete catchment channels. Turbidity was highest in Lakes Pukaki, Henrietta and Denny (Figure 5-10). Causes of high turbidity were likely to be mostly phytoplankton blooms or (re-)suspended sediment in the smaller Lakes Henrietta and Denny, and river delivery of inorganic sediments (including glacial flour) in large glacial- fed lakes such as Pukaki, Tekapo, and Ōhau.

178 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure 5-10: Range and median of turbidity at 35 high-country lakes in Canterbury (2013-2017)

5.1.5 Lake water colour The dominant wavelength of remotely sensed light reflected by lakes (i.e. apparent water colour) was highly variable across Canterbury’s high-country lakes, reflecting the diversity of factors that can affect light reflectance in high country lakes such as suspended sediment, dissolved organic carbon, and phytoplankton (Wetzel, 1983) (Figure 5-11). In most smaller high-country lakes, median dominant wavelength was green-blue, or green for the more productive lakes. Green reflectance can be imparted both by light reflecting off phytoplankton (more productive lakes), but also by absorption of blue wavelengths by dissolved organic matter in less productive lakes (Kirk, 1994). Lakes Pearson, Emily, Evelyn, Emma, Denny and the Front (West) Māori Lake tended to be more yellow/orange in colour, reflecting the higher turbidity measured in these lakes. These lakes had very few or no ‘clear blue water episodes’, possibly indicating a ‘conspicuous change in colour or clarity' compared to their ‘natural state’. Most smaller high-country lakes had a large degree of variability in water colour between 480 nm (blue- green) and 570 nm (yellow-orange), possibly due variation in sediment inputs associated with high rainfall events or snowmelt, as well as seasonal variation in phytoplankton. ‘Higher-altitude Lake Minchin and Lake Letetia tends to have more ‘blue” episodes than most other small high-country lakes, but were ‘sampled’ less frequently.

Large, deep, nutrient-poor, glacial-fed lakes (Lakes Coleridge, Tekapo, Ōhau, Pukaki) were in the blue spectrum, and had little seasonal and year-to-year variability. These lakes have large water volumes, are very low in nutrients and phytoplankton and are subject to high wind-induced mixing. Therefore, their water colour is likely less affected by seasonal changes in phytoplankton concentrations and flood events than smaller and shallower lakes. In addition, water colour in these glacial-fed lakes is partly driven by concentrations of glacial flour, which gives them a ‘milky-blue’ colour.

Coastal lakes (shown for comparison) were mostly yellow/orange in colour and had little variability. Limitation of the Landsat derived water colour data include the variable and unevenly spaced temporal resolution of “samples”.

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Figure 5-11: Range and median of Landsat-derived dominant wavelength in selected Canterbury lakes (2013-2018) Environment Canterbury Technical Report Technical Canterbury Environment Supplied by Moritz Lehmann, University of Waikato

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state and trends, 2005 trends, and state - 2019

Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

5.1.6 Presence of the exotic organism Lindavia, and the effect of “lake snow” Lake snow (also referred to as ‘lake snot’ in New Zealand) are macroscopic (i.e. visible) aggregates suspended in the water column, which usually include ‘sticky’ extracellular polysaccharides, phytoplankton, bacteria and detritus. Lake snow is similar to the more commonly observed marine snow which plays an important role in organic matter transformation in the ocean. Lake snow was first observed in New Zealand in Lake Wanaka in the early 2000s. Lindavia intermedia is the centric diatom assumed to produce the mucus that causes lake snow formation, and it is usually present in large numbers in lake snow in New Zealand lakes (Novis et al., 2017a). Recent research indicates that Lindavia intermedia is an invasive organism (Novis et al., 2017b).

Although lake snow has been observed in a number of Canterbury’s high-country lakes, until recently there were no reports of nuisance events such as those observed in Central Otago lakes such as Lake Wanaka and Wakatipu. However, in 2017, lake snow was reported in Lakes Coleridge and Benmore in a pumped water take and observed collecting on fishing lines. As part of a biosecurity monitoring effort by Environment Canterbury in 2017-19, the presence of Lindavia intermedia was confirmed in several small and large lakes in the Upper Waitaki and most of the Waimakariri Lakes (Table 5-6). Lindavia was not present in Lakes Ōhau and Middleton, nor in Lake Hawdon. However, since the method used to confirm the presence of Lindavia is not designed to capture single, suspended cells of phytoplankton, Lindavia intermedia could still be present in low cell densities in lakes even though it was not observed in the surveys.

Not all lakes with Lindavia intermedia have visible or nuisance growths of lake snow. It is uncertain why some lakes that contain this species do not have more aggregated cell growth forms that form “snow” whereas other do. The actual formation of lake snow may be strongly linked to shear stress from internal waves, as turbulence is needed to ’clump together’ the algal secretions of transparent extracellular polysaccharides (TEPs) into snow (e.g. Grossart & Simon, 1993; Alldredge et al., 1998). This may help explain why in New Zealand, lake snow has been observed mostly in large, deep lakes in the South Island that have deep thermoclines and can experience internal waves. In addition, a threshold cell concentration appears to be needed for aggregation to occur, and marine or lake snow is often observed during large diatom blooms (Passow et al., 2012). Not enough is currently known about the causes of lake snow production by this diatom to make predictions regarding the likely or possible impact of lake snow on our high-country lakes.

More research is needed to predict which lakes may be vulnerable to lake snow formation, and to improve our understanding of the triggers of lake snow formation and the impact of lake snow on lake ecosystem function in New Zealand.

Environment Canterbury Technical Report 181 Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 5-6: Lake snow presence in Canterbury high-country lakes

Order of Lindavia sampling Site Date sampled observed? Relative abundance Environment Canterbury lake surveys 2017-2020 1 Lake Camp 21-Feb-17 no 2 Lake Roundabout 21-Feb-17 no 3 Māori Lake East 21-Feb-17 no 4 Lake Clearwater 22-Feb-17 no 5 Lake Heron 22-Feb-17 Yes occasional 6 Lake Denny 23-Feb-17 no 7 Lake Emma 23-Feb-17 no 8 Lake Emily 23-Feb-17 no 9 Māori Lake West 23-Feb-17 no 10 Spider Lakes 24-Feb-17 no 11 Lake Donne 24-Feb-17 no 12 Lake Ōhau 26-Apr-17 no 13 Lake Tekapo 27-Apr-17 Yes abundant 14 Lake McGregor 27-Apr-17 Yes common 15 Kellands Pond 28-Apr-17 Yes rare 16 Lake Middleton 29-Apr-17 no 17 Lake Alexandrina 29-Apr-17 Yes common 18 Lake Ruataniwha 14-Jun-17 Yes abundant 19 Lake Aviemore not sampled n/a 20 Lake Waitaki 15-Jun-17 Yes abundant 21 Lake Georgina 14-May-18 No 22 Lake Evelyn 13-May-18 No 23 Lake Ida 14-May-18 Yes 24 Lake Lyndon 15-May-18 Yes 25 Lake Sarah 15-May-18 Yes 26 Lake Grasmere 16-May-18 Yes abundant 27 Lake Pearson 16-May-18 Yes rare 28 Lake Hawdon 17-May-18 No 29 Lake Georgina February 2020 No 30 Lake Evelyn February 2020 No 31 Lake Selfe February 2020 Yes

