Carlson's TSI index - how to assess trophic status with a number
A lake’s trophic status can be described in words (oligo-, meso-, eutrophic) or with a number - and the latter is served by Carlson’s TSI index (Trophic State Index). It is convenient, because it lets you compare water bodies and track changes over time on a single, consistent scale.
What it is made of
TSI is calculated from three closely linked indicators of fertility:
- transparency (the depth of the Secchi disc, SD),
- chlorophyll-a (Chl) - algal biomass,
- total phosphorus (TP).
Each of them gives a TSI value on a scale of 0-100 - you can use one or compare all three.
The formulas (after Carlson)
The relationships are logarithmic (SD in metres, Chl and TP in µg/l):
TSI(SD) = 60 − 14.41 · ln(SD)TSI(Chl) = 30.6 + 9.81 · ln(Chl)TSI(TP) = 4.15 + 14.42 · ln(TP)
In the ideal case all three give a similar result; the divergence between them is in itself sometimes informative (e.g. about what limits algal production).
How to read the scale
As a rough guide:
- < 40 - oligotrophic (poor, clear),
- 40-50 - mesotrophic (intermediate),
- 50-70 - eutrophic (fertile, blooms),
- > 70 - hypertrophic (extremely fertile).
The higher the TSI, the greater the fertility.
What to watch for
- season and a single measurement - TSI is best calculated from growing-season data and as a series,
- divergence of the indicators - a large difference between TSI(SD), TSI(Chl) and TSI(TP) is a signal that something other than algae is affecting transparency (e.g. mineral suspension).
🧮 Tool: Carlson’s TSI calculator - enter transparency, chlorophyll-a or total phosphorus and see the index and the trophic status at once.
In practice
For TSI you need consistent series of transparency, chlorophyll and phosphorus. In LimnoLog you record these indicators in one project and track their trend; the TSI value itself you can keep as a custom indicator and observe its change season after season.
← Knowledge base