Trophic State Index Chart

The Trophic State Index (TSI) is estimated by using water quality measurements and a mathematical formula (Carlson, 1977). TSI numbers are calculated using three different lake measurements: water transparency, and phosphorus levels, and chlorophyll-a.

Chlorophyll-a is the green pigment that algae use to capture the energy of the sun. It is used as an indication of how much algae are in a lake.

All lakes fall into one of three trophic states: eutrophic, mesotrophic, or oligotrophic. Eutrophic lakes have a TSI value greater than 55 and are considered highly fertile, or productive. They often have an abundance of algae and high phosphorus levels. This high algal growth decreases the transparency of the water and gives the water a greenish or brown color. Mesotrophic lakes have a TSI value from 40 to 55. Due to lower nutrient availability in mesotrophic lakes, they are less productive. Being less productive, there is less algal growth and clearer water. Oligotrophic lakes have a TSI value of less than 40. These are the least productive of the lakes and have the clearest water.

All lakes in the Conservancy monitoring program are oligotrophic. The least productive are North Lake Leelanau and Big Glen Lake. Little Glen Lake is the most productive as a result of its shallow depth. Cedar Lake is also showing sign of eutrophication likely due to high residential development in the watershed.

Carlson, Robert E. 1977. A trophic state index for lakes. Limnology and Oceanography. 22: 361- 368.