Carlson’s Trophic State Index (TSI) is a common means for characterizing a lake’s trophic state (overall health) and associating Secchi, chlorophyll-a, and phosphorus measurements. The term “trophic status” refers to the level of productivity in a lake as measured by phosphorous content, algae abundance, and depth of light penetration. Carlson’s Trophic State Index is one means available to examine the relationship between total phosphorus, chlorophyll-a, and Secchi disk readings in a lake and its overall productivity. Individual TSI values can be calculated from the following equations:
Total phosphorus and chlorophyll-a are measured in micrograms per liter (µg/L) and Secchi disk transparency is measured in meters (3.281 feet per meter). The ln function in these equations is the “natural log” which is different than the “log” function. [The ln key is generally found next to the log key on most calculators.] The TSI scale ranges from 0 (ultra-oligotrophic) to 100 (hypereutrophic). High and or increasing trophic status values indicate more eutrophic conditions (higher productivity).
Carlson’s Trophic State Index
R.E. Carlson
TSI < 30 Classical Oligotrophy: Clear water, oxygen throughout the year in the hypolimnion, salmonid fisheries in deep lakes.
TSI 30 - 40 Deeper lakes still exhibit classical oligotrophy, but some shallower lakes will become anoxic in the hypolimnion during the summer.
TSI 40 - 50 Water moderately clear, but increasing probability of anoxia in hypolimnion during summer.
TSI 50 - 60 Lower boundary of classical eutrophy: Decreased transparency, anoxic hypolimnia during the summer, macrophyte problems evident, warm-water fisheries only.
TSI 60 - 70 Dominance of blue-green algae, algal scums probable, extensive macrophyte problems.
TSI 70 - 80 Heavy algal blooms possible throughout the summer, dense macrophyte beds, but extent limited by light penetration. Often would be classified as hypereutrophic.
TSI > 80 Algal scums, summer fish kills, few macrophytes, dominance of rough fish.