Carlson’s Trophic State Index converts total phosphorus, chlorophyll-a and Secchi depth into a single comparable number — the first quantitative step in any lake assessment. We use it not just to classify a lake, but to read which factor is controlling it from the way the three indices disagree.
Trophic-state classification is where our lake assessment begins — it sets the magnitude of the problem and the distance a restoration must move the lake. Every later step, from the loading model to the intervention choice, is calibrated against this baseline.
Explore Our ProcessCarlson built the index so that each measurable variable maps onto the same 0–100 trophic scale
TSI(TP) = 14.42 ln(TP) + 4.15, with total phosphorus in µg/L. Phosphorus is usually the limiting nutrient in fresh water, so it sets the ceiling on how much algal biomass a lake can support — the index anchors the diagnosis to the controllable variable.
TSI(Chl) = 9.81 ln(Chl) + 30.6, with chlorophyll-a in µg/L. This is the realised algal biomass — the biological response. Where it tracks TSI(TP), phosphorus is being fully expressed as algae; where it lags, something else is holding growth back.
TSI(SD) = 60 − 14.41 ln(SD), with Secchi depth in metres. Transparency integrates algal and non-algal turbidity. When TSI(SD) exceeds TSI(Chl), non-algal particles — clay, silt or colour — are degrading clarity independently of the algae.
The diagnostic power is not in any single index but in how the three disagree. When all three agree, a simple phosphorus–algae system is at work. When TSI(Chl) > TSI(TP), the lake is producing more algae than its phosphorus alone explains — nitrogen fixation or internal loading is in play. When TSI(SD) > TSI(Chl), non-algal turbidity is suppressing clarity, pointing to resuspension or catchment sediment. When TSI(Chl) > TSI(SD), large colonial or filamentous algae are present that scatter light less per unit mass. The numerical bands — < 40 oligotrophic, 40–50 mesotrophic, 50–70 eutrophic, > 70 hypertrophic, consistent with the OECD boundaries — classify the lake; the deviations tell us why it sits where it does, which is what shapes the restoration.
The classification is a starting point, not a conclusion
We fix the current trophic state and the state a successful restoration must reach, giving a measurable objective rather than a vague “improve water quality&rdquo.
The index deviations point to whether nutrients, non-algal turbidity or species composition dominate — directing the next analytical step.
Tracking the indices through the season and across years separates a one-off bloom from a structural trophic shift.
The Vollenweider mass balance that predicts the in-lake phosphorus behind the index.
Read MoreWhy a lake can produce more algae than its external phosphorus explains.
Read MoreEuphotic depth and the N:P ratio that explain the index deviations.
Read MoreReynolds & Bauhm places your lake on the trophic scale, reads the controlling factor from the index deviations, and carries it through to a quantified restoration target.
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