Ecological status of Okains Bay estuary
Report No R08/53 ISBN: 978-1-86937-853-0
Lesley Bolton-Ritchie
June 2008
Report R08/53 ISBN 978-86937-853-0
58 Kilmore Street PO Box 345 Christchurch Phone (03) 365 3828 Fax (03) 365 3194
75 Church Street PO Box 550 Timaru Phone (03) 688 9069 Fax (03) 688 9067
Website: www.ecan.govt.nz Customer Services Phone 0800 324 636
Ecological status of Okains Bay estuary
Executive Summary
This report presents detailed information on the ecological status of the Okains Bay estuary. The Okains Bay estuary is the tidally influenced lower 2.8 km of the Opara Stream. Over the last 150 years it has been highly impacted by sedimentation and modification of the estuary margins, and it has potentially been supplied with considerable quantities of nutrients. That is, the present estuarine environment is not in a pristine state. It is likely that the size and depth of the present-day stream channel and the size and extent of the aquatic vegetation, mud/sand and sand flats reflect past impacts on this estuary. The present day impacts include unnatural disturbance particularly by vehicle use, high recreational use particularly over the summer months and nutrient and sediment inputs.
This estuary has three main habitat types. These are: • saltmarsh - approximately 21.5% of the area • mud/sand flats (firm, soft and very soft mud/sand) - approximately 35% of the area • sand flats (firm, soft and mobile sand) – approximately 29% of the area There are also areas of live cockles and/or cockle shell, artificial structures including roads and fences and the stream channel. Sizeable areas of saltmarsh occur in the upper reaches and along the margins of the middle and lower reaches. The mud/sand sediment predominates from the upper estuary to around 450 m from the estuary mouth with the sediment in the lower 450 m predominantly sand. Where the sediment was mud/sand the sediment was typically firm at the high shore, becoming softer with distance down the shore to be soft to very soft at the waters’ edge. In the sandy area the sediment type was patchy, with patches of soft sand, patches of mobile sand and patches of firm sand.
The sediments within the estuary had a low organic matter content, low-moderate nutrient enrichment and low copper, chromium, lead, nickel and zinc concentrations. Thirty four invertebrate taxa were found to live on and in the sediments. There were typically more taxa and individuals in the lower reaches than the middle and upper reaches. Cockles were the most abundant taxa occurring at all sites sampled. However, they were most abundant in the lower reaches where they were found at densities up to 5,343/m2 with half of these new recruits to the population.
This study has identified that: 1. this estuary is the larger of the two estuaries of Banks Peninsula. 2. there is a considerable area and diversity of aquatic vegetation and a variety of sand and mud flat habitats in this estuary. These provide habitats and food for birds, fish and a diversity and abundance of invertebrates. These attributes along with the findings from a landscape study (Boffa Miskell, 2007) support the classification of the estuary as an area of significant natural value and an area of high natural physical, heritage or cultural value. To ensure that the recognised values of this estuary are protected and enhanced the issues of nutrient inputs, degradation of marginal habitats and unnatural disturbances and any other risks to these values need to be addressed.
A number of recommendations have been made including water quality, saltmarsh, broad-scale and fine-scale monitoring and provision of information to residents and visitors about the values of this estuary and how they might be protected. The issue of the modified estuary margins is one that Environment Canterbury and local residents could address in the future.
The information in this report provides: • a baseline against which changes in the sediments and macrobiota can be assessed. • detailed information that the local community can use if planning enhancement activities. • information that will be useful to future resource consent applicants for, for example, subdivision developments within the bay and discharges into the stream or estuary. • data against which data from urbanised estuaries in the region can be compared.
Environment Canterbury Technical Report i Ecological status of Okains Bay estuary
ii Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Table of contents
Executive Summary...... i
1 Introduction ...... 1 1.1 Okains Bay...... 1 1.2 Background information...... 2
2 The estuarine environment ...... 3 2.1 Evaluation of Okains Bay estuary...... 3 2.1.1 Description of the estuary ...... 3 2.1.2 Flushing of the estuary...... 4 2.1.3 Birds of the estuary ...... 4 2.1.4 Fish and shellfish of the estuary ...... 5 2.1.5 Cultural significance...... 5 2.1.6 Status of the estuary ...... 5 2.1.7 Recreation...... 5 2.1.8 The Okains Bay estuary as a feature of Banks Peninsula...... 6 2.2 Past and present stressors ...... 6 2.2.1 Sediment inputs ...... 6 2.2.2 Nutrient inputs ...... 6 2.2.3 Habitat loss - alteration of the estuary margins ...... 9 2.2.4 Microbiological contamination...... 11 2.2.5 Toxic contamination ...... 11 2.2.6 Unnatural disturbance of the estuary ...... 11
3 Broad-scale intertidal habitat assessment ...... 12 3.1 Methods ...... 12 3.2 Results ...... 13 3.2.1 Salt marsh ...... 13 3.2.2 Mud and sand habitats...... 16 3.2.3 Shellfish beds...... 17 3.2.4 Artificial structures...... 17 3.2.5 Terrestrial margin (200 m)...... 17
4 Fine scale intertidal assessment ...... 18 4.1 Methods ...... 18 4.1.1 Sampling sites...... 18 4.1.2 Sample collection ...... 18 4.1.3 Sample processing...... 19 4.2 Data analyses ...... 20 4.3 Results ...... 21 4.3.1 Sediments ...... 21 4.3.2 Macrobiota ...... 25 4.3.3 Sediment characteristics and the macrobiota...... 30 4.4 Discussion...... 31 4.4.1 Sediments ...... 31 4.4.2 Macrobiota ...... 32 4.4.3 Sediments and macrobiota ...... 33
Environment Canterbury Technical Report iii Ecological status of Okains Bay estuary
5 Conclusions...... 33
6 Recommendations ...... 35 6.1 Saltmarsh mapping ...... 35 6.2 Broad scale mapping of the non vegetated area...... 35 6.3 Fine scale monitoring...... 35 6.4 Water quality monitoring ...... 35 6.5 Information ...... 35 6.6 Estuary margins ...... 35
7 Acknowledgements ...... 36
8 References...... 36
Appendix I: Broad scale habitat classification definitions ...... 39
Appendix II: Description of key estuary habitat features...... 40
Appendix III: Co-ordinates of the sites and location of the stations at each site ...... 42
Appendix IV: Sediment grain size analysis – Methodology...... 42
Appendix IV: Sediment grain size analysis – Methodology...... 43
Appendix V: Sediment grain size of each sample at each site...... 44
Appendix VI: Sediment quality data of each sample at each site...... 45
Appendix VII: Total number of individuals of each taxa in all cores from a site ...... 46
iv Environment Canterbury Technical Report Ecological status of Okains Bay estuary
List of Figures
Figure 1-1 Location of Okains Bay, Banks Peninsula...... 1 Figure 2-1 The Okains Bay estuary ...... 3 Figure 2-2 Erosion scars within the Okains Bay catchment ...... 7 Figure 2-3 Vegetation of the Okains Bay catchment ...... 8 Figure 2-4 Indictors of nutrient enrichment of the estuary (photos taken June 2008) ...... 9 Figure 2-5 Modified margins of the Okains Bay estuary...... 10 Figure 2-6 Modified margins of the Okains Bay estuary...... 11 Figure 2-7 Disturbance of the mudflats...... 12 Figure 3-1 Habitats of the Okains Bay estuary ...... 14 Figure 3-2 Vegetation of the Okains Bay estuary ...... 15 Figure 3-3 Non vegetated areas of the Okains Bay estuary...... 16 Figure 4-1 Sampling sites in Okains Bay estuary ...... 19 Figure 4-2 Composition of the sediment at each station at each site...... 22 Figure 4-3 LOI (%), Cu (mg/kg), Cr (mg/kg), Ni (mg/kg), Pb (mg/kg), Zn (mg/kg), TKN (mg/L) and TP (mg/L) at each site ...... 23 Figure 4-4 Non-metric muti-dimensional scaling ordination of sediment data from nine stations at each site ...... 25 Figure 4-5 Number of taxa and individuals in the cores from each site...... 27 Figure 4-6 Total number of coelenterates, molluscs, annelids, arthropods and vertebrates in all cores from each site ...... 27 Figure 4-7 Cockle shells and live individuals in Okains Bay estuary...... 28 Figure 4-8 Size frequency of cockles at sites C and D...... 29 Figure 4-9 Non-metric multi-dimensional scaling (MDS) ordination of the presence and abundance of the macrobiota at each station at each site...... 30
List of Tables
Table 2.1 Summary of daily mean flows (l/s) in Opara Stream...... 4 Table 3.1 Major habitat types of Okains Bay estuary...... 13 Table 3.2 Area (Ha) of each vegetation type in the Okains Bay estuary ...... 13 Table 4.1 Taxa present in the core samples ...... 26 Table 4.2 Correlation between the presence and abundance of the macrobiota and sediment characteristics...... 31
Environment Canterbury Technical Report v Ecological status of Okains Bay estuary
vi Environment Canterbury Technical Report Ecological status of Okains Bay estuary
1 Introduction Environment Canterbury (ECan) has the statutory role of monitoring and managing coastal habitats in the Canterbury region. Estuaries are one of the coastal habitats within the region with the Okains Bay estuary being a small estuary with a predominantly rural catchment. Estuaries are vulnerable to sedimentation, nutrient enrichment, habitat loss, toxic and microbiological contamination and damage from vehicles and human recreational use (Robertson and Stevens, 2007). An evaluation of the current stressors to and the ecological state of an estuary, and in this case the Okains Bay estuary, provides information that will assist ECan fulfil their statutory obligations.
1.1 Okains Bay Okains Bay, a north-eastern bay of Banks Peninsula (Figure 1-1) is one of the bigger bays of the peninsula. The distance from the top of the valley (elevation 573 m) to the bays’ sandy beach is 8 km and from the beach to the heads it is 2.3 km. From the head of the valley the steep sloping hillsides grade to more gentle slopes which give way to the low gradient valley floor. The 3.5 km long low gradient valley floor widens with distance down the valley with the beach 0.9 km long. Beyond the beach the bay width is between 1.4 and 2 km.
Figure 1-1 Location of Okains Bay, Banks Peninsula
Environment Canterbury Technical Report 1 Ecological status of Okains Bay estuary
The lower 3 km of the valley floor is a sequence of fine sand dune and beach ridges formed by infilling of the bay over the last 2000 years or so (Stephenson and Shulmeister, 1999). Mineralogical evidence indicates that the fine sand is derived from sediment carried around Banks Peninsula in the Southland Current and washed into Okains Bay by wave action. While the rate of infilling has been variable over time, it has been calculated that the coastline migrated seaward at a rate of 2.35 m/year between 1860 and 1993 (312 m) (Stephenson and Shulmeister, 1999). Flowing from the hillside and along the northern side of the valley floor is the Opara Stream. The low gradient of the valley floor results in tidal inflows to some 2.8 km up the stream from the sea. The mixing of the seawater with the freshwater from the stream has resulted in estuarine conditions in the lower reaches of this stream. It is this estuarine area that is the focus of this study.