Other records Lake Benmore Apr-17 Yes abundant Lake Opuha 11-Feb-13 Yes occasional Lake Coleridge 2012 Yes Abundant Lake Sumner* 20-Feb-20 Yes * in phytoplankton sample Loch Katrine* 20-Feb-20 Yes * in phytoplankton sample Lake Taylor* 20-Feb-20 Yes * in phytoplankton sample

5.1.7 Cultural health Overall Cultural Health Index (CHI) or takiwā assessments were ‘good’ or ‘very good’ for 4 out of 11 lakes, ‘moderate’ for 5 lakes and ‘poor’ for two lakes (Table 5-7).

182 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Table 5-7: Summary of lake cultural health assessments Lake Takiwā Overall CHI Stream health Mahinga Kai Site status Alexandrina Mod Mod Mod A-1 Aviemore Mod Mod Poor A-1 Camp Poor Clearwater Mod Poor Poor Very poor B-1 Emma Good Heron Good Poor Very good A-1 Māori East Mod Māori West Good McGregor Mod Mod Mod A-1 Sumner Very good Sources: Tipa and Associates & Williams, 2015, Te Rūnanga o Arowhenua et al., 2010, Statistics New Zealand (https://statisticsnz.shinyapps.io/cultural_health/), Lenihan 2011. Note: CHI assessments refer to outlet streams.

5.2 Regional trends in water quality

5.2.1 Trends in phytoplankton biomass, turbidity and nutrients Water quality declined as significant trends in over a third of the high-country lakes that we monitor: Between 2007 and 2019, nine lakes had significant increasing trends of phytoplankton biomass (chl-a) (Figure 5-12), 6 lakes increasing total nitrogen (TN) (Figure 5-13) and 4 lakes were increasing in total phosphorus (TP) (Figure 5-14) or a combination thereof (Table 5-8). On the contrary, in four large, deep lakes total nitrogen concentrations decreased significantly between 2007 and 2019 (Figure 5-13). Turbidity increased significantly in 16 of the monitored lakes, but decreased significantly in the large glacial lakes Pukaki, Benmore and Aviemore (Figure 5-15).

Most lakes with increasing trends of chl-a or nutrients are part of the Waimakariri or Rakaia lakes clusters, or located in the Ashburton lakes basin. Lakes Grasmere, Sarah, Pearson, Hawdon, Lyndon, Ida as well as Heron, Māori, and Camp all had increasing trends in nutrients and/or chl-a between 2007 and 2019 (Table 5-8). In the Upper Waitaki Kellands Pond and Lake Benmore had trends of increasing nitrogen or chl-a.

Since failure to reject the null hypothesis (i.e. lack of significant trend) does not always mean that water quality is stable or has ‘been maintained’ (Larnard et al., 2015), we also plotted the relative Sen slope (RSS) with 90% confidence intervals to visualise the general direction and magnitude of trends in water quality (Figures 4-8 to 4-10).

Phytoplankton biomass (chl-a) Increasing trends for phytoplankton biomass were significant in nearly a third of monitored lakes. Furthermore, while not significant, trends analysis indicates a probable increase in chl-a in more than half the monitored lakes (Figure 5-12). Most lakes with significant chl-a trends also had increasing trends in at least one nutrient, indicating increased nutrient load/availability as a highly likely driver of increased algal growth.

Several lakes in the monitoring dataset are relatively unimpacted by anthropogenic factors and thus indicative of changes in natural conditions for the region. Among these, Lake Lyndon showed significant increases in chl-a. Changes in climate patterns are likely to have a strong influence on phytoplankton biomass trends as it impacts upon water and sediment influxes, as well as mixing and thermal stratification patterns. In some lakes phytoplankton biomass closely tracks flood-delivered TP (e.g. Lake Ōhau, Coleridge). Patterns in summer rainfall appear to be cyclic rather than linear (App 3.2), and match chl-a patterns in a number of lakes. However, large increases in chl-a, in particular in Lake Heron, Lake

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Camp, Lake Pearson, Lake Grasmere and Lake Benmore, since 2014 do not track the summer rainfall. While changes in light climate and stratification patterns may be a factor in these increasing chl-a trends, these lakes also had increasing trends in at least one nutrient, so it is likely that increased loads/availability of bioavailable nutrients are a contributing factor.

Turbidity in Lake Benmore decreased over the monitoring period, possibility resulting in higher light availability for phytoplankton (if mixing patterns over summer remained unchanged). If phytoplankton were light limited in Lake Benmore in summer, the increase in chl-a from 2009-2019 could be linked to less turbid (clearer) water and higher light availability. At the same time intensification in land use upstream of Lake Benmore occurred, and both DIN and TN increased significant in the Ahuriri River 2013-2019 (Figure 4-140). It is possible we are seeing the combined effects of changes in flow patterns resulting in increased clarity (due to climatic change and changes in water use) and increased nutrient availability due to land use intensification.

Figure 5-12: Trends chl-a concentrations in 25 high-country lakes in Canterbury (percentage change)

A large number of our high-country lakes are likely to have experienced changes in phytoplankton and zooplankton community composition with the introduction of the invasive centric diatom Lindavia intermedia (Table 5-6), and the invasive zooplankter Daphnia pulex. Their combined impact on phytoplankton communities and biomass in Canterbury high-country lakes remains largely unknown, and very limited data on historic plankton composition is available to determine the impacts. Phytoplankton community structure may also have shifted over the last two decades in response to changes in climate and nutrient status with unknown consequences for overall biomass in the top 10 m of water column: Overseas research has shown an increase in the dominance of centric diatoms over larger diatoms with changes in depth and strength of stratification due to increasing lake temperatures (Rühland et al., 2015; Winder & Sommer, 2012). No long-term temperature records are available for our high-country lakes.

Nutrients Total nitrogen increased significantly in 6 small to medium sized lakes, and decreased significantly in four large deep lakes (Figure 5-13). Increases were highest in Kellands Pond and the Front Māori lakes, coinciding with intensification of pastoral agriculture.

