The Trophic Level Index (TLI) is a method of characterising the ecological health of lakes based on the amount of nutrients and algae growing in them. It provides an integrated measure of water quality that can be tracked over time and that can be used to estimate biological productivity. It is presented as a score, the lower the score the better the condition of the lake.
What do the TLI scores mean?
The Trophic Level Index gives an indication of lake water quality. The range of TLI scores can be divided into categories in Table 1 below.
Table 1. Lake Trophic Level Index scores.
How is TLI calculated?
The TLI score for a lake is calculated using four separate water quality measurements – chlorophyll a, total nitrogen, total phosphorus and water clarity.
Chlorophyll a is an indicator of phytoplankton (algae) growth. Knowing how much chlorophyll a there is in a lake gives us a good idea of how much algae the lake has. It’s okay to have some algae in a lake and is in fact important for ecosystems, just not too much. The more algae present, the poorer the water quality.
Nitrogen and phosphorus are key nutrients that algae and plants thrive on. Large amounts of these nutrients in lakes encourage excessive growth of algae which can lead to poor water quality.
Water clarity is a measurement of how far down the water column you can see. In general, the clearer the water, the better the water quality. Water clarity is measured as Secchi disc depth.
The Trophic Level Index combines these four measurements into one score and makes it easier for us to compare and understand the overall condition of New Zealand's lakes. See this factsheet on how state is calculated for lakes.
Does a lake’s TLI change?
The TLI is calculated yearly using data collected between 1 July and 30 June (hydrological year). Changes in a lake’s water quality will be reflected in the TLI score.
Increasing TLI scores indicate that a lake is becoming nutrient enriched with an increasing likelihood of algae bloom events, which are associated with reduced water clarity.
Efforts made by councils or landowners to reduce the amount of phosphorus and nitrogen entering the lake may lead to a lower TLI score over time. You can view a lake’s TLI history on LAWA by clicking the “Show more information” button.
Are there other ways to assess lake health?
LakeSPI and state attribute bands are also used to assess lake health.
TLI scores are available on LAWA for more sites than state attribute bands, as the sampling frequency requirement is less demanding. For example, Canterbury high-country lakes are sampled in a frequency (December to April only) that complies with LAWA’s data requirements for TLI, but not state attribute band calculations.
Regional councils, unitary authorities and lake managers usually use all available information to assess a lake’s overall water quality.
Where do I find more information?
Burns NM, Bryers G, Bowman E (2000) Protocol for monitoring trophic levels of New Zealand lakes and reservoirs. Ministry for the Environment, Wellington. 122 p.
Shallenberg M, van der Zon K (2021). Review of the Lake Trophic Level Index. Prepared for the Regional Council Lakes Focus Group.
NIWA (2021). Water quality state and trends in New Zealand lakes. Analysis of national lakes data ending in 2020. Prepared for Ministry for the Environment.