1.2 Background information The finding of a large quantity of Maori artifacts within Okains Bay suggests that at one time this bay supported a Maori population. These earliest settlers would have made use of the abundance of trees, birds, fish and shellfish within the bay and the easy access to the valley via the estuary. The first European settlers of 1851 arrived to a densely forested Okains Bay. The podocarp forest of totara, matai, kahikatea and rimu was alive with native birds (Kirk, 1978). By 1854 pit sawing of the forest was underway with sawmilling continuing into the 1890s, i.e. for some 40 years. In 1954 the river was navigable at high tide to fair size cutters (Kirk, 1978). However, punts were also used to convey timber down the stream and out over the bar to whaling ships waiting in the bay. These punts could carry 6000 – 7000 feet of timber. One of the largest sawmills of the Peninsula began operation within Okains Bay in 1872 (Ogilvie, 1992). Once the large sawmill was in operation up to 20,000 m of timber per week was transported from Okains Bay to Christchurch. Much of this timber was transported by the vessels which travelled up stream to the landing place located just downstream of the Schoolmasters bridge. As the land was cleared it was used for dairying and as a consequence year by year there was an increase in dairying within the bay (Jacobsen, 1976). The Okains Bay cheese factory was commissioned in 1894 and by 1902 this factory was producing 61 tonnes of cheese and 4 tonnes of butter per year (Ogilvie, 1990). While dairying dominated, sheep were grazed within the bay from 1869. The depression of the 1930s saw a downturn in dairying and an upturn in sheep farming on Banks Peninsula. Nonetheless there was considerable dairying on the valley floor up until the closure of the cheese factory in 1968. Nowadays the pastures of the bay are grazed by sheep, cattle and some dairy cows while the cheese factory has been transformed into the renowned Okains Bay Maori and Colonial museum. In 2001 there were 438 households and a total of 1092 people living in Okains Bay (Statistics New Zealand). In addition, there is an influx of campers particularly over the summer months and more so over the Christmas New Year period, and day visitors to the museum and hence the bay. The campers, that frequent the motor camp, nestled in the pine trees behind the beach and adjacent to the estuary, come to enjoy the sheltered bay, shallow sandy beach and the sheltered, safe waters of the lower estuary. This bay has been an attractive place for holiday makers since the 1890’s (Ogilvie, 1990). In the early days the holiday makers came to enjoy the peace and quiet of this scenic bay, fish for blue cod off the wharf (built in 1891), spear flounder in the estuary and go eeling in the stream (Ogilvie, 1990).
2 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
2 The estuarine environment
2.1 Evaluation of Okains Bay estuary
2.1.1 Description of the estuary This estuary consists of the tidally influenced lower 2.8 km of the Opara Stream (Figure 2-1). The mixing of the fresh water from the stream with sea water has resulted in estuarine conditions including mudflats and estuarine biota in the channel and on the banks of the lower reaches of the stream. The mudflats widen with distance downstream; in the upper reaches (Schoolmasters bridge) the stream channel and its banks are 7 m wide, at the Chorlton Road bridge they are 133 m wide and at the widest point, some 400 m from the sea, they are 370 m wide.
Figure 2-1 The Okains Bay estuary
Environment Canterbury Technical Report 3 Ecological status of Okains Bay estuary
2.1.2 Flushing of the estuary The Opara Stream flows through the mudflats and into the sea via a 25 m wide stream mouth. This stream has a spring fed baseflow component but a highly variable flow due to the influence of rainfall on the quantity of water in the stream. The variability in flow (l/s) is shown by the data presented in Table 2.1.
Table 2.1 Summary of daily mean flows (l/s) in Opara Stream Data collected by Environment Canterbury
January February March April May June July August September October November December 2000 Median 60.5 46 45 48.5 95 117.5 99 80 636.5 330 178.5 86 Minimum494040405479796826922611463 Maximum 83 248 1127 278 276 792 174 13834 6184 14224 375 178 2001 Median 77 46 39 36.5 60 74 90 205 219 298 220.5 85 Minimum 49 42 31 31 38 60 62 147 127 106 139 73 Maximum 189 61 45 47 231 139 6027 594 2362 1064 725 163 2002 Median 275 153 97 115 145 202 308 273 174 112 89 91 Minimum 62 119 77 94 99 124 207 191 129 89 36 63 Maximum 17032 295 138 1634 545 1686 1303 928 351 352 4262 275 2003 Median 59 43.5 52 166.5 107 78.5 181 110 314.5 320 134 69 Minimum 42 39 44 87 89 67 100 82 169 176 86 56 Maximum 103 53 411 3584 160 105 377 889 4135 1092 322 81 2004 Median 54 62 49 54.5 125 132.5 93 842 416 233 110 167 Minimum 51 54 41 47 51 81 76 198 271 140 81 87 Maximum 62 194 170 150 570 677 1549 7936 1115 425 362 874 2005 Median 96 79 68 66.5 168 228.5 175 149 107 177 88.5 77 Minimum 80 65 55 55 74 142 134 105 93 109 73 61 Maximum 261 382 95 572 1137 883 685 361 425 2431 119 342 2006 Median 48 44 66 54.5 153 435 350 822 179 148 147.5 121 Minimum 38 35 51 40 68 171 207 190 133 107 99 89 Maximum 113 164 227 166 4377 1406 1254 6556 380 1122 1066 1413 2007 Median 144 82 62 54.5 45 54.5 315 315 133.5 166 85 73 Minimum 91 67 49 47 34 37 156 175 91 94 66 59 Maximum 461 105 259 182 184 3535 10100 2725 276 2514 123 235
Sea water flows into the estuary as the tide rises reaching the uppermost marginal vegetation and channels on spring tides. Given the length and shape of this estuary it is likely that all seawater that flows upstream with the rising tide leaves the estuary as the tide recedes. That is, this is a well flushed estuary.
2.1.3 Birds of the estuary The birds that frequent the estuary are: Blue heron White-faced heron Caspian tern (nest on the island in the bay) Pied and black oyster catcher Pied stilt Paradise duck Mallard duck Spur-winged plover Red billed gull Black backed gull Kingfisher
4 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Pukekos live on the banks and in the estuarine vegetation particularly in the vicinity of the Millenium walkway bridge (approx. opposite the museum).
2.1.4 Fish and shellfish of the estuary This estuary is: • the access way for whitebait to the middle reaches of the Opara Stream • the access way for elvers and mature eels to and from the middle reaches of the Opara Stream • likely a nursery area for flatfish • an area that adult flatfish can inhabit • an area where schools of yellow-eyed mullet can occur • the habitat for an abundance of cockles
Adult whitebait or īnanga (Galaxias species) come down the stream to lay their eggs among the plants of the upper estuary in late summer and autumn, and then die. On hatching, the young are swept out to sea, where they spend five or six months. They then enter the estuary mouth in spring, and swim upstream to fresh water further inland. Adult eels also come down the stream and pass through the estuary to spawn at sea. Their larvae then make their way back to coastal waters in spring, transform into a juvenile stage (glass eels) and enter the estuary. They settle and feed in the estuary for a while before swimming upstream into fresh water.
In the 1960s eeling in the middle reaches of the Opara Stream was a common practice as was floundering in the lower reaches of the estuary and off the beach (pers.obs.). Nowadays adult flounder are a rare sight within the estuary (J. Thacker, pers. obs.). During the whitebait season whitebaiters typically fish in the vicinity of Schoolmasters bridge (J. Thacker, pers. obs.).
2.1.5 Cultural significance Historically the stream and estuary would have been used as an access way to the abundant bush, birdlife and eels up the valley. The estuarine reaches would have been valued for the abundance of cockles and fish.
On the 6th of February each year the Okains Bay Maori and Cultural museum is a focus for Waitangi day celebrations. One aspect of the celebrations is the paddling of a waka within the estuary.
2.1.6 Status of the estuary Okains Bay estuary (up to Schoolmasters bridge) is listed as an area of significant natural value and an area of high natural physical, heritage or cultural value (RCEP, 2005). This classification is based on the following aspects: 1. protected areas; 2. wetland, estuaries and coastal lagoons; 3. ecosystem flora and fauna habitats; 4. scenic sites; 5. areas adjacent to areas of significant conservation value.
2.1.7 Recreation Part of the attraction of Okains Bay as a holiday destination is the coastal environment, including the estuary. Not only is the coastal area scenic, it provides numerous recreational opportunities. The lower reach of this estuary has very high recreational value with swimming, scavenging (children gathering crabs etc.), kayaking, sunbathing and walking being popular activities. There is also potential for shellfish gathering, but there is no information on the popularity of this activity or the quantities collected.
Local residents, campers and day visitors alike enjoy the safe and warm waters of the estuary for swimming. Because this is a popular swimming spot Environment Canterbury undertakes routine microbiological water quality monitoring for contact recreation in the lower reach of the estuary over
Environment Canterbury Technical Report 5 Ecological status of Okains Bay estuary
the summer months. The site has a GOOD1 grading. However, the water quality at this site is impacted by rainfall, i.e. high concentrations of faecal indicator bacteria occur during and after moderate to heavy rainfall.
Okains Bay has been identified as the most popular bay in Banks Peninsula because of its outstanding recreational values, its cultural significance and its visual diversity (Boffa Miskell, 2007).
2.1.8 The Okains Bay estuary as a feature of Banks Peninsula Okains Bay and Le Bons Bay are the only Banks Peninsula bays that have a sizeable estuarine area. In both bays the estuarine environment consists of the stream channel and mudflats flanking the channel. In Okains Bay the estuarine area extends some 2.8 km, and in Le Bons Bay it extends some 2.2 km, inland from the sea. Along the Le Bons stream the width of the stream channel and mudflats is at most 100 m but more typically around 75 m. Along the Opara Stream in Okains Bay the width of the estuarine area increases with distance downstream with the greatest width being 370 m.
2.2 Past and present stressors
2.2.1 Sediment inputs Details on past and present land use within the catchment are described in section 1.2. The clearing of the podocarp forest would have resulted in large loads of sediment flowing down the stream, into the estuary and then out into the bay. Evidence of the soil loss from the hillsides, likely due to land clearance, are the historic erosion scars (Figure 2-2). Many of these historic erosion scars are in the gullies adjacent to waterways but the larger areas are not immediately adjacent to waterways. Nowadays the erosion scared hillsides are dominated by pasture with small remnants of native bush and considerable areas of scrub (Figure 2-3).
It is likely that there was also a considerable input of sediment to the estuary during the construction of the main road to Chorlton (including the construction of the bridges – a succession of bridges have been built) and the back road flanking the true left bank of the stream between Schoolmasters bridge and the Chorlton Road bridge (Figure 2-1).
One of the key issues affecting the condition of New Zealand estuaries in the present day is sedimentation (Robertson and Stevens, 2007). Sedimentation of estuaries causes a multiplicity of impacts including changes to the grain size distribution. The result is muddier and more nutrient- enriched sediments. This may cause initial smothering of the biota followed by a shift in the biological community to one that is mud-tolerant. Sedimentation was a significant historic issue for this estuary with major impacts from sedimentation likely occurring during the land clearance/sawmilling era (1850’s – 1890’s). The depth and width of the present day channel and mudflats was probably shaped by this sedimentation. Present day sediment inputs to this estuary come from hillside erosion during heavy rainfall and potentially from land development.
2.2.2 Nutrient inputs With dairying there may have been a considerable input of nutrients to the stream and estuary, with this coming from stock access to the water ways and discharges from milking sheds and the cheese factory. At present the potential sources of nutrients include the phosphorus-rich volcanic rock in the catchment, runoff from grazed land and seepage from septic tanks. The households within Okains Bay are not on a reticulated sewage system rather each household has a septic tank. For those households in proximity to either the small hillside waterways or the Opara Stream there is potential for seepage from the tanks to reach the estuary. The occurrence of a diatom film over the mud within the channel in the upper reaches and patches of the algae Gracilaria chilensis in the lower reaches (Figure 2-4) are indicators of nutrient enrichment of this estuary.