Total phosphorus was the most ‘stable’ parameter we analysed, with 4 lakes showing increasing trends (Figure 5-14). Lakes Grasmere, Sarah, Ida, Lyndon and Pearson had increasing trends 2007-2019. For Lakes Coleridge, Tekapo, Ōhau, Benmore (near dam), and Aviemore more than 40% of TP data points

184 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

were below the detection limit and detection limits changed several times for the monitoring period. TP trends were not determined for any of the microtrophic lakes.

Figure 5-13: Trends TN concentrations in 25 high-country lakes in Canterbury (percentage change)

Figure 5-14: Trends in TP concentrations in 25 high-country lakes in Canterbury (percentage change)

Turbidity Nearly all of the lakes in the Selwyn-Waihora Zone (Waimakariri and Rakaia lakes) have undergone a significant increase in turbidity (reduction in clarity) between 2007 and 2019 (Figure 5-15), possibly linked to a significant increase in total annual rainfall (as measured at Arthurs Pass (2007-2016, probability 98%, p = 0.02)) or potential increased sediment input after the Canterbury earthquakes in 2010/2011 (Christchurch) and 2016 (Kaikoura). Turbidity is often linked to external inputs of sediment or glacial flour in high-country lakes, but may also be partly autochonous (i.e. due to high algal

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biomass/chl-a). Lakes Lyndon, Pearson and Ida all had increasing trends of both chlorophyll a and turbidity, along with at least one increasing nutrient, suggesting that increases in algal biomass– at least in part- be contributing to increased turbidity. Lake Pearson had undergone a step change (increase) in all measured parameters and showed little sign of recovery by May 2019.

Figure 5-15: Trends in turbidity in 25 high-country lakes in Canterbury (percentage change)

Trends in turbidity in the large glacial fed-lakes (Lakes Ōhau, Tekapo, Pukaki, Summer, Coleridge) are mostly explained by recent climate patterns (likely by changes in glacial flour input or magnitude and timing of floods), but interpretation is complicated by the influence of hydropower (water storage and release) management. Turbidity increased in Lakes Coleridge and Ōhau, but decreased in Lakes Pukaki, Benmore and Aviemore. Data from these lakes illustrates the impact of high rainfall and river flows in December 2009, January 2010 and January 2013 on TP and turbidity. Increases in turbidity in Lake Coleridge may also be linked to increases in turbid flows diverted to the lake as a result of changed management of the diversions.

Light attenuation in these large microtrophic lakes was shown to be linked to turbidity (as a proxy for concentration of glacial flour), rather than algal biomass (Rose et al., 2014), suggesting glacial flour as the main driver of visual clarity. According to the National Institute for Water and Atmospheric Research (NIWA) twelve of New Zealand’s large glaciers are retreating, including the Classen, Hooker, Godley, Grey, Maud, Mueller, Murchison, and Tasman Glaciers, which all eventually feed into Lake Pukaki or Lake Tekapo and the lakes downstream. While the long-term effects of glacial retreat include ultimately less inorganic particle loading to downstream receiving environments, and increased settlement in increasingly sized small glacial terminal lakes, short-term effects can be an increase in the turbidity of the large lakes connected to glaciers due to increased melting (Sommaruga & Kandolf, 2014). However, glacial flour also enters the lakes from overland runoff from catchments, so relatively low summer rainfall could have resulted in reduced input of glacial flour over the summer months. In fact, the decrease in turbidity seems to coincide with a decreasing trend in summer rainfall (December – April) in Tekapo from 2011 to 2016 (Data from NIWA). Despite decreasing summer rainfall between 2007-2017, the average lake level in Lake Pukaki increased between 2007 and 2017 (Data from NIWA/Meridian Energy, and there were no decreasing trends in river flow in Jollie River flowing into Lake Tekapo or the Ahuriri River. Management of lake levels for power generation further modulates changes in inflows and glacial flour loading, and thus turbidity and inorganic nutrients associated with suspended sediment.

Differences in trends in turbidity between lakes such as Tekapo (stable or increasing) and Pukaki (decreasing), which are both glacial-fed, may reflect the proportion of snowmelt, runoff and glacial melt

186 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

feeding into the lakes. The catchment of Lake Tekapo has a high proportion of non-glaciated area, and larger glacial terminal lakes (e.g. Hooker/Tasman Lake) upstream possibly acting as large sedimentation basins that retain some of the glacial flour.

Turbidity also decreased in Lake Benmore. The lower turbidity in Ahuriri Arm was possibly due to a lack of large flood flows in the Ahuriri River since 2014.

Table 5-8: Trends in water quality 2007-2019 (based on deseaonalised trend analysis)

Zone Lake TP TN Chl-a Turbidity Hurunui - Waiau Katrine decreasing increasing increasing Hurunui - Waiau Sumner ND decreasing decreasing increasing Hurunui - Waiau Taylor increasing increasing increasing Selwyn - Waihora Sarah increasing increasing increasing Selwyn - Waihora Grasmere increasing increasing increasing increasing Selwyn - Waihora Pearson increasing increasing increasing increasing Selwyn - Waihora Hawdon increasing increasing increasing Selwyn - Waihora Lyndon increasing increasing increasing increasing Selwyn - Waihora Georgina increasing increasing increasing increasing Selwyn - Waihora Ida increasing increasing increasing increasing Selwyn - Waihora Selfe increasing increasing increasing Selwyn - Waihora Coleridge ND decreasing increasing Ashburton Emily increasing increasing increasing decreasing Ashburton Māori Front decreasing increasing decreasing increasing Ashburton Māori Back decreasing increasing increasing Ashburton Heron increasing increasing increasing Ashburton Emma increasing increasing increasing decreasing Ashburton Camp increasing increasing increasing Ashburton Clearwater increasing decreasing decreasing Upper Waitaki Alexandrina decreasing increasing increasing Upper Waitaki Tekapo ND decreasing decreasing increasing Upper Waitaki Pukaki ND decreasing decreasing Upper Waitaki Ōhau ND decreasing increasing Upper Waitaki Benmore-Haldon ND decreasing increasing decreasing Upper Waitaki Benmore-Ahuriri ND decreasing increasing decreasing Upper Waitaki Benmore-Dam ND decreasing increasing decreasing Upper Waitaki Aviemore ND decreasing increasing decreasing Upper Waitaki Kellands increasing increasing Blue/orange indicates significant decreasing/increasing trends. Lake in red fail to meet LWRP objective in 2019, lakes in green meet LWRP in 2019. Note: Trends in TP were not determined for the large microtrophic lakes as too many datapoints were below the limit of detection and step changes in in limit of detection occurred over the sampling period.

