Lake Brunner water quality
A project funded by West Coast Regional Council has increased our knowledge of the role of phosphorus in determining water quality in Lake Brunner.
Lake Brunner is a large (41 km²), deep (maximum depth 109 m) lake in the northwestern part of the South Island. Its water quality is high and it is valued for its trout fishing.
Development of dairy farms in the area and increased pasture drainage has led to concern that its water quality will decline. Therefore, it is important to know more about the natural variability of the water quality in the lake, and whether the lake's water quality may have been changing in recent years.
NIWA scientists have examined changes in water quality in Lake Brunner since the early 1990s, and the effects of nutrients carried in from the surrounding land on the lake's water quality. Water samples have been collected from the lake almost monthly since 1992 by NIWA and West Cost Regional Council and analysed for concentrations of nutrients and chlorophyll a (a measure of algal biomass).
Water samples from the three main inflows of the lake have also analysed for nutrient concentrations.
In addition, Secchi Disc depth (a measure of water clarity) was measured, and oxygen and temperature profiles were recorded from the surface to the bottom of the lake. This work shows how important it is to monitor water quality in lakes, before problems with increasing algal productivity through deteriorating water quality arise.
There has been a decrease in Secchi Disc depth between 1992 and 2011, corresponding with an increase of chlorophyll a and the concentration of nutrients measured at the lake's surface. The depletion of oxygen in the bottom water during the summer (a measure of algal productivity) increased since 1992. Together these changes indicate increased algal growth in the lake caused by increased amounts of nutrient washing in from the surrounding land. The critical mean Total Phosphorus concentration in the lake inflows that will cause the lake to shift from its current oligotrophic state to a more enriched, or mesotrophic state, is 22.5 mg m-3; about 1.7 times the present rate of phosphorus loading.
The two major streams draining dairy farms in the area are more phosphorus-rich than the other inflows, and contribute 85% of the lake's total phosphorus load. Hence managing the phosphorus in the runoff from dairy farms is important for controlling algal productivity and the water quality of Lake Brunner.
A monitoring study has been underway in one of the dairying sub-catchments of the Lake Brunner catchment (Inchbonnie) since 2004. Results from that study have shown very high levels of runoff of phosphorus and nitrogen going towards the lake: they are well above average for intensively farmed land elsewhere in New Zealand. These runoff levels are undoubtedly driven by the very high rainfall in the area (on average 4.7 m per year). Future work in the Lake Brunner catchment is focusing on ways to mitigate phosphorus runoff from dairy farming.
Lake Brunner is oligotrophic (relatively pristine) because algal productivity is strongly limited by the availability of phosphorus throughout the year – this availability is indicated by the ratio of nitrogen to phosphorus in the lake water (N:P).
The lake has a relatively long residence time (the time taken to replace the volume of the lake with current inflows) of 1.14 years. This enhances the ability of the lake to keep nutrients. The present volume-weighted mean concentration of Total Phosphorus in the inflows is estimated at about 13 mg m‑3, slightly more than twice the concentration in the lake. The lake therefore retains 50 to 55% of phosphorus transported from the surrounding areas, through burial in the sediments. In contrast, 30% of nitrogen is retained in the lake through burial, or removed by denitrification (the conversion of nitrate to nitrogen gas by bacteria).