1 There are five grades for swimming sites. These are Very good, Good, Fair, Poor and Very Poor. A Good grading means that a site is satisfactory for swimming most of the time. On occasions (such as after high rainfall) there may be an increased risk of faecal contamination from run-off.
6 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Figure 2-2 Erosion scars within the Okains Bay catchment
Environment Canterbury Technical Report 7 Ecological status of Okains Bay estuary
Figure 2-3 Vegetation of the Okains Bay catchment
8 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
A B
Figure 2-4 Indictors of nutrient enrichment of the estuary (photos taken June 2008) A – Gracilaria chilensis and Ulva sp. (green), lower reach B – Diatom film on surface of mud in the channel, upper reach
2.2.3 Habitat loss - alteration of the estuary margins With the advent of shipping to transport sawn timber from the bay came the development of an upper section of the estuary margin as a landing area (below Schoolmasters bridge). The details of the alterations made to the estuary margins at this time are not known.
At present the true right margin of the upper reaches and sections of the lower reaches are covered with estuarine vegetation, notably rushes. Along sections of this rushland the landward edge is behind fence lines and within grazed paddocks (Figures 2-5 A and E and 2-6 B). The demand for grazing land has resulted in the modification of the landward edge of the estuary margin including impacts on the rushland. In addition the demand for grazing land could have resulted in the margins of, the wetlands adjacent to, and the drainage channels into, the estuary being converted to pasture. Nowadays some land adjacent to the estuary and drainage channels becomes inundated with water during extreme high tides. This suggests that in the past there was conversion of wetlands and/or mudflats to pasture. Other modifications to the estuary margin are: • a stonewalled, grassed area which is used during Waitangi Day celebrations (Figure 2-5 B) • rails from the waka storage area, across the mudflat to the waters edge • the hardfill area on either side of the Chorlton Road bridge (Figure 2-5 C) • culverts from the small tidal channels within the grazed paddocks (Figures 2-5 D and 2-6 A) • a roadway on the true right bank downstream from the Chorlton Road bridge, through the rushland (Figure 2-5 F) • modifications to small tidal channels (Figure 2-6 A) It is also likely that the gravel road on the true left bank between Schoolmasters bridge and the Chorlton Road bridge has resulted in considerable modification to that stretch of estuary margin.
The margins of this estuary have been highly modified over time. These modifications have resulted in loss of marginal habitat for estuarine plants and animals.
Environment Canterbury Technical Report 9 Ecological status of Okains Bay estuary
A B
C D
F E
Figure 2-5 Modified margins of the Okains Bay estuary A – Rushland behind a fence line B – Stone walled grassed area on true right bank C – Road and hardfill area leading to Chorlton Road bridge D – Culvert from a tidal channel E – Rushland behind a fence line F – Roadway through rushland
10 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
A B
Figure 2-6 Modified margins of the Okains Bay estuary A – Culvert from a modified tidal channel B – Fence line across tidal flat with tidal vegetation behind the fence
2.2.4 Microbiological contamination The results from the routine microbiological water quality monitoring for contact recreation in the lower reach of the estuary over the summer months indicate that the water quality is influenced by rainfall, i.e. there is faecal contamination in the estuary during and after moderate to heavy rainfall. The potential sources of this faecal contamination are: • runoff from land grazed by sheep, cattle and cows • seepage from septic tanks • runoff from impervious surfaces (roads, roofs)
2.2.5 Toxic contamination Contaminants that can be toxic to estuarine life include toxic heavy metals, polycyclic aromatic hydrocarbons (PAH’s), polychlorinated biphenyls (PCB’s) and pesticides. These contaminants reach the estuarine environment via agricultural stormwater runoff, stormwater runoff from roads and other impervious surfaces, industrial discharges and air pollution. The chemicals collect in fine sediments and bio-accumulate in fish and shellfish. Of the contaminants listed above likely pesticides and heavy metals could affect this estuary.
2.2.6 Unnatural disturbance of the estuary Historically, shipping and associated activities would likely have caused considerable disturbance to the mudflats and stream bed. Past and any present stock movement across the mudflats and small tidal channels would result in disturbance and compaction of the sediment and impacts on the biota. In more recent times it is the driving of motor vehicles across the mudflats that has raised concerns for the environment (Figure 2-7). Vehicle use can have a significant impact on marine biota (Stephenson, 1999). The motor vehicles can ‘rip-up’ surface sediment and compact sediment as well as kill and disturb the biota. Vehicle access to the mudflats seaward of the Chorlton Road bridge is now prohibited (RCEP, 2005), and the placement of large warning signs there should cause a reduction in vehicles being driven over the mudflat.
High recreational use, i.e. shellfish gathering, scavenging (children gathering crabs etc.), boating activities, walkers and swimmers can result in considerable disturbance of the sediments and biota.
Environment Canterbury Technical Report 11 Ecological status of Okains Bay estuary
A B
Figure 2-7 Disturbance of the mudflats A – Vehicle tracks just below the Chorlton Road bridge, January 2007 B – Animal tracks just above the Chorlton Road bridge, November, 2004 C – Vehicle track in very soft sand near the motor camp, June 2008 D – One of the warning signs near the motor camp, June 2008
3 Broad-scale intertidal habitat assessment Broad-scale habitat mapping is used to describe the intertidal environment according to dominant habitat types. These are based on surface features of substrate characteristics and vegetation type. The habitat information is used to describe the current condition of this estuary.
3.1 Methods Aerial photographs in combination with detailed ground truthing were used for this mapping. The aerial photos were those in the Environment Canterbury GIS system and were taken in March 2004. The classification of habitat types was based on those described in the National Estuary Monitoring Protocol (Robertson et al., 2002) (Appendix I). A description of key estuary habitats is provided in Appendix II.
12 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
3.2 Results The broad-scale habitat map of the intertidal area of the Okains Bay estuary is shown in Figure 3-1. The approximate area and percentage of each major habitat type are given in Table 3.1.
Table 3.1 Major habitat types of Okains Bay estuary
Hectares % area
Saltmarsh 9.9 21.5
Mud/sand (firm, soft, very soft) 16.1 35.0
Sand (firm, soft, mobile) 13.5 29.3
Cockles (shellbank and/or live) 1.1 2.4
Artificial structures 0.8 1.8 Water 4.6 10.0
3.2.1 Salt marsh Rushland made up the largest area of vegetation in this estuary (Table 3.2). There were also sizeable areas of sedgeland and herbfield within the estuary and an edging of marram grass at the estuary mouth.
The margins of the upper reach were dominated by rushland. Rushland also occured along the south- eastern margins of the middle and lower reaches of the estuary. The jointed wire rush Apodasmia similis and the sea rush Juncus kraussii. subsp.australiensis were the dominant rushland species (see Figure 3-2) with only occasional plants of the knobbed rush Isolepis nodosa in the lower reaches of the estuary. At the highest levels of the shore the saltmarsh ribbonwood Plagianthus divaricatus typically grew alongside the rushes.
Table 3.2 Area (Ha) of each vegetation type in the Okains Bay estuary
Area (Ha) Sedgeland 2.2 Schoenoplectus pungens
Rushland 4.6
Apodasmia similis Juncus kraussii. s ubsp. australiensis Isolepis nodosa
Herbfield 2.8
Sellaria radicans Sarcocornia quinqueflora
Marram Grass 0.3 Ammophila arenaria
Environment Canterbury Technical Report 13 Ecological status of Okains Bay estuary
Figure 3-1 Habitats of the Okains Bay estuary
14 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
A B
C D
E F
Figure 3-2 Vegetation of the Okains Bay estuary A – Jointed wire rush, true right bank B – Sea rush, true right bank C – Sea rush, jointed wire rush and saltmarsh ribbonwood, true right bank D – Three-square with patches of sea rush, true left bank E – Three-square downstream from Chorlton Road bridge, true left bank F – Patch of herbfield, true right bank
Environment Canterbury Technical Report 15 Ecological status of Okains Bay estuary
The sedgeland was dominated by the blue-green three-square Schoenoplectus pungens. On the true left bank there were two sizeable areas of sedgeland and a number of very small patches (not mapped) downstream of the Chorlton Road bridge. There was also a patch of three-square on the true right bank downstream of the Millenium walkway bridge.
The herbfield was dominated by two species, the glasswort Sarcocornia quinqueflora and the creeping Selliera radicans. There was a sizeable patch of herbfield in the lower reaches on the true right of the estuary. Both of the herb species occurred at a low density within the rushland.
3.2.2 Mud and sand habitats Upstream of the Chorlton Road bridge the sediment was mud/sand. Typically the sediment was firm mud/sand at the high shore becoming softer with distance down the shore to very soft mud/sand at the waters edge. With distance upstream from the bridge the sediment became more slippery and sticky.
A B
C D
Figure 3-3 Non vegetated areas of the Okains Bay estuary A – Mudflat, true right bank, downstream of Chorlton Road bridge B – Cockle shells adjacent to the channel C – Very soft sand, true right bank D – Mobile sand, true right bank, near the mouth
Downstream of the Chorlton Road bridge the sediment was variable with a transition from mud/sand to sand with distance downstream (Figure 3-1). Mud/sand occurred to around 450 m from the estuary mouth with the lower 450 m predominantly sand. The mud/sand sediment was typically firm across
16 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
much of the intertidal flat but soft to very soft at the waters edge. The sandy area consisted of patches of very soft sand, firm sand and mobile sand.
3.2.3 Shellfish beds Along the banks of the stream there were patches of cockle shells and live cockles. The red seaweed Gracilaria chilensis was also present in these areas.
3.2.4 Artificial structures The artificial structures include the: • stonewalled, grassed area which is a viewing area for Waitangi Day celebrations; • rails from the waka storage area, across the mudflat to the waters edge; • hardfill area on either side of the Chorlton Road bridge; • roadway on the true right bank downstream from the Chorlton Road bridge, through the rushland; • walkway to the Millennium walking bridge; • fences.
3.2.5 Terrestrial margin (200 m) True Left Bank Immediately downstream of Schoolmasters bridge enhancement of the true left bank is underway. This enhancement which consists of the planting of native vegetation in a park setting (called Victoria Park) extends some 100 m downstream and 10-30 m away from the waters edge.
Between Schoolmasters Bridge and the Chorlton Road bridge is a gravel road with a grassy/weedy margin between the road and the saltmarsh. There is one house and farm yard area above the road with the remaining area above the road grazed hillside. Downstream of the Chorlton Road bridge the estuary margin is hillside with grazed land, rocky shore and steep rock faces at the estuary margin (Figure 3-4).
Figure 3-4 Examples of the terrestrial margin on the true left below the Chorlton Road bridge
True right bank Just downstream of Schoolmasters bridge the steep sloping bank is flanked by a grassed paddock. Downstream of the paddock there is a church, the community hall, the Museum, tennis courts, the school and some houses within the 200 m margin. The main Okains Bay road to at least the store (opposite the school) is also within the margin. Down stream of the school the margin is predominantly farmland with several houses immediately adjacent to the estuary. Near the mouth of the estuary is the camping ground with the facilities building within the 200 m margin.
Environment Canterbury Technical Report 17 Ecological status of Okains Bay estuary
4 Fine-scale intertidal assessment Fine scale intertidal assessment is the use of a suite of physical, chemical and biological indicators to assess the condition on an estuary. The indicators are the sediments and benthic macrobiota as these provide information on biodiversity, sedimentation and contamination and hence health of an estuary.