5.2.2 Patterns in Trophic Level Index Since the Trophic Level index is an annual summary index we limit this assessment to patterns not trends. Patterns in TLI generally followed changes in its individual components and TLI scores can be quite variable for a lake from year to year. This is especially the case in shallower lakes. But while year- to-year variability in local climate can be an important driver of year-to-year variability in TLI, the ‘5 year

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average’ TLI that lakes fluctuate around is largely determined by physical lake characteristics, catchment type and land use. As trends in TLI components show (previous section) this ‘average’ TLI is increasing in a number of lakes and TLI objective exceedances are becoming more common in several lakes when looking at 2015-2019 vs 2009-2014 (e.g. Lake Pearson, Hawdon, Lyndon, Georgina, Ida, Selfe, Heron, Emma, Camp, Alexandrina) (Table 5-2). Some lakes (e.g. Lakes Heron, Camp Pearson, Lyndon, Georgina, Ida, Selfe) seem to follow a pattern of increasing TLI with some degree of variability, but concerning consistency. Many of our small to medium sized high-country lakes have significant parts of their catchment in pastoral agriculture and intensification has continued in a number of lake catchments (such as Lake Heron, Kellands Pond, Māori lakes, and a number of the ‘Ryton lakes’ near Lake Coleridge) much like continuing intensification pressure in farming throughout much of Canterbury and New Zealand.

TLI scores in our-high country lakes often display a large degree of year-to-year variability, which is often driven by variability in local climate. As stated above, TLI components and thus the TLI scores are increasing in a number of lakes. The following paragraphs discuss observed TLI pattern and year-to- year variability and their link to local climate, which is unlikely to be the underlying cause of long-term increases in TLI scores for those lakes affected by changes in anthropogenic nutrient loading (i.e. those in catchments affected by land use intensification). Our data-set is not extensive enough to assess impact of climate change on long-term water quality and this is not an assessment of climate change impacts on our high-country lakes.

In the period between 2008 and 2012 many lakes had higher than usual levels of chl-a and nutrients. As a result, a number of lakes across the region had increasing TLIs between 2007 and 2010/2011, and recorded comparatively high TLIs in the period 2009-2012 (e.g. Lakes Katrine, Taylor, Sumner, Heron, Clearwater, Camp, Grasmere, Ida, Pukaki, Benmore - Haldon). Summer rainfall generally increased between 2007 and 2011 (Appendix 3.2), possibly resulting in an increase in nutrient load inputs.

After 2011 TLI results often stabilised or decreased (see figures in Section 4 for individual lakes). This reduction frequently ended with TLI scores higher than those observed prior to 2007. In a number of lakes TLI increased again between 2014 and 2019, in some cases coinciding with increases in summer rainfall between 2014 and 2018 in the Northern part of the region (Appendix 3.2).

Fluctuations of TLI in large, deep oligotrophic lakes (e.g. Sumner, Tekapo, Pukaki, Ōhau, Benmore until 2015, Aviemore) were largely driven by TP which is in turn probably linked to sediment input with floods. For instance, low flows in summer 2013/2014 in the Ahuriri River were coincided with atypically low TLI scores in Lakes Benmore, Ōhau, Aviemore and Pukaki (Clarke, 2015).

Finally, when discussing patterns in TLI since 2005, it is important to consider two limitations of our data: a) We observed very low TLIs in 2005 and 2006 in a large number of lakes. This could partially be due to a change in sampling technique, as samples were initially taken further up in the water column, near the surface. From 2007/2008, samples were depth-integrated from the top 10 m. Especially in clear lakes, phytoplankton biomass (and associated nutrients) can be lower near the surface than deeper in the mixed layer (e.g. Noges et al., 2010). For this reason, our trend analysis was restricted to 2007-2019 data. b) Some lakes had a large seasonal variability of algal biomass. Therefore, average yearly chl-a concentrations were sensitive to whether 4 or 5 samples were collected in a year and to which months they were collected in. Because of inter-annual differences in sampling frequency (4 or 5 months) and sampling months (December or May were not included in some years) year-to- year variations and patterns in seasonal Trophic Level Index (TLI) need to be interpreted with caution.

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5.3 Summary of high-country lakes state and trends Water quality in our large, deep high-country lakes is generally very good. However, more than two thirds of our small to medium sized lakes have nutrient concentrations or algal biomass that are above the objectives set in the Canterbury Land and Water Regional Plan. Less than a quarter of lakes monitored (7 lakes) met LWRP objectives both for Trophic Level Index (TLI) and lake macrophyte ecological condition (as measured by LakeSPI). Just under half the lakes met TLI objectives (based on 5-year average between 2015-2019, Table 5-1). Half of the surveyed lakes were in high or excellent lake ecological condition (according to the LakeSPI classification), but the other half were only in moderate condition and did not meet the objectives set in the LWRP. Slightly more lakes are improving in ecological condition than deteriorating (Table 5-4).

Based on the frequency and magnitude of departure from TLI objectives we can identify lakes that are (under current management) likely to continue to fail to reach their LWRP objectives in the future (Table 5-2): Lakes Denny, Lake Emma, Lake Pearson, Lake Grasmere, Kellands Pond and Lake Clearwater. Kellands Pond and Lake Denny in particular reflect the intensification of land use in the catchment. Lake Pearson has experienced a step-change in turbidity, nutrient and phytoplankton biomass in 2013. The Front Māori Lake, Lake Sarah, Lake Hawdon and Lake Georgina are also identified as lakes of concern, but may have started to improve in water quality recently.

TLI is increasing in a number of lakes and TLI objective exceedances are becoming more common when looking at 2015-2019 vs. 2009-2014 (e.g. Lake Pearson, Hawdon, Lyndon, Georgina, Ida, Selfe, Heron, Emma, Camp). Between 2007 and 2019, nine lakes had significant increasing trends of phytoplankton biomass (chl-a), 6 lakes increasing total nitrogen (TN) and 4 lakes were increasing in total phosphorus (TP) or a combination thereof (Table 5-8). In addition, more than half the monitored lakes have experienced significant increases in turbidity since 2007.

Many of the lakes that are failing to meet their LWRP TLI objectives (Table 5-2) also had significant or probable increasing trends of at least on nutrient and/or chla (Table 5-8). These lakes include Kellands Pond, many lakes in the upper Waimakariri catchment (Sarah, Grasmere, Pearson, Hawdon) and Lake Lyndon, as well as most lakes in the Ashburton basin (Front Māori Lake, Lake Heron, Lake Camp, Lake Clearwater). Should current trends continue, even lakes of this group that are currently meeting their objectives are unlikely to reach their LWRP objectives in the long-term. Most of these lakes have significant parts of their catchment in pastoral agriculture and intensification has occurred in a number of lake catchments (such as Lake Heron, Kellands Pond, the Māori lakes, and of number of the ‘Ryton lakes’ near Lake Coleridge).

