You are here: Environmental reporting 
Fresh water Lake water quality
Factors affecting lake water quality
The nutrient status of lakes is strongly related to their depth and the types of land use and human activities in the catchment.
Lake depth
Deep lakes have a greater capacity to absorb incoming nutrients before showing definite signs of deterioration in water quality. The monitored lakes that have high levels of nutrients tend to be shallow. The monitored lakes with the lowest levels of nutrients are nearly all deep lakes in mountain country in the South Island (for example, Lakes Coleridge, Pūkaki, Wānaka and Tekapō) and do not have particularly intensive farming or urban activity in their catchments.
Land use
Levels of nutrients (nitrogen and phosphorus) and algae tend to be higher in lakes in pastoral catchments than in lakes in natural catchments.
Because algal concentrations affect water clarity, the lakes in natural catchments have water that is clearer than water in lakes in pastoral catchments. For example, lakes in the mountainous terrain of the South Island commonly have underwater visibility for more than 10 metres, although this is lowered naturally in some cases by tannins (pigments) leaching from beech forests or by fine glacial sediment.
Many lakes that lie in intensively used catchments are the subject of management programmes to stem the inflow of pollutants from the surrounding land.
The following two graphs use the most recent data available to show the similarities in patterns for both water quality and ecological condition.
Nutrient level (Trophic Level Index) in lakes with different land cover types
Read a text description of the chart
The Trophic Level Index (TLI) uses six categories from microtrophic (very low nutrient levels) to hypertrophic (very high nutrient levels). The bar graph shows the number of lakes in each trophic category.
| Land Cover | Microtrophic TLI 1-2 |
Oligotrophic TLI 2-3 |
Mesotrophic TLI 3-4 |
Eutrophic TLI 4-5 |
Supertrophic TLI 5-6 |
Hypertrophic TLI >6 |
Total |
|---|---|---|---|---|---|---|---|
| Alpine | 2 | 1 | 3 | ||||
| Exotic | 1 | 7 | 9 | 1 | 18 | ||
| Native | 5 | 16 | 12 | 5 | 1 | 39 | |
| Pastoral | 1 | 7 | 10 | 10 | 12 | 10 | 50 |
| Urban | 1 | ||||||
| nd | 1 | ||||||
| Total | 8 | 25 | 30 | 25 | 13 | 11 | 112 |
| Total % | 7 | 22 | 27 | 22 | 12 | 10 |
nd= land cover is not determined
Ecological condition (LakeSPI) in lakes with different land cover types
Read a text description of chart
LakeSPI uses five different categories ranging from non-vegetated to excellent. The bar graph shows the number of lakes in each category.
| Land cover | Excellent >75% |
High 50-75% |
Moderate 20-50% |
Poor >0-20% |
Non-vegetated 0% |
Total Lakes |
|---|---|---|---|---|---|---|
| Alpine | 1 | 1 | ||||
| Exotic | 6 | 6 | 10 | 1 | 2 | 25 |
| Native | 8 | 11 | 18 | 14 | 4 | 55 |
| Pastoral | 8 | 7 | 18 | 6 | 27 | 66 |
| Urban | 1 | 1 | 2 | |||
| nd | 3 | 1 | 2 | 6 | ||
| Total | 23 | 28 | 47 | 21 | 36 | 155 |
| Total % | 15 | 18 | 30 | 14 | 23 |
nd= land cover not determined
Other factors affecting lake water quality
Natural factors such as air temperature and wind are also important determinants of water quality in lakes. Algal blooms are more likely to occur in lakes in warmer climates (those at lower elevations and in the north) and in the summer. Wind can create waves and currents, particularly in shallow lakes, which lift sediments from the lake bed into the water. As well as reducing the clarity of water, this can increase the amount of nutrients available for algal growth. Clarity and the appearance of lake water may be affected by soil type. For example, lakes surrounded by peaty soil, such as those commonly found in Westland and Waikato, have water that is naturally brown-stained or ‘dirty’ looking.
This information has come from the latest state of the environment report, Environment New Zealand 2007 and Lake water quality in New Zealand 2010: Status and trends
Return to the environmental indicators for lakes page
Last updated: November 2010