This fine scale intertidal assessment in the Okains Bay estuary aims to: 1. Quantify the present condition of the sediments (grain size, organic matter content, Cr, Cu, Ni, Pb and Zn concentrations and total Kjeldahl nitrogen (TKN) and total phosphorus (TP) concentrations)
2. Quantify the present condition of the benthic macrobiota
3. Identify any issues about the health of the sediments and macrobiota
4.1 Methods
4.1.1 Sampling sites Samples were collected from four sites in Okains Bay estuary (Figure 4-1). Site A was located on the true left, while sites B, C and D were located on the true right, of the channel. The sites were at the mid-low tide shore as recommended in the Estuarine Environmental Assessment and Monitoring Protocol (Robertson et al., 2002). At each site a 50 m (along the shore) by 30 m (down the shore) area was marked out and a GPS reading taken at the seaward inshore corner (Appendix III). The 50 m by 30 m area was subdivided into fifteen 10 m by 10 m areas. Details on the numbering of the stations at each site are given in Appendix III. Randomly generated cartesian co-ordinates determined the location of the sampling station within each 10 m by 10 m area.
4.1.2 Sample collection Sampling was carried out on the 8th (site C) and 9th (site A), 10th (site B) and 11th (site D) of October, 2006. At each site the following samples were collected: Macrobiota a. fifteen core samples - area 0.011 m2, depth 150 mm Sediments b. Nine surface (to a depth of 20 mm) samples for grain size analysis c. Nine surface (to a depth of 20 mm) samples for chemical analyses The depth profile of the sediment was examined at each sampling station. At each sampling station the biological and sediment samples were collected from as close together as possible. One macrobiota core sample and one sediment depth profile was examined from each of the fifteen stations. Sediment samples for grain size and chemical analyses were collected from nine of the stations; three from each level down the shore (Appendix III). The three stations at each shore level were randomly selected. Macrobiota samples were preserved in 70% alcohol containing the fixative glyoxal. The sediment samples were stored in chilly bins (with chilla pads) for transportation to the laboratory undertaking the analyses.
18 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Figure 4-1 Sampling sites in Okains Bay estuary
4.1.3 Sample processing 4.1.3.1 Macrobiota In the field each core sample was sieved through a 500 µm mesh. The material remaining on the sieve was preserved in 70% alcohol containing the fixative glyoxal. The animals present in each core sample were sorted from the debris using a binocular microscope. They were then identified and counted. Organisms were identified, where possible, to species level. All cockles (Austrovenus stutchburyi) and mud flat snails (Amphibola crenata) were counted and then measured to the nearest mm using vernier callipers or an eyepiece micrometer. The shell length of the cockles and the shell height of the mud flat snail were measured. 4.1.3.2 Sediments The sediment samples for grain size analysis were analysed by staff from the Department of Geology, University of Canterbury. Details of the analytical method are given in Appendix IV. Each sediment sample for chemical analysis was homogenised and then split into two portions one portion for each of the following analyses: • organic matter content (loss on ignition (LOI)) • copper (Cu), chromium (Cr), lead (Pb), nickel (Ni), zinc (Zn), total Kjeldahl nitrogen (TKN) and total phosphorus (TP) concentrations
To measure the sediment organic matter content, the APHA 2540 G method, modified to ignition temperature of 450 ºC was used (Environment Canterbury Laboratory).
Environment Canterbury Technical Report 19 Ecological status of Okains Bay estuary
The sediment, for the TKN, TP and metal analyses, was wet sieved through a 63 цm nylon mesh (sample preparation EPA 3050B). The sediment retained on the mesh, i.e. < 63 цm was divided into three portions and prepared and analysed as follows. Sediment for TKN was digested and analysed following the ASTM D3590 B method (Environment Canterbury Laboratory). Sediment for TP was prepared and analysed following the APHA 21st Edition 2005 4500-P,F method. Sediment for Cu, Cr, Ni, Pb and Zn analyses were strong acid digested then analysed by Flame Atomic Adsorption Spectroscopy following Standard method 3111B in APHA 21st Edition 2005. The sediment profiles were inspected for any evidence of stratification.
4.2 Data analyses Microsoft Excel 2003, STATISTICA (version 7) and the software package PRIMER (Plymouth Routines in Multivariate Ecological Research), (version 6) (Clarke and Warwick, 2001) were used for the production of charts, box plots and univariate and multivariate statistical analyses. To determine if there was a significant difference in percent LOI and Cu, Cr, Pb, Ni, Zn, TKN and TP concentrations and the number of taxa and total number of individuals between sites, the Kruskal- Wallis ANOVA (Analysis of Variance) (Statistica V7) was used. Multi Dimensional Scaling (MDS) ordination was used to produce a two-dimensional plot depicting the relative similarity between samples and sites based on sediment characteristics (percent of each grain size fraction, percent LOI and TKN, TP, Cu, Cr, Ni, Pb and Zn concentrations) and on presence and abundance of the macrobiota. To generate the (MDS) ordination plot based on the sediment characteristics, the data were first normalised. The Euclidean distance measure was applied to the normalised data to produce a similarity matrix. To generate the (MDS) ordination based on the macrobiota, the data were log(x+1) transformed. The Bray-Curtis similarity measure was applied to the transformed data to produce a similarity matrix. From the similarity matrix a two-dimensional non-metric MDS ordination of samples and sites was generated. Interpretation of the MDS ordination is based on the closeness of samples/sites on the plot. The closer the samples/sites are, the more similar they are with respect to the parameters used to generate the plot. For each plot a stress value is given. Stress (goodness-of-fit) is a measure of the accuracy of the two-dimensional ordination of points on the MDS plot in representing the actual values in the similarity matrix (Clarke and Warwick, 2001). Low stress values indicate a good ordination with no indication that the plot is a misleading interpretation of the data. The BIO-ENV Primer routine was used to investigate the relationship between the presence and abundance of the macrobiota and the sediment characteristics. The sediment characteristics that were used were percent very coarse sand, coarse sand, medium sand, fine sand, very fine sand, coarse silt, medium silt, fine silt, very fine silt, clay and LOI and TKN, TP, Cu, Cr, Ni, Pb and Zn concentrations. The sediment data were normalised. The macrobiota data were log (x+1) transformed. The BIO-ENV routine identifies the subsets of environmental variables which give the best match between the biological (taxa abundances) and environmental (sediment characteristics) (dis)similarity matrices, as measured by Spearman Rank Correlation (Ps).
20 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
4.3 Results
4.3.1 Sediments
4.3.1.1 Grain Size The percentage of granule, sand, silt and clay in each sample are shown in Figure 4-2. The raw data are presented in Appendix V. The sediment grain size typically changed from sandy mud ( mud = silts + clay) to muddy sand with distance down the estuary (Figure 4-2).
At site A the sediment closer to the channel (stations A1, A2 and A4) was muddier that that some 20- 30 m up the shore from the channel (stations A11, A13, A15). At site C the sediment closer to the channel (stations C1, C2 and C3) was sandier that that some 10-30 m further up the shore (stations C6, C8, C10, C11, C12 and C15). At site D the sand content was higher at stations closer to (D5, D10 and D15) than further away from (stations D1, D6 and D11) the sea.
4.3.1.2 Organic matter (LOI) The raw data are given in Appendix VI. Organic matter content was highest at site A and lowest at site D (Figure 4-3). That is, the organic matter content generally decreased with distance down the estuary.
4.3.1.3 Nutrients (TKN and TP) The raw data are presented in Appendix VI. There was a significant (p = 0.05) increase in TP concentrations with distance down the estuary (Figure 4-3). That is, concentrations at site D > site C >site B > site A. TKN concentrations were significantly (p = 0.05) higher at site D than at site B. There were no significant differences in TKN concentrations between sites A, C and D.
4.3.1.4 Heavy metals The raw data are presented in Appendix VI. Copper, chromium, nickel, and zinc concentrations were significantly higher (p = 0.05) at sites C and D than at sites A and B (Figure 4-3). There were no significant differences in concentrations between sites A and B and between sites C and D. That is, concentrations were higher in the lower reaches than in the upper reaches. For lead the concentrations at sites B, C and D were significantly higher (p = 0.05) than at site A and concentrations at site C were significantly higher than those at site B (Figure 4-2).
The copper, chromium, nickel, lead and zinc concentrations at all sites were below the ANZECC (2000) ISQG-low sediment quality guideline values. The ANZECC (2000) ISQG-low guideline values are: Copper – 65 mg/kg, Chromium – 80 mg/kg, Nickel – 21 mg/kg, Lead – 50 mg/kg, Zinc – 200 mg/kg.
Environment Canterbury Technical Report 21 Ecological status of Okains Bay estuary
100%
80%
60%
40%
Percentage 20%
0% A1 A2 A4 A7 A9 A10 A11 A13 A15
100%
80%
60%
40%
Percentage 20%
0%
B1 B2 B5 B6 B7 B8 B12 B13 B15
100%
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40% Percentage 20%
0% C1 C2 C3 C6 C8 C10 C11 C12 C15
100%
80%
60%
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Percentage 20%
0% D1 D3 D5 D6 D7 D10 D11 D13 D15
Site and station
Granule Sand Silt clay
Figure 4-2 Composition of the sediment at each station at each site
22 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
3.5 10
3.0 8 2.5 2.0 6
LOI 1.5 4
1.0 2 0.5 Cu concentration (mg/kg) 0.0 0 ABCD ABCD Site Site
30 15
25 12
20 9 15 6 10 3 5 Ni concentration (mg/kg) Cr(mg/kg) concentration 0 0 ABCD ABCD Site Site
30 70
25 60 50 20 40 15 30 10 20 5 10 Zn concentration(mg/kg)
Pb concentration (mg/kg) 0 0 ABCD ABCD Site Site
1400 1000 1200 800 1000
800 600
600 400 400 200 200
TP concentration(mg/L) TKN concentration (mg/L) 0 0 ABCD ABCD Site Site Figure 4-3 LOI (%), Cu (mg/kg), Cr (mg/kg), Ni (mg/kg), Pb (mg/kg), Zn (mg/kg), TKN (mg/L) and TP (mg/L) at each site Note: horizontal bar = median, box = interquartile range, whisker ends = 5% and 95%iles, * = outlier values, o = extreme values
Environment Canterbury Technical Report 23 Ecological status of Okains Bay estuary
4.3.1.5 Sediment depth profiles
Site A
0.5 – 1 cm deep brown-grey surface layer. Grey to dark grey with increasing depth. A layer of grit at approx. 7 cm depth at three of the stations close to the channel.
Site B
0.5 – 2 cm deep brown-grey surface layer. Below surface layer grey with a lot of iron staining. The sediment below the surface was sandier at sites near the channel than a sites higher on the shore.
Site C
1-3 cm deep brown-grey surface layer. Close to the channel the sediment below the surface layer was grey sand with cockles shells through it. Higher on the shore the sediment below the surface layer was grey to dark grey mud/sand with iron staining.
Site D
1-2 cm brown-grey surface layer at stations close to the channel but not at those highest on the shore. The sediment below the surface layer was grey becoming darker with depth. Numerous live cockles and cockle shells present. At the stations furthest from the channel the sediment graded from light to dark grey with depth with some iron staining at depth.
