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Lake water quality

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. Councils currently monitor the nutrient status of about 120 lakes around the country (although there are 134 lakes for which recent nutrient data are available). These lakes are typically sampled monthly or four times a year. About half of all the lakes monitored are shallow (less than 10 metres deep) and about half are smaller than 50 hectares.

Of the 229 lakes in New Zealand that are larger than 50 hectares, about one-third are monitored. 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.

Environmental indicator

The Trophic Level Index (TLI) is used to report on the lake water quality indicator. In New Zealand, the TLI is widely used to measure changes in the nutrient (trophic) status of lakes. This index considers:

  • phosphorus levels
  • nitrogen levels
  • visual clarity
  • algal biomass.

The TLI incorporates both particulate and dissolved nitrogen (nitrate) and phosphorus in the water, which 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 additional measurements can be made, such as 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.

Did you know?

  • New Zealand has 3820 lakes that are larger than 1 hectare. Of these, 229 are larger than 50 hectares.
  • Two-thirds of New Zealand's lakes are in natural or partially developed catchments, such as native bush, and are likely to have good to excellent water quality.
  • About 40 per cent of all lakes are in catchments in which the predominant land cover is pasture and about 2 per cent of lakes are in towns and cities. These lakes commonly have degraded water quality.

 


Information on the limitations of the lake water quality indicator.

This information has come from the latest state of the environment report, Environment New Zealand 2007.

 

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Last updated: December 2007