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Factsheet: Lake Trophic Level Index

The Trophic Level Index (TLI) is used on LAWA to give an overall picture of the health of New Zealand lakes. Each lake is assigned a number between 1 and 7, the lower the number, the better the water quality in the lake. 

How is TLI calculated?

The TLI number is calculated using four separate water quality measurements – total nitrogen, total phosphorous, water clarity, and chlorophyll-a.

Total nitrogen and total phosphorous are nutrients that plants thrive on. Large amounts of these nutrients in the lakes encourage the growth of algae which can lead to poor water quality.

Water clarity is a measurement of how clear the water in the lake is. In general, the clearer the water, the better the water quality.

Chlorophyll-a is the green colour in plants. Knowing how much chlorophyll there is in a lake gives us a good idea of how much algae the lake has. It’s okay to have algae in a lake, just not too much. The more algae present, the poorer the water quality.

The Trophic Level Index combines these four measurements into one number.

What do the TLI numbers mean?

The Trophic Level Index gives an indication of lake water quality. Each range of numbers translates into a scientific description as explained below:

Supertrophic: The lake is fertile and saturated in phosphorus and nitrogen, often associated with poor water clarity

Index

Description

LAWA Icon

Less than 2  Microtrophic: The lake is clear and blue with very low levels of low levels of nutrients and algae.   VG (1)
2-3  Ogliotrophic: The lake is clear and blue, with low levels of nutrients and algae

 

 Good (1)
3-4  Mesotrophic: The lake has moderate levels of nutrients and algae.

 

 Average
4-5  Eutrophic: The lake is green and murky, with higher amounts of nutrients and algae.

 

 Poor
Greater than 5 Supertrophic:  The lake is fertile and saturated in phosphorus and nitrogen, often associated with poor water clarity.   Verypoor

 

Does a lake’s TLI change?

The TLI displayed on LAWA is calculated from data collected over the past 12 months. Changes in a lake’s water quality will impact the overall TLI. For example 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. 

 TLI