Water quality in the Waitaki Lakes and the Waitaki River
Graeme Clarke
Topics
• Nutrients? • Nutrient uptake in lakes • Waitaki River - current state • Summary
• Lakes –Focus on total nutrients (TN,TP) –Includes nutrients bound in phytoplankton • Rivers –Focus on dissolved plant available nutrients (DIN,DRP) –Very little / no phytoplankton –DIN and DRP drive benthic algal growth Phytoplankton – convert dissolved plant available nutrients to organic matter
Benthic algae – take up nutrients less efficiently Influence of lake residence time on algal response
57 days
75 days 14 days
1.1 days
Longer residence time = greater uptake of nutrients Lake Benmore at dam 0-10m deep. Nitrogen concentrations 350
300
250 Lake 200 DIN 150 Lake
100 TN
50
0
Low concentrations of plant available nutrients – most converted to phytoplankton Lake Benmore at dam 0-10m deep. Phosphorus concentrations 12
10
8
6
4 Lake DRP
2 Lake TP
0
Low concentrations of plant available nutrients – most converted to phytoplankton Lake Benmore at dam 60m deep. Nitrogen concentrations
140
120
100 Lake DIN 80 Lake TN 60
40
20
0
More DIN at greater depth – nutrient cycling and less assimilation/uptake by phytoplankton 30
25 Lake 20 DIN
15 River DIN 10
5
0
Lake Benmore at dam and upper Waitaki River DIN concentrations roughly correlated 1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
DissolvedInorganic Nitrogen (mg/L) 0.2
0.0
Padkins Strm
Waitaki River Kurow Awakino River@SH83 Waitaki River above SH1 Penticotico Stream@SH83 Hakataramea River Main Rd Otiake River@Mt Bell Station Hakataramea River Cattle Creek Maerewhenua River@Duntroon Otekaieke River D/S of Bushy Crk Otekaieke River Eastern Road Bridge Currently very low concentrations of DIN in upper Waitaki River • Current nutrient concentrations in upper Waitaki River are very low • A more productive lake system is likely to contribute more nutrients to the Waitaki River but… • Nutrient discharge from lake to the river can be limited by managing lake state • Uncertainties……….
What if nutrient loads into the upper Waitaki River increased significantly?… • Increased risk of ‘non-didymo’ nuisance algae growths in Waitaki River – less assimilative capacity (possible implications for North Bank tunnel) • Less effective augmentation of Wainono Lagoon
Summary • Current river nutrient concentrations in upper Waitaki River very low. • Lake and river nutrient concentrations related. • Discharge of nutrients into upper Waitaki River from lake can be managed by managing Lake Benmore state (TLI limits)