Water quality indicators for lakes

Trophic Level Index (TLI)
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.

LakeSPI
The LakeSPI Index uses Submerged Plant Indicators (SPI) to assess the ecological condition of New Zealand lakes.  Key features of aquatic plant structure and composition are used to capture both the native and invasive character of vegetation in a lake.

Current situation

Nutrient level (Trophic Level Index)

Of the monitored lakes, 112 have nutrient data between 2005 and 2009 available.  Of these:

  • forty-four per cent have high to very high levels of nutrients, meaning the water quality is degraded
  • thirty-three per cent have low or very low levels of nutrients.

Eleven of the 112 lakes are classed 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.

The following map and graph present results using the most recent data available in 2010.

Nutrient level (Trophic Level Index) of monitored lakes

Trophic Level Index Map of New Zealand

 

trophic level of 112 monitored lakes in New Zealand

The map shows the locations of 112 monitored New Zealand lakes colour coded according to their most recent (to 2005) trophic status.

Most lakes in the nutrient-enriched trophic categories are in the north of the North Island or the lowlands of the South Island while the least nutrient enriched lakes are most commonly found in high country of both islands (eg, Lake Taupo and the Southern Lakes).

The Trophic Level Index (TLI) includes six categories from microtrophic (very low nutrient levels) to hypertrophic (very high nutrient levels). The bar graph shows the number of the lakes in each trophic category.

Tropic level of 112 monitored lakes
  Number of lakes
Hypertrophic 11
Supertrophic 13
Mesotrophic 30
Oligotrophic 25
Microtrophic 8

Data source: Ministry for the Environment.

The nutrient status of lakes is strongly related to their depth and the types 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 (presented above) as well as other factors such as catchment land use, climate and lake depth. It is estimated that:

  • forty-three per cent of all lakes are likely to have low concentrations of nutrients and excellent water quality, often because they lie in natural, or only partially developed, catchments
  • twenty-five per cent are likely to have moderate levels of nutrients.
  • thirty-two per cent of lakes are likely to have high levels of nutrients and poor water quality (classed as eutrophic or worse).

 Ecological condition

Of the 155 lakes that had data on ecological condition available:

  • thirty-seven per cent have poor ecological condition or had no submerged plants
  • thirty-three per cent have high or excellent ecological condition.

ecological condition of 155 monitored lakes

The ecological condition (LakeSPI) includes five categories from non-vegetated to excellent. The bar graph shows the number of the lakes in each category.

  Number of lakes
Non-vegetated 36
Poor 21
Moderate 47
High 28
Excellent 23

Recent trends in water quality

Trends between 2005 and 2009 were assessed for 68 lakes, and found that 19 (28 per cent) of the lakes had deteriorated and eight lakes (12 per cent) had improved. 

This pattern varied across lakes with different land covers.  Around 40 per cent of lakes with predominantly native catchment cover had deteriorated, compared with 25 per cent of lakes with predominantly pastoral catchment cover. 

Trends in nutrient levels (Trophic Level Index) of monitored lakes and land cover, 2005-09

trends-nutrient-levels

The bar graph shows the numbers of 67 monitored lakes that are either deteriorating, not changing or improving over time by land cover.

tropic level trends of monitored lakes by land cover 2005-2009
Land cover Declining quality No change Improving quality
Alpine   2  
Exotic 1 4 1
Native 9 12 3
Pastoral 9 22 4

Note: For one lake, land cover was not determined, so only 67 lakes are used for this graph.

Data source: Ministry for the Environment.

Trends in water quality between 2000 and 2009 have been assessed for 18 lakes that have sufficient data. The levels of nutrients in most of these lakes have shown no signs of change since 2000. However, three of the 18 lakes show signs of deterioration (that is, an increase in nutrient or algae levels or a decrease in visual clarity), and one shows signs of improvement (Lake Omapere in Northland).

What you can do

  • Fence off the banks of streams and river banks that drain into lakes, as well as the shores of the lakes themselves.
  • Plant native vegetation or other plants to improve water quality by filtering sediment, faecal bacteria and nutrients from surface water run-off.
  • Farmers can prevent stock from entering rivers, lakes and wetlands and manage the use of fertilizers. The Dairying and Clean Streams Accord provides more information.
  • For more advice on lake and wetland protection contact your local or regional council [LGNZ website] or the New Zealand Landcare Trust [Landcare Trust website].
  • Before entering a lake or river, boat owners should 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 and Lake water quality in New Zealand 2010: Status and trends.

 

Reviewed:
17/07/13