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6 Conclusions

While our large, deep high-country lakes in Canterbury remain in good trophic (nutrient) condition, many small to medium sized lakes are degrading in condition and/or are failing to meet plan nutrient and trophic state targets: Of the small to medium sized lakes, more than two thirds have nutrient concentrations or algal biomass that are now above the objectives set in the Canterbury Land and Water Regional Plan.

Trophic water quality of about a third of the smaller lakes is likely to be deteriorating. While in about half of the surveyed lakes the community of aquatic plants (macrophytes) was in high or excellent condition, six lakes showed a deterioration in aquatic plant ecological condition.

It is of particular concern that several of the lakes currently failing to meet their LWRP objectives also have increasing trends in nutrient and/or phytoplankton biomass and so are even more unlikely to meet their objectives in the future.

A large number of the lakes identified as not meeting their LWRP objectives and limits and/or having increasing trends in chl-a or nutrients are located in the Ashburton Lakes Area or the Selwyn-Waihora Water Management Zone. We have identified a number of lakes of concern (including Lakes Pearson, Grasmere, Emma, Denny, Clearwater and Kellands Pond) and ‘at risk’ of further degradation (Lakes Heron, Sarah, Georgina, Camp and the Front Māori Lake). Most of these lakes have significant parts of their catchment in pastoral agriculture and intensification has occurred in several lake catchments.

Many of the high-country lakes are likely to be phosphorus limited or co-limited by nitrogen and phosphorus at times. While nitrogen loads can be relatively poor predictors of phytoplankton biomass in many lakes (Kelly et al., 2014), macrophyte depth and species richness (indicating the condition of the aquatic macrophyte community) is often more strongly influenced by nitrogen than phosphorus (Kelly et al., 2014, Moss et al., 2013). Therefore, loads of both phosphorus and nitrogen need to be limited and managed to maintain or improve lake trophic state, macrophyte communities and ecosystem health.

The LWRP identifies most of these lakes as in mapped “Sensitive Lake Zones” where further controls on farming are required. The failure of many of these lakes to meet plan objectives and continuing to degrade indicates the current “sensitive lake zone” controls and the current limits set for catchment nutrients contributions in the LWRP do not allow these lakes to meet their LWRP TLI objectives or their in-lake limits for nutrients or algal biomass in the long-term.

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7 Recommendations

While many large high-country lakes are in good condition, better management is required of the small and medium sized lakes that are failing to meet the objectives set in our Land and Water Regional Plan for both trophic level (TLI) and condition of the aquatic macrophyte community.

The data presented in this report highlight the importance of sensitive lake catchments (Lake Zones) and their need to be managed in a way that minimises nutrient (both phosphorus and nitrogen) and sediment inputs to our high-country lakes. It also highlights that for a number of lakes current provisions around these “sensitive lake zones” (e.g. setting nutrient load limits to a baseline of 2009-2013 levels) appears not to safeguard long-term ecosystem health or “halt” degradation of water quality. Reductions in nutrient loading should be considered especially for ‘at risk” lakes where either water quality objectives are frequently exceeded by a large margin, or where water quality is likely to be deteriorating. Catchment nutrient limit setting should also consider the difference in impact of immediately bioavailable dissolved nutrients vs. less bioavailable particulate loads, especially for lakes with short water residence times and where current catchment limits of total nutrient do not appear to archive LWRP outcomes.

Additional management strategies could include reserving Lake Zones for land uses with minimal or low sediment and nutrient losses, more rigorous stock exclusion policies, as well as maintaining or restoring fringing wetlands around lakes and inflowing streams. In particular lakes of concern (including Lakes Pearson, Grasmere, Emma, Denny, Clearwater and Kellands Pond) or ‘at risk’ (Lakes Sarah, Lake Heron, Georgina and the Front Māori Lake), are likely to benefit from these measures. In addition, it would be beneficial if a lake failing to meet a LWRP outcome would trigger an ‘action plan’, i.e. a process to identify the source and magnitude of nutrient inputs, define restoration targets and devise a strategy for improving lake water quality.

This report also highlights several potential shortcomings of our current monitoring programme. A full review of the high-country lakes sampling programme is currently underway and will be reported in a separate document. The review will also address the representativeness of the lakes in the sampling programme, and whether more reference sites need to be included.

