The long-term monitoring of lakes in New Zealand is undertaken primarily by regional councils, but also by the National Institute of Water and Atmospheric Research (NIWA). Councils currently monitor the nutrient status of about 120 lakes around the country (although there are 135 lakes for which recent nutrient data are available).
These lakes are typically sampled monthly or four times a year. The sample is not representative of all lakes in New Zealand, with a bias towards monitoring large, deep, low altitude lakes.
Many of the large unmonitored lakes are in national parks (such as Fiordland) or are reservoirs for hydro-electric power generation. Regular monitoring is not regarded as being necessary at these lakes because they generally do not experience the same land-use pressures as lakes in developed catchments, and changes to the quality of their water are expected to be much slower. In addition, access to the more remote high-altitude lakes means that regular monitoring is impractical.
The Trophic Level Index (TLI) is used as an environmental indicator to report on lake water quality and to measure changes in the nutrient (trophic) status of lakes. This index considers:
Nitrogen and phosphorus inputs to lakes may be natural or caused by humans. Fertilizer run-off and sewage effluent are two examples of man-made inputs. These nutrients cause excessive plant and algae growth, leading to reduced visual clarity.
The TLI incorporates both particulate and dissolved nitrogen (nitrate) and phosphorus in the water. This is important because water generally remains in lakes for long periods, so that even particulate nutrients can eventually be used to boost growth in aquatic plants.
While TLI is perhaps the most important indicator of the impact of land use on lake water quality in New Zealand, many other measurements can be made. These include suspended solids and pH, and the occurrence and abundance of fish and aquatic plants.
Bacteria levels are also monitored in some lakes throughout New Zealand over the summer to report on recreational water quality.
LakeSPI (Submerged Plant Indicators) is an indicator which is being increasingly adopted in lake monitoring programmes to assess ecological quality. Features of aquatic plant structure and composition are used to assess both the native and invasive character of vegetation in a lake.
The results from TLI and LakeSPI are complementary in that TLI is a more direct assessment of water quality, whilst LakeSPI provides additional information about the submerged plant community.
Last updated: November 2010