Trophic state index - Plant Management in Florida Waters

Trophic State Index (TSI)

Trophic state index (TSI) is a scale of numbers from 1 to 100 that can be used to indicate the relative trophic state of a waterbody. Low TSI values indicate lower levels of biological productivity, and higher TSI values indicate higher levels. The use of TSI is an attempt to make evaluations of biological productivity easier to understand.

Using mathematical formulas, TSI values can be calculated individually for four parameters: total nitrogen concentrations, total phosphorus concentrations, total chlorophyll concentrations, and Secchi depth. Sometimes a single TSI value for a waterbody is calculated by combining selected individual TSI values.

The State of Florida classifies waterbodies according to "designated uses" that have been assigned to each. (See Water quality in this website.)

The Florida Department of Environmental Protection (FDEP) assesses water quality in Florida by evaluating whether each waterbody is able "to support its designated use". The FDEP assessment is based solely on TSI values as follows:

waterbodies with TSI values from 0 to 59 are rated as "good and fully support use"
those waterbodies with TSI values between 60 to 69 are rated as "fair and partially support use"
waterbodies with TSI values from 70 to 100 are rated as "poor and do not support use"

Individual TSI values may be further combined in a special type of averaging to produce an Average Trophic State Index (abbreviated TSI_ave). Governmnet and regulatory agencies responsible for water management often use the average value, overlooking the fact that the designing author, Dr. Robert Carlson of Kent State University in Ohio, never intended TSI values to be reduced to a single number. TSI values for individual parameters could differ markedly within any specific waterbody, and this significant variation will be obscured when the TSIave is calculated.

Dr. Carlson has noted that TSI values should not be averaged; consideration of the differences in individual TSI values in a waterbody can bring insight and a better understanding of its biological productivity .

Pitfalls of using TSI?
Applying words like good, fair, and poor to TSI ranges has contributed to the unfortunate misconception that trophic state is synonymous with the concept of water clarity. While higher TSI values indicate waterbodies with high levels of biological productivity, this is not necessarily a "poor" condition.

This can lead to confusing evaluations. For example, consider a waterbody with a TSI of 80. Its high rating tells us that the lake has a high level of biological productivity, a capacity to support abundant populations of fish and wildlife.

However, if we use Environmental Protection standards, the same TSI rating of 80 puts the waterbody in the category described as "poor and does not support use", regardless of the fact that it is actually able to support an abundance of fish and wildlife. While this lake may not be ideal for swimming or diving, it is fully able to support recreational activities such as fishing and bird watching.

The information on this page was derived in part from the following publication of
Florida Lakewatch of the University of Florida Department of Fisheries and Aquatic Sciences.
This booklet may be downloaded in its entirety as a PDF file by clicking here.
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This project is a collaboration of
the Center for Aquatic and Invasive Plants, University of Florida,
and the Bureau of Invasive Plant Management, Florida Department of Environmental Protection