Field tool · Version 1.0.0
Trophic state index calculator
Carlson's trophic state index (TSI) turns Secchi depth, chlorophyll a, and total phosphorus into one productivity scale from about 0 to 100. Below 40 reads oligotrophic, 40 to 50 mesotrophic, 50 to 70 eutrophic, and 70 or more hypereutrophic. Each step of 10 is a doubling of algal biomass.
- Inputs
- Secchi (m), chlorophyll a and total phosphorus (µg/L)
- Primary index
- Chlorophyll a, when entered
- Boundary
- Productivity only, not impairment or safety
Confirm the prerequisite
Review the governing method before entering or interpreting data.
Return to the source guide
Shared route boundary
Does not average indices, establish impairment, recreation safety, regulatory status, cause, or treatment need, or replace seasonal records, local reference conditions, and the responsible authority's criteria.
Carlson trophic state index · Version 1.0.0
Enter any of the three indicators.
Use growing-season, near-surface results from the same station and period. Each index is reported separately; the tool does not average them.
Results appear here after you choose Calculate.
Save the work and continue
Carry the result into its governed record and broader workflow; these links do not approve the work.
Open the data validation checklist Validate the Secchi, chlorophyll, and phosphorus results before index values are reported or released.
Continue the lake manager foundations path Place productivity indicators inside the full basin-to-decision workflow instead of acting on one index.
Shared route boundary
Does not average indices, establish impairment, recreation safety, regulatory status, cause, or treatment need, or replace seasonal records, local reference conditions, and the responsible authority's criteria.
Three indices, compared rather than averaged.
The calculator uses the published simplified Carlson (1977) equations exactly as written below, with Secchi depth in meters and chlorophyll a and total phosphorus in micrograms per liter. By design, 1 m of Secchi depth and 48 µg/L of total phosphorus each give a TSI of 60, and the scale moves 10 units each time transparency halves.
TSI(SD) = 60 − 14.41 ln(SD)TSI(CHL) = 9.81 ln(CHL) + 30.6TSI(TP) = 14.42 ln(TP) + 4.15
| TSI | Trophic state |
|---|---|
| Below 40 | Oligotrophic |
| 40 to below 50 | Mesotrophic |
| 50 to below 70 | Eutrophic |
| 70 and above | Hypereutrophic |
Carlson cautions against averaging the three values. Chlorophyll is the best predictor of algal biomass, so it is the primary index; differences between the Secchi or phosphorus index and the chlorophyll index point to conditions worth investigating, such as non-algal turbidity or a nutrient other than phosphorus limiting growth. A TSI value does not establish impairment, recreation safety, regulatory status, or a treatment need. Use seasonal records, local reference conditions, and your authority's criteria for those decisions, as described in the trophic state and reference conditions guide.
- A Trophic State Index for LakesLimnology and Oceanography
- Trophic State Equations (excerpted from A Coordinator's Guide to Volunteer Lake Monitoring Methods)North American Lake Management Society
Controlled preview. Internal technical review is complete. Independent review is pending, so this artifact remains available for review but is excluded from search-engine discovery. Next review: 2027-04-30.