24 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
4.3.1.6 Similarity between sites based on sediment characteristics The % sand, silt, clay, LOI and Cu, Cr, Ni Pb, Zn, TKN and TP concentrations were used to generate a multi dimensional scaling (MDS) ordination of each of the nine stations at each site (Figure 4-4). The stress level (0.06) indicates that the ordination is a very good representation of the similarities between stations.
2D Stress: 0.06 site A B C D
Figure 4-4 Non-metric muti-dimensional scaling ordination of sediment data from nine stations at each site
The sediment at site A stations was distinct from that at site B, C and D stations (Figure 4-4). There was some similarity between the sediment at some site C and site D stations. The sediment at one site C station was similar to that at some of the site B stations while the sediment at one of the site D stations was very distinct from that at all other stations.
The site A, C and D stations have a wide spread in the ordination, i.e. the sediment differed between stations (Figure 4-4). By comparison the site B stations did not have such a wide spread indicating considerable similarity in the sediment at all stations.
4.3.2 Macrobiota
4.3.2.1 Taxa present The animal taxa found in the samples are listed in Table 4.1.
4.3.2.2 Number of taxa Zero (site A and B) to 11 (site D) taxa occurred per core (Figure 4-5). There were significantly more taxa per core at site D stations than at site A and B stations.
4.3.2.3 Number of individuals Zero (site A and B) to 264 (site D) individuals occurred per core (Figure 4-5). There were significantly more individuals per core at site D stations than at site A, B and C stations.
Environment Canterbury Technical Report 25 Ecological status of Okains Bay estuary
Table 4.1 Taxa present in the core samples
Phylum Scientific name common name Coelenterata Edwardsia sp. sea anemone
Mollusca Bivalvia Arithritica sp. Austrovenus stutchburyi cockle Mactra ovata Paphies australis pipi
Paphies subtriangulata tuatua Tellina liliana wedge shell Gastropoda Amphibola crenata mud flat snail Cominella glandiformis whelk
Notoacmea helmsi
Potamopyrgus estuarinus Annelida Polychaeta Aonides sp. Aquilospio sp.
Boccardia (paraboccardia) syrtis
Capitella sp. Glycera ovigera Glycera sp.
Heteromastus filiformis
Perinereis ?brevicirris Scolecolepides benhami Scolelepis? sp.
Oligochaeta Unidentified sp.
Nemertea Unidentified sp. Arthropoda Insecta Insect pupa Amphipoda Amphipod sp.A Amphipod sp.B
Amphipod sp.C Corophid sp. Isopoda Isopod sp.A Decapoda Helice crassa
Halicarcinus sp. pill box crab Macrophthalmus hirtipes stalk eyed crab Cirripedia Elminius modestus Mysidacea Unidentified mysid
Vertebrata Fish Pelotretis flavilatus lemon sole Peltorhamphus latus speckled sole
26 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
14 300 12 250
10 200 8 150 6 100 4 taxa of Number 50 2 Number of individuals 0 0 ABCD ABCD Site Site
Figure 4-5 Number of taxa and individuals in the cores from each site Note: horizontal bar = median, box = interquartile range, whisker ends = % and 95%iles, * = outlier values, o = extreme values
4.3.2.4 Abundance of taxa Molluscs (shellfish and snails), annelids (worms) and arthropods (crabs, shrimps, hoppers) occurred at each site (Figure 4-6). Coelenterates (anemones) occurred at sites A, C and D and vertebrates (fish) occurred at sites B, C and D.
Coelenterata Mollusca Annelida Arthropoda Vertebrata 1400
1200
1000
800
600
of individuals Number 400
200
0 ABCD Site
Figure 4-6 Total number of coelenterates, molluscs, annelids, arthropods and vertebrates in all cores from each site
Environment Canterbury Technical Report 27 Ecological status of Okains Bay estuary
Site A At this site worms were the most abundant animals in the cores. The worm community was dominated by oligochaetes (64% of the worms; about 4/core) with around one Scolecolepides benhami and one Capitella sp. per core. Molluscs were the next most abundant group. The small black snail Potamopyrgus estuarinus was the most abundant mollusc (about 2/core) with around one small bivalve Arthritica sp. and one cockle per core. Arthropods made up 11% of the animals with the crab Helice crassa (about 1/core) the most abundant arthropod. The small mud anemone Edwardsia sp. was present in 10 of the 15 cores. There were more Edwardsia sp. at this site than at any other site.
Site B At this site arthropods were the most abundant animals, with the hopper Amphipod sp.A (1-2/core) the most abundant arthropod. There were comparable numbers of worms and molluscs. The worm community was dominated by Scolecolepides benhami (about 1/core) and the mollusc community had comparable numbers of cockles and mud flat snails (<1 per core). No anemones were found at this site. Overall there were a low number of animals at this site.
Site C At this site there were comparable numbers of molluscs and worms, making up 43 and 46% respectively of the community. The rest of the animal community was made up of arthropods (9%), a coelenterate and a fish. The most abundant worm was Aonides sp. (about 5/core) with Scolecolepides benhami and Boccardia syrtis occurring at a density of 1-2/core. Molluscs were dominated by cockles with about 7/core. The most abundant arthropod was the crab Helice crassa (about 1-2/core).
Site D At this site molluscs were the most abundant animals, with cockles (about 55/core) the most abundant mollusc. Worms were the next most abundant group, with Scolecolepides benhami (about 28/core) the most abundant worm. Arthropods made up 2 % of the animal community while there were few coelenterates and fish present. The most abundant arthropod was the hopper Amphipod sp.A (about 1/core).
4.3.2.5 Size distribution of molluscs
Austrovenus stutchburyi (Cockles) There were sufficient cockles at sites C and D but not sites A and B, for size frequency analysis (Figure 4-8). At site C 42% of cockles were smaller than 3 mm with most 1 mm or smaller. At site D 59% of cockles were smaller than 3 mm with over half of these individuals 1 mm or smaller. That is, there had been recent recruitment of juveniles to the population at both sites. The remaining cockles at each site covered a similar size range with indications of at least one other year class present.
Figure 4-7 Cockle shells and live individuals in Okains Bay estuary
28 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Amphibola crenata (mud flat snail) There were insufficient mud flat snails in the cores for size frequency analysis.
50 Site C
40
30
20
10 Number of individuals
0 0 3 6 9 12 15 18 21 24 27 30 33
Size (mm)
600
Site D 500
400
300
200
Number ofindividuals 100
0 0 3 6 9 12 15 18 21 24 27 30 33 Size (mm)
Figure 4-8 Size frequency of cockles at sites C and D (site C n = 105; Site D n = 812)
4.3.2.6 Similarity of sites and stations based on the macrobiota Taxa with a total of ten or more individuals over all core samples (15 taxa) were used to generate a multi dimensional scaling (MDS) ordination of the samples from each site (Figure 4-9). The samples with no animals were not included in this analysis (two samples from each of site A and B).The stress level (0.18) indicates that the ordination is a reasonable representation of the similarities between samples.
The wide spread of site A samples in the ordination indicates considerable variability in the presence and abundance of taxa between samples. The spread of the site B and C samples in the ordination also indicate variability, while the limited spread of site D samples indicate considerable similarity, in the presence and abundance of taxa between samples.
Environment Canterbury Technical Report 29 Ecological status of Okains Bay estuary
On the ordination there was no clear separation of the samples from one site from those at other sites. That is, there was similarity between sites in the presence and abundance of taxa.
The ordination of the samples and sites based on the macrobiota (Figure 4-9) differs from that based on the sediment characteristics (Figure 4-4).
2D Stress: 0.18 site A B C D
Figure 4-9 Non-metric multi-dimensional scaling (MDS) ordination of the presence and abundance of the macrobiota at each station at each site
4.3.3 Sediment characteristics and the macrobiota The relationship between the presence and abundance of the macrobiota and the sediment characteristic was investigated (BIO-ENV Primer routine). The best correlation between the macrobiota and one to a combination of five sediment characteristics are presented (Table 4.2).
The highest correlation of 0.522 was between the macrobiota and a combination of five sediment characteristics. That is, these five sediment characteristics account for around 50% of the difference in the presence and abundance of the macrobiota between stations and sites. This means that other 50% of the difference is due to other unmeasured factors.
30 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Table 4.2 Correlation between the presence and abundance of the macrobiota and sediment characteristics
Correlation Sediment characteristics
0.377 coarse silt
0.459 LOI, Cu 0.509 LOI, Cu, coarse silt
0.514 TKN, LOI, Cu, coarse silt
0.522 TKN, LOI, Cu, coarse silt, very fine sand
4.4 Discussion
4.4.1 Sediments
4.4.1.1 Sediment grain size Fine muddy sediments accumulate in areas where water flow is slow enough to allow fine silt and clay grains to settle to the seabed. Coarser sand grains are to be found in areas with faster water flow. There was a change from sandy mud to muddy sand with distance down the estuary which indicates water flow in the upper reaches is slower than in the lower reaches. The sediment at sites A, B and C are typically muddier than those found in other New Zealand estuaries (Robertson et al., 2002).
4.4.1.2 Organic matter content The organic matter content (LOI) in Okains Bay estuary ranged from 0.5 to 3.1 percent. The muddier sediment typically contained more organic matter than the sandier sediment.
The organic matter content was low-moderate, which is typical of New Zealand well-flushed estuaries (Robertson et al., 2002).
4.4.1.3 Nutrients The increase in TP concentration with distance down the estuary is likely a function of the change in sediment grain size distribution down the estuary. TKN concentration was not related to sediment grain size distribution down the estuary.
The TP concentrations indicate moderate phosphorus enrichment of the sediments (Robertson and Stevens, 2008). The likely source of the phosphorus is the phosphorus rich volcanic rock of the region. The TKN concentrations indicate low-moderate enrichment of the sediments (Robertson and Stevens, 2008). The exact source of the nitrogen is not known but the potential sources are runoff from grazed pastures, stock and septic tank seepage.
4.4.1.4 Heavy metals The copper, chromium, lead, nickel and zinc concentrations were typically higher in the lower reaches than the upper reaches of the estuary. This is likely a function of the difference in sediment grain size distribution between sites.
Environment Canterbury Technical Report 31 Ecological status of Okains Bay estuary
The concentrations of copper, chromium, lead, nickel and zinc were low, with these concentrations below the ANZECC (2000) ISQG-low sediment quality guideline values. The concentrations are typical of New Zealand well-flushed estuaries outside of heavily urbanised areas.
4.4.1.5 Sediment depth profiles The sediment depth profiles give an indication of the recent sedimentary history of this estuary. The layer of grit approx. 7 cm below the surface at some site A stations could have been deposited during the construction of the back road (Figure 2-1).
The muddy sediment beneath the surface layer at all sites indicate land derived sediment, rather than sediment derived from continental shelf fine sand. These muddy sediments predominated at sites A and B and occurred at stations furthest from the channel at sites C and D. That is, the land derived sediment settled close to and to a distance of at least 30 m from the channel in the present-day upper to middle reaches, and from at least 20 m from the channel in the lower reaches, of the estuary.
Given Okains Bays’ history of land clearance and changes in land use over a period of 150 years, analysis of estuarine sediment cores sampled from a number of different sites and to a depth of several metres would provide useful information on the sedimentary history of this estuary.