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phosphorus cycle. Geology, 27(2), 171. Gibbs, M., & Measures, R. J. (2017). Lake Opuha: In lake processes and aeration - a review. NIWA Client Report 2017024HN. Prepared for Opuha Water Limited Grossart, H.-P., & Simon, M. (1993). Limnetic macroscopic organic aggregates (lake snow): Occurrence, characteristics, and microbial dynamics in Lake Constance. Limnology and Oceanography, 38(3), 532–546. https://doi.org/10.4319/lo.1993.38.3.0532 Guildford, S. J., & Hecky, R. E. (2000). Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship? Limnology and Oceanography, 45(6), 1213–1223. https://doi.org/10.4319/lo.2000.45.6.1213 Hawes, I., & Schwarz, A.-M. (1997). Monitoring the effects of sediment inputs on the aquatic macrophytes of Lake Coleridge. NIWA Client report No: CHC97/24. Hayes, J. W. (1980). An Ecological Survey of Lake Alexandrina. Department of Zoology, University of Canterbury, Christchurch. Hayward, S., Meredith, A., & Stevenson, M. (2009). Review of proposed NRRP water quality objectives and standards for rivers and lakes in the Canterbury region. Environment Canterbury Report R09/16. Jolly, V., & Brown, J. (1975). New Zealand Lakes. Auckland University Press, New Zealand. Kelly, D., Robertson, H., & Allen, C. (2014). Nutrient loading to Canterbury high-country lakes for sustaining ecological values. Cawthron Report No. 2557. Prepared for Department of Conservation and Environment Canterbury. Kpodonu, A. T. N. K., Hamilton, D. P., Lusk, C. H., Hartland, A., Laughlin, D. C., Verburg, P., Theodore, A., Hamilton, D. P., & Lusk, C. H. (2019). Long-term changes in the water quality of a deep temperate oligotrophic lake in response to catchment disturbance: evidence from sediment cores. New Zealand Journal of Marine and Freshwater Research, 53(4): 571–587. https://doi.org/10.1080/00288330.2019.1577279 Larned, S., Unwin, M., Mcmillan, H., Snelder, T., McBride, G., & Verburg, P. (2015). Analysis of water quality in New Zealand lakes and rivers. National Institute of Water and Atmosphere, Report, CHC2015-03. Lehmann, M.K.; Nguyen, U.; Allan, M.; van der Woerd, H.J. (2018) Colour Classification of 1486 Lakes across a Wide Range of Optical Water Types. Remote Sensing, 10, 1273. Lenihan, T. M. (2011). Cultural Monitoring of Hurunui River, Lakes & Tributaries. Presentation to Hurunui‐Waiau Zone Committee, Http://files.ecan.govt.nz/public/cw-Hw-Zip- reports/Lenihan%202011%20Cultural%20Monitoring%20of%20HURUNUI%20RIVER_Presentati on%20to%20HZC_12May2011.pdf. Meredith, A.S., (2004). Monitoring of the Water Quality of Canterbury High Country Lakes. Environment Canterbury Technical Report: U04/34. Meredith, A., & Wilks, T. (2006). Canterbury high country lakes water quality monitoring programme: Results of 2006 monitoring and assessment of nutrient indices. Environment Canterbury Report No. U06/34. Meredith, A., & Wilks, T. (2007). Canterbury high country lakes water quality monitoring programme: results of the third year monitoring 2007. Environment Canterbury Report U07/50. Ministry for the Environment. (2003). Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas. Publication Reference Number: ME 474. Ministry for the Environment. (2014). National Policy Statement for Freshwater Management. Issued 4 July 2014. Moss, B., Jeppesen, E., Søndergaard, M. et al. Hydrobiologia (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: resolution of a current controversy? Hydrobiologia, 710(1): 3-21. http://dx.doi.org/10.1007/s10750-012-1033-0 Nõges, P., Poikane, S., Kõiv, T., & Nõges, T. (2010). Effect of chlorophyll sampling design on water quality assessment in thermally stratified lakes. Hydrobiologia, 649(1), 157–170. https://doi.org/10.1007/s10750-010-0237-4 Norton, N., & Snelder, T. (2003). Options for numeric water quality objectives and standards for rivers and lakes of Canterbury. NIWA Client Report CHC2003-026. Prepared for Environment Canterbury

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Norton, N., Spigel, B., Sutherland, D., Trolle, D., Plew, D., & NIWA. (2009). Benmore Water Quality: a modelling method to assist with assessments of nutrient loadings. Environment Canterbury Report Report No: R09/70. NIWA Client Report: CHC2009-091 prepared for Environment Canterbury. Novis, P., Schallenberg, M., Saulnier-Talbot, É., Kilroy, C., & Reid, M. (2017a). The diatom Lindvia intermedia identified as the producer of nuisance pelagic mucilage in lakes. New Zealand Journal of Botany, 8643(September), 1–17. https://doi.org/10.1080/0028825X.2017.1377263 Novis, P., Mitchell, C., & Podolyan, A. (2017b). Lindavia intermedia, the causative organism of New Zealand lake snow: relationships between New Zealand, North American and European populations according to molecular and morphological data. Landcare Research. Prepared for the Otago Regional Council. Novoa, S., Wernand, M. R., & van der Woerd, H. J. (2013). The Forel-Ule scale revisited spectrally: preparation protocol, transmission measurements and chromaticity. Journal of the European Optical Society-Rapid Publications, 8, 13057, http://dx.doi.org/10.2971/jeos.2013.13057. Passow, U., Ziervogel, K., Asper, V., & Diercks, A. (2012). Marine snow formation in the aftermath of the Deepwater Horizon oil spill in the Gulf of Mexico. Environmental Research Letters, 7(3). https://doi.org/10.1088/1748-9326/7/3/035301 Pickrill, R. A., & Irwin, J. (1986). Circulation and sedimentation in Lake Benmore, New Zealand. New Zealand Journal of Geology and Geophysics, 29(1), 83–97. https://doi.org/10.1080/00288306.1986.10427525 Rose, K. C., Hamilton, D. P., Williamson, C. E., Mcbride, C. G., Fischer, J. M., Olson, M. H., … Cabrol, N. (2014). Light attenuation characteristics of glacially-fed lakes. Journal of Geophysical Research: Biogeosciences, 119, 1446–1457. https://doi.org/10.1002/2014JG002674. Ruehland, K. M., Paterson, A. M., & Smol, J. P. (2015). Lake diatom responses to warming: reviewing the evidence. Journal of Paleolimnology, 54, 1–35. https://doi.org/10.1007/s10933-015-9837-3 Sanoamuang, La-orsri (1992). The ecology of mountain lake rotifers in Canterbury, with particular reference to Lake Grasmere and the genus Filinia Bory de St. Vincent. PhD thesis, University of Canterbury. https://ir.canterbury.ac.nz/handle/10092/5726 Schallenberg, M., Friedrich, U., & Burns, C. W. (2001). Postulated responses of phytoplankton and bacteria to predicted increases of inorganic suspended sediments in oligotrophic lakes. New Zealand Journal of Marine and Freshwater Research, 35(4), 763–779. https://doi.org/10.1080/00288330.2001.9517041 Schallenberg, M., & Sorell, B. (2009). Regime shifts between clear and turbid water in New Zealand lakes: Environmental correlates and implications for management and restoration. New Zealand Journal of Marine and Freshwater Research, 43(3), 701–712. https://doi.org/10.1080/00288330909510035 Schwarz, A.-M., & Hicks, M. (1995). Secchi Depth readings in Lake Coleridge and their relationship with turbid inflows. NIWA Consultancy report No: 27. Prepared for Lake Coleridge Working Party. Smith, F., Kelly, D.,Wilks, T., Broady, P. & S. Gaw (2011): Distribution of Scytonema (cyanobacteria) and associated saxitoxins in recreational lakes in Canterbury. July 2011. Environment Canterbury Report No. R11/36. Sommaruga, R., & Kandolf, G. (2014). Negative consequences of glacial turbidity for the survival of freshwater planktonic heterotrophic flagellates. Scientific Reports, 4, 4113. https://doi.org/10.1038/srep04113 Spencer, M. (1977). Trophic status of twenty-one New Zealand high country lakes. New Zealand Journal of Marine and Freshwater Research, 12(2), 139–151. Spigel, B., Plew, D., Hamilton, D., Sutherland, D., & Howard-Williams, C. (2015). Updated model assessment of the effects of increased nutrient loads into Lake Benmore. NIWA Client Report No CHC2015-089. Prepared for Environment Canterbury. Stout, V. (1973). Lake Grasmere: a study of a small South Island lake. Unpublished Report, Department of Zoology, University of Canterbury, Christchurch. Sutherland, D., & Burton, T. (2015). Ecological assessment of 6 Canterbury lakes using LakeSPI and weed surveillance in 23 waterbodies. NIWA Client report No: CHC2015-052. Prepared for Environment Canterbury, Meridian Energy Limited, Genesis Energy Limited. Sutherland, D., Edwards, T., Wells, R., & Kelly, G. (2013). Assessment of the ecological condition of 8 Canterbury lakes using LakeSPI - and weed surveillance in 12 lakes. NIWA Client Report No:

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CHC2013-057. Prepared for Environment Canterbury. Taranaki Catchment Commission (1987). Lake Alexandrina water quality study. Report Prepared for the Waitaki Catchment Commission. Tau, T. M., Goodall, A., Palmer, D., & Tau, R. (1990). Te whakatau kaupapa. Ngai Tahu resource management strategy for the Canterbury Region. Wellington: Aoraki Press. Te Rūnanga o Arowhenua, Pauling, C., & Norton, T. (2010). Cultural Health Assessment of Ō Tu Wharekai / The Ashburton Lakes. https://www.takiwa.org.nz/docs/2010_AshburtonLakes.pdf Timms, B. V. (1983). Benthic macro in vertebrates of seven lakes near Cass, Canterbury high country, New Zealand. New Zealand Journal of Marine and Freshwater Research, 17(1), 37–49. https://doi.org/10.1080/00288330.1983.9515985 Tipa and Associates, & Williams, E. (2015). The Cultural Health of the Waitaki Catchment. Prepared for Environment Canterbury. van der Woerd, J. H., & Wernand, R. M. (2018). Hue-Angle Product for Low to Medium Spatial Resolution Optical Satellite Sensors. Remote Sensing, 10(2): 180, https://doi.org/10.3390/rs10020180 Ward-Smith, R., Stout, V., & Coombridge, W. (1985). Lake Alexandrina Interim Guidelines for Management of the Catchment. Unpublished Report, The Lake Alexandrina Catchment Steering Committee, 67pp. Wetzel, R.G. 1983. Limnology. 2nd Edition, Saunders College Publishing, Philadelphia. White, E. 1983. Lake eutrophication in New Zealand—a comparison with other countries of the organisation for economic co‐operation and development, New Zealand Journal of Marine and Freshwater Research, 17(4), 437-444, DOI: 10.1080/00288330.1983.9516018 Winder, M., Reuter, J. E., & Schladow, S. G. (2009). Lake warming favours small-sized planktonic diatom species. Proceedings of the Royal Society, 276, 427–435. https://doi.org/10.1098/rspb.2008.1200

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Appendix 1: Protocol for sampling Lindavia intermedia Protocol for sampling Lindavia intermedia (‘lake snow’) presence in 20 high country Canterbury Lakes

Sampling equipment

 Didymo plankton net (40 µm mesh), 240 mm diameter  30 m tow line  Variable weights  Collection mesh (40 µm)  Pottles  Ice  Chilly bin

Deep lakes (>10 m) This includes Lakes Camp (18.9 m), Clearwater (19 m), Heron (36.2 m), Ōhau (129 m), Tekapo (120 m), Aviemore (222 m), Waitaki (135 m), Alexandrina (30 m), McGregor (12 m). Deep lakes will be sampled by a combination of vertical hauls, ideally to encompass the top of the thermocline, and horizontal tows at different depths. Tows will be continued until there is a visible residue on the collection mesh, but not necessarily evidence of mucilage indicative of ‘lake snow’. If there is any sign of slime that could be Lindavia mucilage, a note will be made. The sample will be transferred to a 130 ml container comprising the material on the mesh, the mesh itself, plus a little lake water rinsed from the inside of the net. Samples will be maintained on ice and frozen as soon as possible.

Shallow lakes (<10 m) This includes Lakes Denny (2.1 m), Donne (1.1 m), Emily (2.3 m), Emma (3 m), Kellands (5.4 m), Māori west (2.6 m), Māori East (1.2m), Middleton (4.9 m), Roundabout (1.7 m), Spider (0.8 m), Ruataniwha (depth undetermined). Shallow lakes will be sampled by horizontal tows at a range of depths chosen to avoid fouling of sampling equipment by vegetation. In lakes where a vessel cannot be deployed, tows will be undertaken by snorkel/scuba divers propelled by scooters (Diver Propulsion Vehicles). Tows will be continued until there is a visible residue on the collection mesh, but not necessarily evidence of mucilage indicative of ‘lake snow’. Samples will be maintained on ice and frozen as soon as possible.

Addressing cross contamination between lakes and samples To prevent the transfer of live cells we will follow the Check, Clean, Dry treatment recommended for fine mesh nets; specifically by immersing in a 5% solution of dishwashing liquid for as long as it takes to thoroughly saturate the net, plus for at least an additional minute. In addition, to prevent contamination of subsequent samples with dead cells from an earlier positive sample, the nets will be rinsed and brushed thoroughly prior to each new deployment. The net will be rinsed with two successive 5 l quantities of tap water, with collection of material after of the final rinse as a ‘blank’ for checking using the analysis procedure below. The order of lake sampling will be recorded.

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Analysis Each sample will be thawed, the collection net rinsed off in the lab, and settled before examination. A new disposable pipette will be used for each sample to prevent cross-contamination. The microscope protocol will be multiple (at least 3) scans of subsamples of each collected wet sample, at 200x to 400x magnification. We will note the presence of centric diatoms, and will first compare them with wet samples of Lindavia, which NIWA holds. Provisional ID of Lindavia from the wet samples will be confirmed using permanent slides for QA and for future record. Slides will be compared to a confirmed reference slide for Lindavia intermedia held by NIWA. QA will be carried out by Cathy Kilroy (NIWA) as required. The permanent slides from each lake will be archived in our diatom collection at NIWA.

