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Nutrient enrichment of shallow lakes

Photograph of Lake RotopikoWhy we monitor nutrient enrichment of shallow lakes

There are over 100 lakes in the Waikato region, which vary greatly in their physical, chemical and biological characteristics. The majority of the region’s lakes are shallow and less than 10 hectares in size.

Many of these shallow lakes are valuable conservation refuges for unique plant and animal species. They are valued for their unique genetic diversity, cultural and spiritual importance, scientific interest, recreational opportunities and intrinsic value.

This indicator focuses on nutrient enrichment (which influences trophic state) as a measure of water quality in shallow lakes. Monitoring nutrient enrichment in shallow lakes tells us about the ability of a lake to support native plant and animal life and identifies long-term trends in water quality. This information helps us develop management responses to avoid or reverse adverse affects on shallow lake water quality.

The indicator also tells us about the presence of potentially harmful blue-green algae in certain lakes.

What's happening?

Shallow lakes are defined as generally:

  • having an average depth of less than three metres
  • being able to support submerged aquatic plants over large areas of the lake bed
  • not being stratified - their shallow depth means the lake’s water is stirred up regularly due to wind and wave action.

The Waikato region’s shallow lakes are generally nutrient enriched. They have high levels of nutrients such as phosphorus and nitrogen. The amount of nutrients entering a lake from its catchment mainly determines its trophic state. Nutrient enrichment results in poor water quality and a high trophic state.

Most of the 12 shallow lakes monitored are highly to extremely nutrient enriched (a trophic status of eutrophic, supertrophic or hypertrophic). They have high nutrient levels and poor water clarity. In contrast lakes with low-to-moderate nutrient levels and clear water are classed as oligotrophic or mesotrophic.

Increasing nutrient enrichment or ‘eutrophication’, results from run off and leaching of contaminants such as effluent, fertiliser and sediment from land use in a lake's catchment. Nutrients can also be recycled from the bottom sediments of shallow lakes, adding to the levels found in the overlying water. Farmland now surrounds most shallow lakes in the region.

>>Find out more about these data and trends

More information

More detail on this indicator, including how and where Waikato Regional Council  collects this information, is available in the Technical Information page.

Useful links

See the Ministry for the Environment’s performance indicators for lakes.

The Ministry for the Environment website also has information on a case study on the Lake Ngaroto restoration project undertaken by the Waipa District Council.

 Documents available from Waikato Regional Council

You can order any of these documents from our library. Most documents will incur a charge.

Barnes, G.E. 2002: Trends in the water quality of selected shallow lakes in the Waikato Region, 1995-01. Environment Waikato Technical Series 2002/11.

Burns, N.M.; Rutherford, J.C. and Clayton, J. S. 1999:  A monitoring and classification system for New Zealand lakes and reservoirs.  Journal of lake and reservoir management 15:  255–271.

Hamilton, D. P.; Vant, W. N. and Neilson, K. A. 2010:  Lowland lakes.  In: Collier, K. J.; Hamilton, D. P.; Vant, W. N. and Howard-Williams, C. (eds), The Waters of the Waikato:  ecology of New Zealand’s longest river. Environment Waikato, Hamilton. pp. 245–264.

When this indicator is updated

This indicator is updated every two years.

Contact at Waikato Regional Council

Water quality scientist - Science and Strategy Directorate