4.4.2 Macrobiota
4.4.2.1 Taxa The invertebrate taxa present in Okains Bay estuary are typical of South Island estuarine environments (Jones and Marsden, 2005; Fenwick et al., 2006; Robertson et al, 2002). Thirty-four invertebrate taxa were present at the mid-low shore. This is more than the 30 taxa at the mid-low shore in the Avon-Heathcote Estuary/Ihutai (EOS Ecology, 2007), and the 16 taxa the mid-low shore in the Ashley estuary (Fenwick et al., 2006). However, if the four amphipod taxa recorded from the Okains Bay estuary were grouped together as Amphipoda, as were the Amphipoda from the Avon- Heathcote Estuary/Ihutai, there would be 31 taxa which is comparable to the number of taxa in the Avon-Heathcote Estuary/Ihutai. 4.4.2.2 Individuals The number of individuals present ranged from 0 to 264 per core or 0 to 24,076/m2, with a mean density of 1,088/m2 at site A, 486/m2 at site B, 1,964/m2 at site C, and 8,372/m2 at site D. By comparison, the mean density of individuals at the mid-low shore in the Ashley estuary was 300 – 7,000/m2 (Fenwick et al., 2006) and at the mid-low shore in the Avon-Heathcote Estuary/Ihutai it was 400 – 4851/m2 (EOS Ecology, 2007). That is, the mean density of individuals at sites in Okains Bay estuary is similar to that at sites in the Ashley estuary, but the highest mean density in Okains Bay estuary, i.e. site D, is about twice that in the Avon-Heathcote Estuary/Ihutai in 2007. 4.4.2.3 Taxa abundance and distribution Four taxa, cockles and the worms Scolecolepides benhami, Aonides sp. and oligochaetes made up 81% of the individuals in the cores. The remaining 30 taxa made up 19% of individuals with the mean density of these taxa typically low. Cockles were the most abundant taxa making up 48.5% of all individuals. Cockles occurred at all sites but were most abundant at site D, this indicates that cockles can tolerant to a range of environmental conditions but conditions at site D were the most suitable. The mean density of cockles at site A was 67/m2, at site B was 61/m2, at site C was 675/m2 and at site D was 5,343/m2. Size frequency analysis of the cockles from sites C and D revealed that around half of the individuals were very small, i.e. new recruits to the population. However, there was a wide size range for the other 50% of individuals. S. benhami was the next most abundant taxa making up 25% of all individuals. This worm occurred at all sites but was most abundant at site D. The mean density of S.benhami at site A was 97/m2, at site B was 116/m2, at site C was 207/m2 and at site D was 2,560/m2. Aonides sp. was the third most abundant taxa making up 4.1% of all individuals. This worm occurred at sites C and D with a mean density of 456/m2 at site C and 30/m2 at site D. Oligochaetes were the fourth most abundant taxa making up 3.1% of the individuals. Oligochaetes only occurred at site A where they had a mean density of 371/m2. Six taxa occurred only at site A with the most abundant of these being Oligochaetes and the little black snail Potamopyrgus estuarinus. Three taxa occurred only at site B with 7 or fewer individuals of these
32 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
taxa recorded in the cores. One taxa represented by two individuals, occurred only at site C. Six taxa, the most abundant of which was the pipi represented by 9 individuals, occurred only at site D. Another nine taxa occurred only at sites C and D. These results suggest that the environmental conditions in this estuary influence the distribution of 25 of the 34 taxa. 4.4.2.4 Factors affecting the abundance and distribution of taxa Five sediment characteristics - TKN and copper concentration and percent organic matter, coarse silt and very fine sand, account for 52% of the variability in the presence and abundance of taxa in this estuary. Salinity and other unmeasured environment factors and biological interactions (predation, grazing, inter and intra-specific competition) likely account for the remaining 48% of the variability. Of these other factors it is probably salinity that has the greatest influence on the presence and abundance of the taxa down the estuary. In the Okains Bay estuary salinity likely increases with distance downstream, hence low salinity tolerant taxa could live at site A while low salinity intolerant taxa, that require salinities just below or comparable to seawater, could not live at site A but could live at site D. Taxa tolerant to a range of salinities would occur throughout the estuary. Seven taxa were present at all four sites, two taxa were present at three sites while the remaining 25 taxa had a limited distribution.
4.4.2.5 The influence of human activities on the sediments and biota The lowest number of taxa and individuals occurred at site B. Site B was just downstream of the Chorlton Road bridge. There would have been considerable disturbance of the environment at this site during bridge construction (several bridges over time). More recently the sediments in this area have been disturbed and likely compacted by vehicles being driven on the mudflats (Figure 2-6). In addition, any water flowing through the culvert at the top of the shore, behind which is a tidal channel through grazed land, is a potential source of nutrients, micro-organisms and sediment to site B. That is, low number of taxa and individuals at site B could have been as a result of human activities in this area. Vehicle access to the mudflats is now a prohibited activity and the presence of signage of this prohibition near the bridge should reduce disturbance to this area of mudflat.
Recreational activities and in particular shellfish collecting and walking could have a significant influence on the cockle population in the lower reaches. The abundance of cockles in this area makes it attractive as a harvesting ground. However, excessive harvesting of the larger cockles will influence cockle population structure and reduce population density. A reduction in the adult population could over the longer term influence cockle recruitment. Furthermore, disturbance of the sediments through harvesting cockles will affect the presence and abundance of the other taxa living there.
4.4.3 Sediments and macrobiota The results indicate that the sediments and macrobiota of this estuary are typically in a healthy condition. It is not possible to give an overall condition rating to the estuary as there are no existing criteria for rating the overall condition of New Zealand estuaries.
5 Conclusions The 46 ha Okains Bay estuary is the larger of the two estuaries of Banks Peninsula. This estuary is the tidally influenced lower 2.8 km of the Opara Stream. The estuarine environment is characterised by sizeable areas of aquatic vegetation in the upper reaches and along the margins of the middle and lower reaches. Rushes, sedges and herbs were present with rushes in over half of the vegetated area. The considerable areas of aquatic vegetation are a significant ecological feature. Not only does the vegetation provide habitat and food for birds, fish and some invertebrates, it also stabilises the estuary margins, influences estuary hydrodynamics and traps sediment. The remaining estuarine area consists of the mud/sand and sand flats that flank the stream channel.
There were a range of sediment types within the estuary with the sediment changing from sandy mud to muddy sand with distance down the estuary, i.e. it was typically muddy in the upper reaches and sandy near the mouth. The sediment varied with distance down the shore typically being firm at the high shore and becoming softer with distance down the shore. The sediments within the estuary had a low organic matter content, low-moderate nutrient enrichment and low copper, chromium, lead, nickel and zinc concentrations.
Environment Canterbury Technical Report 33 Ecological status of Okains Bay estuary
Thirty four invertebrate taxa were found to live on and in the sediments of this estuary. These taxa were coelenterates (anemones), molluscs (shellfish and snails), annelids (worms) and arthropods (crabs, shrimps and hoppers). There were typically more taxa and individuals in the lower reaches than the middle and upper reaches of the estuary. Cockles were the most abundant taxa and occurred at all sites. However, they were most abundant in the lower reaches where they were found at densities up to 5,343/m2 with half of these small individuals, i.e. new recruits to the population. The sediment characteristics of TKN concentration, organic matter content, copper concentration, percent coarse silt and percent very fine sand accounted for 52% of the variability in the distribution and abundance of the invertebrate taxa. It is likely that salinity also has a considerable influence on the distribution and abundance of the invertebrate taxa.
The aquatic vegetation, mud/sand and sand flats are a habitat for fish, birds and invertebrates while the aquatic vegetation and invertebrates are a source of food for the bird and fish species that use the estuary. A number of bird species frequent the estuary while mullet and flat fish utilise, and whitebait/ īnanga and eels transit through, the estuary. In the past floundering was a common activity in the lower reaches but nowadays there are few adult flounder to be found. In season people fish for whitebait/ īnanga in the vicinity of the Schoolmasters bridge.
Okains Bay has been identified as the most popular bay in Banks Peninsula because of its outstanding recreational values, its cultural significance and its visual diversity. The estuary contributes to this popularity through the high recreational value of the lower reaches, its use on significant cultural occasions and its contribution to the visual diversity of the bay. In the lower reaches swimming, scavenging, kayaking, sunbathing and walking are popular activities. There is also potential for shellfish gathering in the estuary.
Okains Bay estuary is listed as an area of significant natural value and an area of high natural physical, heritage or cultural value. This study has identified that: 1. this estuary is the larger of the two estuaries of Banks Peninsula 2. there is a considerable area and diversity of aquatic vegetation and a variety of sand and mud flat habitats in this estuary. These provide habitats and food for birds, fish and a diversity and abundance of estuarine invertebrates. The work of Boffa Miskell (2007) has identified that: 3. the lower reaches have high recreational value. 4. the estuary is a contributor to the visual diversity of Okains Bay. 5. the estuary has cultural significance. These attributes support the classification of the estuary as an area of significant natural value and an area of high natural physical, heritage or cultural value.
In the last 150 years the Okains Bay estuary has been impacted by sedimentation, modification of the estuary margins, and it has potentially been supplied with considerable quantities of nutrients. That is, the present estuarine environment is not in a pristine state. It is likely that the size and depth of the present stream channel, the size and extent of the aquatic vegetation, mud and sand flats and the present day estuary margins reflect past impacts on this estuary. The present day estuary margins indicate reclamation of estuarine habitat for pasture and the modification of terrestrial vegetation from bush to pasture.
The present day impacts include unnatural disturbance particularly by vehicle use and high recreational use particularly over the summer months, nutrient inputs and sedimentation. It is possible that the low diversity and abundance of animals in the sediment at one of the fine scale sites is as a result of a range of unnatural disturbances at and adjacent to the site. While the issue of vehicle use has been addressed by the erection of signage prohibiting use, there is a need for vigilance to ensure that people do adhere to the prohibition. There is no information on the impact of high recreational use including shellfish gathering on the sediments and biota. The low-moderate nutrient enrichment of the estuary sediments and the occurrence of the diatom film within the channel and patches of Gracilaria chilensis indicate nutrient inputs to this estuary. Present day sedimentation is likely minor compared to historic sedimentation. However, there is potential for sediment inputs as a result of hillside erosion during heavy rainfall and future land use changes including subdivision development. That is, sedimentation is still an issue for this estuary. To ensure that the recognised values of the
34 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Okains Bay estuary are protected and if possible enhanced (Objective 6.1 RCEP) there are a number of issues that need to be addressed. These are the issues of nutrient enrichment, the altered estuary margins, sedimentation and unnatural disturbances.
6 Recommendations The following are the recommendations for future scientific monitoring and management.
6.1 Saltmarsh mapping To determine if the extent of the saltmarsh vegetation and the species composition of the saltmarsh is changing over time, mapping of the saltmarsh vegetation is recommended. Detailed quantitative mapping should be undertaken and then repeated every five years.
6.2 Broad-scale mapping of the non vegetated area To determine if the area of each of the sediment types, area of shellfish bed and area of macroalgae is changing over time, broad-scale mapping of the non-vegetated area is recommended. Detailed mapping should be undertaken and then repeated every five years.
6.3 Fine-scale monitoring To determine if the sediments and biota are changing over time it is recommended that fine scale annual monitoring is undertaken at site D. This monitoring will provide an indication of the nutrient status of the sediment, whether the grain size is changing, for example getting muddier, the state of the cockles and the general state of the estuarine biota. The following should be monitored at this site: • sediment grain size • sediment organic matter content • TN and TP concentrations • presence and abundance of the biota including the abundance and size of shellfish
6.4 Water quality monitoring To determine if the source of the nutrient enrichment of the sediment is the fresh water flowing into the estuary, routine monthly water quality monitoring for a year should be undertaken. The water samples should be analysed for the concentrations of nitrogen and phosphorus-based compounds.