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Appendix 2: Lake catchment proportional cover by LCDB v4.0 (Landcare Research 2012) land cover categories for 27 Canterbury high-country lakes (from Kelly et al., 2014)

cing cing

b (%) (%) (%) (%) (%) (%) (%) (%) (ha) area exotic Wetland grassland 9 vegetation 4 vegetation 4 Lake/pond 5 Gravel/rock3 Native 1 forest Exotic 6 forest grassland/scru Gorse/broom 7 Low produLow Tussock/alpine grassland 8 (%) High producing Lake Lake catchment Hurunui catchment Katrine 1,184 46 19 4 7 7 16 Mason 626 47 43 8 1 Sheppard 1,260 27 6 8 2 9 1 4 21 23 Taylor 1,577 34 13 4 13 0 1 11 23 Waimakariri catchment Grasmere 1,746 6 27 23 3 6 33 Pearson 1,667 11 25 20 11 5 26 Sarah 168 15 7 4 13 61 Rakaia catchment Catherine 225 7 32 24 8 18 11 Evelyn 471 9 37 10 6 4 33 Georgina 406 17 33 4 47 Hawdon 208 27 6 1 18 48 Henrietta 797 22 16 18 1 8 36 Ida 393 21 3 42 3 30 Lyndon 1,425 1 39 18 2 6 34 Selfe 648 17 15 19 10 40 Heron 11,087 1 34 26 3 6 9 21 Ashburton catchment Camp 539 4 8 1 86 Clearwater 4,176 57 1 5 37 Emily 241 5 1 3 8 82 Emma 3,560 1 23 6 5 7 1 58 Māori East 8,357 18 3 1 1 45 33 Māori West 1,492 44 2 1 50 3 Rangitata catchment Denny 1,866 27 2 1 32 38 Waitaki catchment Alexandrina 4,627 1 15 1 4 79 Kellands 1,593 1 1 86 10 McGregor 4,849 1 15 1 4 78 Middleton 1,012 36 4 3 7 31 18

1 ‘Native Forest’ includes LCDB v4.0 categories: Indigenous Forest, Manuka and/or Kanuka, Broadleaved Indigenous Hardwoods, Fernland 2 ‘Tussock / alpine grassland / scrub’ includes LCDB v4.0 categories: Tall Tussock Grassland, Sub Alpine Shrubland, Matagouri or Grey Scrub, Alpine Grass/Herb-field 3 ‘Gravel / rock’ includes LCDB v4.0 categories: Gravel or Rock, Landslide 4 ‘Wetland vegetation’ includes LCDB v4.0 categories: Herbaceous Freshwater Vegetation 5 ‘Lake / pond’ includes LCDB v4.0 categories: Lake or Pond 6 ‘Exotic forest’ includes LCDB v4.0 categories: Exotic Forest, Mixed Exotic Shrubland, Deciduous Hardwoods 7 ‘Gorse / broom’ includes LCDB v4.0 categories: Gorse and/or Broom 8 ‘High producing exotic grassland’ includes LCDB v4.0 categories: High Producing Exotic Grassland 9 ‘Low producing grassland’ includes LCDB v4.0 categories: Low-Producing Grassland, Depleted Grassland

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Appendix 3: Regional climate summary Due considerable differences in climate across the Canterbury region, this regional summary includes data from six meteorological stations that were chosen to represent the five Water Management Zone (WMZ) in which the lakes are located. Note that datatypes were not available for all stations, and not all records extend from 2004 to 2019.

1. Lake Taylor (rainfall only) – Hurunui-Waiau-WMZ 2. Arthurs Pass - Waimakariri Lakes (Selwyn-Waihora-WMZ) 3. Snowdon – Lake Coleridge (Selwyn-Waihora-WMZ) 4. Mt Potts (since 2009) - Ashburton-WMZ 5. Lake Tekapo - Upper Waitaki-WMZ 6. Pukaki (wind speed only) - Upper Waitaki-WMZ

Parameters summarized here are 1. Total annual rainfall 2. Summer rainfall (December to April) 3. Summer air temperature (January to March) 4. Number of days of wind speed above 12 m/s between January and April

A3.1 Total annual rainfall Total annual rainfall was high in 2010-2014, 2016 and 2018 at the Lake Taylor meteorological station (Figure A3-1), and 2006, 2010, 2014 and 2016 in Arthurs Pass (Figure A3-2) At the Snowdon meteorological station (near Lake Coleridge) rainfall was highest in 2006, 2008-2010 and 2013 (Figure A3-3). Lake Tekapo also experienced high rainfall in 2006, 2010, 2012-2013 and 2018 (Figure A3-4).

Figure A3-1: Total annual rainfall, Lake Taylor, 2005-2018

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Figure A3-2: Total annual rainfall, Arthurs Pass, 2005-2018

2017 is missing data for June, July and December

Figure A3-3: Total annual rainfall, Snowdon, 2005-2017

Figure A3-4: Total annual rainfall, Lake Tekapo, 2005-2018

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A3.2 Summer rainfall There seems to be a pattern of increasing summer rainfall from 2007-2011, and decreasing summer rainfall from 2011-2016, and 2018 was a wet year at most monitoring stations (Figure A3-5, Figure A3-6, Figure A3-7, Figure A3-8, Figure A3-9).

Figure A3-5: Summer rainfall, Lake Taylor, 2005-2019

Figure A3-6: Summer rainfall, Arthurs Pass, 2005-2019

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No data in January and February 2012, and January 2009

Figure A3-7: Summer rainfall, Snowdon, 2005-2017

Figure A3-8: Summer rainfall, Mt Potts, 2005-2019

Figure A3-9: Summer rainfall, Lake Tekapo, 2005-2019

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A3.3 Summer air temperature Summer air temperatures were higher than average in 2007, 2008, 2013, 2015, 2016, 2018 and 2019 in Arthurs Pass (Figure A3-10), 2005, 2008 and 2013 at Snowdon (Figure A3-11), 2013, 2016 and 2019 at Mt Potts (Figure A3-12), and 2008, 2010 and 2016 at Lake Tekapo (Figure A3-13).

Figure A3-10: Average summer air temperature, Arthurs Pass, 2005-2019

No data for January 2009 Figure A3-11: Average summer air temperature, Snowdon, 2005-2017

Figure A3-12: Average summer air temperature, Mt Potts, 2011-2019

204 Environment Canterbury Technical Report Canterbury high-country lakes monitoring programme – state and trends, 2005-2019

Figure A3-13: Average summer air temperature, Lake Tekapo, 2005-2019

A3.4 Days with wind speed above 12 m/s The number of days with wind gusts exceeding 12 m/s were high in 2010 at all monitored stations (Figure A3-14, Figure A3-15, Figure A3-16). Since all stations have gaps in data records it is difficult to assess any patterns.

No data in January and February 2012, and January 2009

Figure A3-14: Number of windy days, Snowdon, 2005-2019

No April data in 2005 and 2006, no March data in 2007 and 2008, no January data 2019

Figure A3-15: Number of windy days, Lake Tekapo, 2005-2019

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No data in April 2009, February 2010, March 2011, March 2016

Figure A3-16: Number of windy days, Pukaki, 2009-2019

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Appendix 4: Genera of benthic mats identified in Smith et al., 2011 high Canterbury - country lakes monitoring programme programme monitoring lakes country

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