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Trophic Level Index for lakes

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In New Zealand, the Trophic Level Index (TLI) is widely used to measure changes in the nutrient (trophic) status of lakes in New Zealand. This index considers phosphorus and nitrogen levels, as well as visual clarity and algal biomass.

Current situation

Seventy-five of the 134 lakes for which nutrient data is available have high to very high levels of nutrients. Thirteen per cent of the 134 lakes are known as ‘hypertrophic’, meaning they are saturated with nutrients and their water quality is extremely degraded. In such lakes, algal blooms are common and the health of aquatic animals is often at risk. While some recreation may take place on the surface of these degraded lakes (such as sailing), activities such as swimming are restricted because of the lakes’ prolific weed growth and poor water clarity.

Trophic (nutrient) level of 134 monitored lakes, data compiled in 2006 using the most recent results available

Trophic (nutrient) level of 134 monitored lakes, data compiled in 2006 using the most recent results available

Trophic (nutrient) level of 134 monitored lakes, data compiled in 2006 using the most recent results available

 

 

 

 

 

 

Data (text description) for graph and map

 

 

 

 

 

 

 

Data source: Ministry for the Environment.

 

The nutrient status of lakes is strongly related to their depth and the type of land use and human activity in the catchment. Natural factors such as air temperature and wind are also important determinants of water quality in lakes.

 

An estimate of water quality in unmonitored lakes

The majority of the 3820 lakes greater than 1 hectare are not monitored. However, we can estimate their condition using the results for monitored lakes as well as other factors such as catchment land use, climate and lake depth. It is estimated that:

  • the majority (about two-thirds) of all lakes are likely to have relatively low concentrations of nutrients and good to excellent water quality because they lie in natural, or only partially developed, catchments
  • the remaining lakes are likely to have high levels of nutrients and poor water quality.

 


Recent trends

Trends in water quality have been assessed for 49 lakes. The levels of nutrients in most of these lakes have shown no signs of change since 1990. Ten of the 49 lakes show possible or definite signs of deterioration (that is, an increase in nutrient or algae levels or a decrease in visual clarity), and six show signs of improvement. Many of the lakes showing signs of deterioration are already moderately nutrient-enriched (meso-eutrophic) and lie in largely developed catchments (for example, Waikere in Northland, and Waikare and Rotomanuka in the Waikato).

Trends in trophic (nutrient) level of 49 monitored lakes, 1990–2006 (data compiled in 2006)

Trends in trophic (nutrient) level of 49 monitored lakes, 1990–2006 (data compiled in 2006)

 

 

 

 

 

Data (text description) for graph

 

 

 

 

 

Data source: Ministry for the Environment.

 

What can you do?

  • Fence off the banks of streams and river banks that drain into lakes, as well as the shores of the lakes themselves.
  • Planting native vegetation or other plants enhances water quality by filtering sediment, faecal bacteria and nutrients from surface water run-off.
  • Farmer? Prevent stock from entering rivers, lakes and wetlands.
  • For more advice on lake and wetland protection contact your local or regional council or the New Zealand Landcare Trust.
  • Boat owner? Before entering a lake or river, remove all weeds, flush out jet units and wash your boat down to avoid introducing new aquatic pests into the waterway. Visit www.biosecurity.govt.nz for more information on stopping the spread of pest species.

 

 


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

 

 

 

Last updated: December 2007