6.5 Information Information about this estuary including its ecological values should be provided to residents and visitors. The local community, including the school children could become involved in the monitoring of the saltmarsh vegetation. At present such a monitoring programme is being developed for Brooklands Lagoon at the mouth of the Waimakariri River. This could provide a model for such work in Okains Bay estuary.
6.6 Estuary margins The margins of this estuary have been highly modified over time. These modifications have resulted in loss of marginal habitat and hence reduced the buffering capacity of the margins against the impacts of very high tides, storm events and sea level rise. The local community and ECan need to address the issue of the modified estuary margins. There is potential for small stretches of modified estuary margin to be returned to the natural state.
Environment Canterbury Technical Report 35 Ecological status of Okains Bay estuary
7 Acknowledgements The author wishes to thank Fay Farrant, Julie Edwards and Robyn Croucher for assistance with the collection of the samples. Thanks go to Fay Farrant for the sorting of macrobiota samples, the measuring of bivalves and data entry. The sediment grain size analyses were undertaken by Jennifer Jackson from the Geology Department of the University of Canterbury and the sediment organic matter content, metal and nutrient analyses were carried out by Paul Woods the senior chemist of the Environment Canterbury laboratory. This report was reviewed by Dr. Barry Robertson from Wriggle Consultants, Nelson. Thanks to Tim Davie from Environment Canterbury for editing this report.
8 References
ANZECC & ARMCANZ (Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand) 2000: Australian guidelines for water quality monitoring and reporting 2000.
Boffa Miskell, 2007. Banks Peninsula Landscape Study. Report prepared for the Christchurch City Council. 265pp.
Clarke, K. R., and Warwick, R.M. 2001. Change in marine communities: An approach to statistical analysis and interpretation, 2nd edition. PRIMER-E; Plymouth
EOS Ecology, 2007. Benthic Survey of the Avon-Heathcote Estuary:2007. EOS Ecology Report 06059-CCC05-01. 38pp.
Fenwick, G., Chagué-Goff, C., Sagar, P. 2006. Ashley Estuary; intertidal sediments and benthic ecology. NIWA Client Report CHC2006-076. 45pp.
Jacobson, H. C. 1976 (3rd ed.) Tales of Banks Peninsula 1840 -1940. Reprinted by Capper Press. Christchurch, New Zealand.
Jones, M. and Marsden, I.D. 2005. Life in the Estuary. Illustrated guide and ecology. Canterbury University Press. Christchurch, New Zealand. 179pp.
Kirk, A. D. 1978. Edward William Harris – pioneer, Okains Bay. Bascands Limited. Christchurch, New Zealand.
Robertson, B M, and Stevens, L. 2007. Major issues for New Zealand estuaries and selected case studies for state of environment report, unpublished, prepared for the Ministry for the Environment. Wellington: Ministry for the Environment.
Ogilvie, G. 1990. Banks Peninsula: Cradle of Canterbury. 3rd Ed. Philips and King Publishers. Christchurch, New Zealand.
Ogilvie, G. 1992. Picturing the Peninsula. Hazard Press Limited. Christchurch, Melbourne
RCEP (Regional Coastal Environment Plan for the Canterbury Region), 2005. Background Information, Issue Resolution and Processes and Monitoring. Volume 1. Environment Canterbury Report R04/13/1. 271pp.
Robertson, B., Gillespie, P., Asher, R., Frisk, S., Keeley, N., Hopkins,G., Thompson, S and Tuckey,B. 2002. Estuarine environmental assessment and monitoring: A national protocol. Part A. Development, Part B. Appendices, and Part C. Application. Prepared for supporting Councils and the Ministry for the Environment, Sustainable Management Fund Contract No 5096. Part A. 93p. Part B 159p. Part C. 40p plus field sheets.
36 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Robertson, B and Stevens, L. 2008. Porirua Harbour. Fine Scale Monitoring 2007/08. Report for Greater Wellington Regional Council and Porirua City Council. 32pp.
Stephenson, G. 1999. Vehicle impacts on the biota of sandy beaches and coastal dunes: a review from a New Zealand perspective. Department of Conservation Report, Wellington.
Stephenson, W. and Shulmeister, J. 1999. A Holocene progradation record from Okains Bay, Banks Peninsula, Canterbury, New Zealand. New Zealand Journal of Geology & Geophysics 42:11- 19.
Stevens, L. and Robertson, B. 2008. Porirua Harbour. Broad Scale Habitat Mapping 2007/08. Report for Greater Wellington Regional Council and Porirua City Council. 29pp.
Statistics New Zealand Te Tari Tatua, 2008 http://www2.stats.govt.nz/domino/external/web/commprofiles.nsf/htmldocs/Okains+Bay+Com munity+Profile
Environment Canterbury Technical Report 37 Ecological status of Okains Bay estuary
38 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Appendix I: Broad-scale habitat classification definitions (from Robertson et al., 2002)
Sedgeland Vegetation in which the cover of sedges (excluding tussock-sedges and reed-forming sedges) in the canopy is 20-100% and in which the sedge cover exceeds that of any other growth form or bare ground. “Sedges have edges.” Sedges vary from grass by feeling the stem. If the stem is flat or rounded, it’s probably a grass or a reed, if the stem is clearly triangular, it’s a sedge. Sedges include many species of Carex, Uncinia, and Scirpus. Rushland Vegetation in which the cover of rushes (excluding tussock-rushes) in the canopy is 20-100% and where rush cover exceeds that of any other growth form or bare ground. A tall grasslike, often hollow-stemmed plant, included in rushland are some species of Juncus and all species of Leptocarpus. Herbfield Vegetation in which the cover of herbs in the canopy is 20-100% and where herb cover exceeds that of any other growth form or bare ground.Herbs include all herbaceous and low-growing semi-woody plants that are not separated as ferns, tussocks, grasses, sedges, rushes, reeds, cushion plants, mosses or lichens. Mobile sand The substrate is clearly recognised by the granular beach sand appearance and the often rippled surface layer. Mobile sand is continually being moved by strong tidal or wind-generated currents and often forms bars and beaches. When walking on the substrate you’ll sink <1 cm. Firm sand Firm sand flats may be mud-like in appearance but are granular when rubbed between the fingers, and solid enough to support an adult’s weight without sinking more than 1-2 cm. Firm sand may have a thin layer of silt on the surface making identification from a distance difficult. Soft sand Substrate containing greater than 99% sand. When walking on the substrate you’ll sink >2 cm. Firm mud/sand A mixture of mud and sand, the surface appears brown, and may have a black anaerobic layer below. When walking you’ll sink 0-2 cm. Soft mud/sand A mixture of mud and sand, the surface appears brown, and many have a black anaerobic layer below. When you’ll sink 2-5 cm. Very soft mud/sand A mixture of mud and sand, the surface appears brown, and many have a black anaerobic layer below. When walking you’ll sink >5 cm. Cockle bed Area that is dominated by both live and dead cockle shells. Artificial structures Introduced natural or man-made materials that modify the environment. Includes rip-rap, rock walls, wharf piles, bridge supports, walkways, boat ramps, sand replenishment, groynes, flood control banks, stopgates.
Environment Canterbury Technical Report 39 Ecological status of Okains Bay estuary
Appendix II: Description of key estuary habitat features (from Stevens and Robertson, 2008)
SALTMARSH HABITAT Description: A salt marsh is classified as being the intertidal area of fine sediment that has been transported by water and is stabilised by vegetation. Extensive saltmarshes tend to be present if the coastal plain is gently sloping and wide. In general, marsh grasses cannot survive below mean tide level (the midway point between MLW and MHW) and are outcompeted by terrestrial plants above spring high tide. Importance: Saltmarsh is one of the most productive environments on earth, and serve as important nursery grounds and wildlife habitat. They provide nutrients to surrounding areas, fuelling other marine food webs. These dynamic ecosystems provide tremendous additional benefits for humans including flood and erosion control, water quality improvements, opportunities for recreation and for atmospheric gas regulation - estuaries tend to be “carbon sinks,” since carbon dioxide is absorbed in the photosynthesis carried out by the prolific plant growth. Threats: Tidal salt marshes have the ability to respond rapidly to physical stressors, and their condition is often a dynamic balance between relative sea level rise, sediment supply and the frequency/duration of inundation. However, if sea level rises too much, or the sediment supply or inundation through flooding is excessive, then the balance can be upset and the saltmarsh is lost or its condition deteriorates. This balance varies between different types of estuaries but their response centres around how each reacts to sediment inputs and inundation (the latter is particularly important in face of predicted accelerated sea level rise through global warming). Saltmarsh is also vulnerable to increased nutrient inputs, particularly nitrogen. Added nutrients stimulate saltmarsh growth but, if excessive, may lower dissolved oxygen levels, change food web dynamics, alter community composition and stimulate the growth of algae and weeds.
MUD HABITAT Description: Mud flats are areas of unconsolidated fine-grained sediments that are either unvegetated or sparsely to densely vegetated by algae and/or diatoms. They are found in sheltered environments and support high biodiversity (snails, crabs, burrowing polychaete worms, shellfish and other macroinvertebrates). Most of the organisms inhabit the upper 10cm, because below that level, mud often becomes anoxic (low in oxygen or oxygen depleted). To adjust to these harsh physical conditions, many organisms build and maintain burrows or tubes to access oxygen in the air or water, or have adaptations such as siphons. Importance: They provide a number of important ecosystem services including; primary and secondary production; habitat for polychaetes, crustaceans, flatfish and shellfish; refuge and nursery habitat for juvenile fish; and interception, uptake and processing of nutrients and contaminants from watershed drainage. Bacteria living in the sediments of estuaries can also help to break down certain pollutants. Threats: The major threats are from agricultural and urban development and include: excessive sedimentation leading to infilling, contamination with toxicants and disease causing microbes, reclamation and drainage, building of structures, and spread of introduced species, e.g. Pacific oyster.
SAND HABITAT Description: This habitat includes both dune areas near the mouth as well as extensive areas of sand flats in the estuary (these often include a mud or silt component and shell fragments) and sandy channel areas. In these highly dynamic environments, sand is moved by tides, winds, and storm surges, and this movement is responsible for shaping these habitats. Sand flats typically occur in higher energy areas than mud flats where the substrate is predominantly sand and is exposed to sorting from wave and current action. Importance: Sand habitat tends to be the area most intensively used by humans for recreation. Shellfish, polychaetes, crustaceans and young fish are typical animals that inhabit sand flats. Sand channels generally occur in open, deeper areas where channels form. These open areas are typically
40 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
inhabited by bivalve shellfish, polychaetes, young flat fish, and sand loving algae. They are also important for provision of refugia and food for anadromous, resident, and marine fishes, and transport of sediments. Threats: Major threats are excessive sedimentation leading to muddy sediments and/or infilling, contamination with toxicants and disease causing microbes, reclamation and drainage, building of structures, and spread of introduced species. Trampling and grazing of dune vegetation can lead to dune demise. Off-road vehicles threaten sandy beach and sand flat inhabitants by compacting the sand, making burying and burrowing more difficult. These vehicles can also crush organisms that live just below the surface, and disturb crabs and nesting birds.
