Trophic State Index

A trophic state index assigns a numerical value to specific measurement found in the lake that are representative of the lake’s overall condition. There are several trophic status indexes that are used but for the purpose of understanding Geneva Lake’s trophic condition, a modified version referred to as the Wisconsin Trophic State Index is used. Generally, the higher the trophic value the worse the water quality.

Trophic state refers to the amount of biological activity in a lake. The more trophic a lake is the more fertile it is. A very biologically active fertile lake with a lot of plants would be considered a eutrophic lake. A lake with very little plant life and not much fertility is called an oligotrophic lake. Lakes the fall between very fertile and a not very fertile are referred to a mesotrophic lakes.

Three measurement used to evaluate a lake’s fertility are; total phosphorus. chlorophyll ”a” and secchi disk or water clarity. Phosphorus is a vital nutrient that is often the limiting nutrient for plant growth in lakes. The more phosphorus in a lake, the more plant growth. Chlorophyll ”a” is a color pigment found in green plants. A measure of the chlorophyll ”a” in the water column is an indirect measurement of plant density in the open water or plankton community. Secchi disc reading measure the depth of light penetration into the water. All three are related, the more phosphorus the more chlorophyll” a” and the more chlorophyll”a”, the more algae in the water and the less light penetrates into the water.

The figure below shows the plotted TSI values for Geneva Lake’s total phosphorus(TP), Chlorophyll (chl”a”) and secchi over the last 18 years. Geneva Lake’s secchi disk TSI is mostly in the oligotrophic range. The Chl”a” TSI value moves between mesotrophic and oligotrophic. Only the total phosphorus TSI ever reaches eutrophic and only on few occasions. It is mostly in the mesotrophic range. Based upon this information, Geneva lake would be classified as a relatively clean lake that is in transition between oligotrophic and mesotrophic states.