SHELLFISH BEDS Description: In dense groupings, bivalve molluscs (e.g. mussels, cockles, oysters and pipi), form a habitat type known as shellfish beds. Small organisms, such as polychaete worms, juvenile crabs and snails find refuge in the spaces between the shells, while other organisms attach to the shells’ hard surfaces, which provide an anchor unavailable in the surrounding soft sediments. Each species of bed-forming shell-fish has different habitat requirements, which means that shellfish beds can be found in a range of depths, salinities, or substrates (surfaces, such as sand, rock, or mud). Importance: Humans, crabs, fish, and seabirds all consume large quantities of shellfish. For coastal residents and tourists, collecting shell-fish is an important pastime. Through filter-feeding, shellfish improve water quality by removing suspended material and particulate pollutants from the water column. Shellfish beds also provide an important link between benthic (bottom) and pelagic (open water) habitats by capturing small food particles from the water column and transferring them to the benthos. Threats: Intensification of landuse and excessive runoff of nutrients, sediment, pathogens and toxicants represent the largest threat to nearshore shellfish beds, through diminished water quality. Increased temperature through global warming is another significant threat. Overfishing of shellfish can also diminish their filtering function, potentially leading to increased turbidity (cloudiness due to sediments or other substances in the water) and diminished light penetration to the seafloor.
WATER COLUMN Description: The water column is a dynamic environment subject to waves, currents, tides, and riverine influences. In New Zealand estuaries it is generally well supplied with sunlight and consequently phytoplankton (tiny plants suspended in the water column) are major primary producers. Phytoplankton include a wide range of species, but are generally dominated by diatoms in healthy waters. The water column also includes a variety of animal life including; zooplankton (tiny animals suspended in the water column), fish and jellyfish. Importance: Human use of the water column is high with swimming, boating and fishing all popular activities. The water column has an important role in nutrient, dissolved oxygen, salinity, and contaminant concentrations and distribution, as well as fish and shellfish habitat. Threats: Non-point source pollution is currently the greatest threat to estuary water quality. Global climate change, and the associated change in weather and current patterns, pose another threat to water column habitats.
Environment Canterbury Technical Report 41 Ecological status of Okains Bay estuary
Appendix III: Co-ordinates of the sites and location of the stations at each site
Co-ordinates for the seaward, upper shore corner of the 50 m by 30 m area at each site.
Easting Northing
Site A 2513440 5722438
Site B 2513755 5722799
Site C 2514085 5723229
Site D 2514398 5723659
50 m Location of sampling stations at site A 15 14 13 12 11
30 m 109876
54321
CHANNEL sea
Location of sampling sea stations at sites B, C CHANNEL and D
12345
30 m 678910
11 12 13 14 15
50 m
42 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Appendix IV: Sediment grain size analysis – Methodology
The following steps were involved in the grain size analysis of each sample.
1. Cone & Quarter 2. Wet Sieve and picking of fraction >0phi. This fraction was then dried and weighed 3. The remains of the sample was treated with 5-20% of 34% H2O2 in distilled water 4. The sample was dried. 5. Cone & Quarter the dried sample 6. The sample was analysed using a Saturn 5200 DigiSizer
DigiSizer specifications are available at: http://www.pss.aus.net/products/micromeritics/equip_particle_size/5200/5200.html#specs)
Environment Canterbury Technical Report 43 Ecological status of Okains Bay estuary
Appendix V: Sediment grain size of each sample at each site
Very coarse Coarse Medium Very fine Medium Very fine Granule sand sand sand Fine sand sand Coarse silt silt Fine silt silt Clay site and station%%%%%%%%%%%
A1 0.00 0.79 0.00 1.52 18.56 15.08 18.59 20.28 9.51 6.73 8.96 A2 2.25 0.54 0.00 1.90 18.16 14.27 16.60 18.94 9.82 7.34 10.17 A4 1.82 0.81 0.00 3.03 15.26 12.44 16.11 19.61 11.11 8.58 11.24 A7 0.00 0.46 0.00 0.31 24.27 18.66 16.89 16.55 8.53 5.91 8.42 A9 0.00 0.65 0.00 7.11 17.02 15.77 17.29 18.31 9.11 6.20 8.54 A10 0.09 0.73 0.00 1.05 18.48 14.91 16.25 18.79 10.76 8.01 10.92
A11 0.00 0.28 0.00 2.16 22.07 24.35 18.34 14.38 6.99 4.88 6.56 A13 0.00 0.07 0.00 0.33 17.89 26.68 20.40 15.27 7.26 4.95 7.16 A15 0.00 0.00 0.00 0.81 21.40 23.48 21.38 15.40 6.91 4.42 6.21 B1 0.00 0.00 0.00 1.76 41.53 11.23 10.22 14.58 7.97 5.38 7.32 B2 0.00 0.00 0.00 3.05 29.23 11.53 10.90 16.02 10.39 7.81 11.08 B5 0.00 0.00 0.00 0.66 25.44 9.98 14.86 21.16 10.78 7.43 9.69
B6 0.00 0.00 0.00 0.25 33.93 15.28 13.40 14.97 8.23 5.69 8.25 B7 0.00 0.00 0.00 0.73 37.44 12.59 12.04 14.78 8.19 5.69 8.53 B8 0.00 0.00 0.00 1.58 31.87 10.03 10.17 16.74 10.51 7.79 11.32 B12 0.00 0.00 0.00 2.12 36.71 12.47 11.22 14.57 8.52 5.95 8.44 B13 0.00 0.00 0.00 1.45 39.09 12.10 11.32 14.52 8.12 5.63 7.77 B15 0.00 0.00 0.00 0.76 36.04 11.49 11.57 16.47 9.19 6.18 8.31
C1 0.00 0.00 0.00 2.37 57.64 12.13 6.19 7.57 4.73 3.91 5.47 C2 0.00 0.00 0.08 7.01 52.14 12.71 6.55 7.37 4.63 3.86 5.65 C3 0.00 0.00 0.00 2.94 45.19 10.75 8.53 11.12 7.01 5.92 8.52 C6 0.00 0.00 0.00 0.52 26.42 9.40 14.24 19.67 11.20 8.27 10.28 C8 0.00 0.00 0.00 3.38 29.32 8.93 11.89 17.75 10.52 7.99 10.21 C10 0.00 0.00 0.00 3.14 29.30 9.66 12.19 17.70 10.45 7.81 9.76
C11 0.00 0.00 0.00 0.49 27.69 8.02 12.22 19.11 11.93 9.13 11.40 C12 0.00 0.00 0.00 1.17 33.68 8.24 11.17 17.96 10.50 7.69 9.59 C15 0.00 0.00 0.00 1.29 31.40 7.64 11.10 18.65 11.17 8.32 10.42 D1 0.00 0.00 0.00 0.98 39.79 10.61 8.99 14.07 9.26 7.31 8.99 D3 0.00 0.00 0.00 0.33 57.08 12.29 5.41 8.60 5.86 4.64 5.81 D5 0.00 0.00 0.01 3.23 67.11 11.92 2.76 4.64 3.59 2.99 3.76
D6 0.00 0.00 0.00 1.73 57.41 10.76 4.25 7.70 6.13 5.15 6.87 D7 0.00 0.00 0.00 4.02 64.75 12.51 3.36 5.08 3.61 2.91 3.77 D10 0.00 0.00 0.01 0.95 69.45 12.58 2.23 4.10 3.45 2.97 4.27 D11 0.00 0.00 0.00 3.40 47.38 7.72 6.45 11.22 8.18 6.83 8.82 D13 0.00 0.00 0.02 4.98 66.83 12.02 2.54 3.75 3.01 2.70 4.15 D15 0.00 0.00 0.00 7.38 66.99 12.24 2.46 3.42 2.41 1.96 3.14
44 Environment Canterbury Technical Report Ecological status of Okains Bay estuary
Appendix VI: Sediment quality data of each sample at each site
LOI Cu Cr Ni Pb Zn TKN TP site and station % mg/kg mg/kg mg/kg mg/kg mg/kg mg/L mg/L A1 3.1 6.6 18 8.5 17 54 920 740 A2 2.6 6 18 7.7 15 52 910 720 A4 3 7 19 8.7 16 57 1200 720 A7 1.8 5.5 17 7.7 16 51 900 700 A9 2.5 5.9 17 8 14 53 860 730 A10 2.1 5.3 16 8 14 50 850 700 A11 1.7 5.4 15 7.2 15 48 780 670 A13 1.6 5.1 15 7.7 16 47 760 710 A15 1.4 5 14 6.1 16 45 780 690 B1 1.7 5.9 17 9.4 20 53 890 790 B2 1.6 5.9 17 10 21 53 890 800 B5 1.7 6 17 9.7 21 53 800 780 B6 1.5 5.2 16 7.6 16 47 790 780 B7 1.5 5.5 17 7.6 17 50 760 770 B8 1.6 5.6 18 9.2 17 52 830 780 B12 1.5 5.2 18 8.3 17 49 730 800 B13 1 5.2 18 7.4 19 49 740 790 B15 1.5 5.4 19 8.5 20 51 850 790 C1 0.8 7.1 20 10 22 55 1100 850 C2 0.8 7.1 20 9.6 23 56 1000 860 C3 1.5 7.2 20 11 23 56 870 820 C6 1.6 6.7 19 10 21 56 880 820 C8 3.2 7.1 19 9.9 24 57 940 840 C10 1.7 6 17 9.6 20 51 780 820 C11 1.8 7.3 22 9.2 20 58 840 810 C12 1.6 7.2 21 9.5 20 57 810 810 C15 1.8 7.1 23 10 20 56 870 850 D1 1.8 7.3 22 11 22 57 830 870 D3 1.5 7.5 22 11 23 57 890 860 D5 1.1 5.4 17 7.9 15 41 940 850 D6 1.4 7.2 22 11 17 57 850 860 D7 1.4 7.2 21 12 20 58 920 880 D10 1.6 7.7 22 12 23 60 1100 870 D11 1.4 7.2 22 11 22 59 980 840 D13 1.2 7.2 23 11 21 58 1100 860 D15 0.5 7.6 24 11 25 56 920 870
Environment Canterbury Technical Report 45 Sediments and macrobiota of Brooklands Lagoon
Appendix VII: Total number of individuals of each taxa in all cores from a site
Taxa\Site Site A Site B Site C Site D Edwardsia sp. 10054 Arithritica bifurca 14 0 8 19 Austrovenus stutchburyi 11 10 111 818 Mactra ovata 2000 Paphies australe 0009 Paphies subtriangulata 00118 Tellina liliana 001010 Amphibola crenata 1831 Cominella glandiformis 1161 Notoacmea helmsi 0011 Potamopyrgus estuarinus 26000 Aonides sp. 00755 Aquilospio sp. 0091 Boccardia syrtis 00209 Capitella sp. 15 2 2 21 Glycera ovigera 0020 Glycera sp. 0002 Heteromastus filiformis 1050 Perinereis ?brevicirris 0001 Scolecolepides benhami 16 19 34 421 Scolelepis sp. 0005 Unidentified oligochaetes 61 0 0 0 Unidentified nemertine 0 0 0 2 Insect pupa 1 0 0 0 Amphipod sp.A122411 Amphipod sp.B 0 1 0 0 Amphipod sp.C 0 0 0 1 Corophid sp.1000 Isopod sp.A 0 1 0 3 Helice crassa 156215 Halicarcinus sp. 1000 Macrophthalmus hirtipes 0043 Elminius modestus 0013 Unidentified mysid 0 7 0 0 Pelotretis flavilatus 0300 Peltorhamphus latus 0013
46 Environment Canterbury Technical Report