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4

Florida Lake and Estuary

Trophic State Index Statewide

Summary

Explanation of Indicator

The Trophic State Index (TSI) is an expression of Florida's lakes and estuaries trophic state. The three

major trophic states are oligotrophy, mesotrophy, and eutrophy. The TSI procedures provide an effective

method of classifying lakes and estuaries based on their chlorophyll concentration, Secchi depth, and

nitrogen and phosphorous concentrations. The TSI was developed under contracts with the University of

Florida in 1982 in response to the federal Clean Lakes Program. Criteria were developed for the 4 index

components based on a regression analysis of data on 313 Florida lakes. The lake criteria cutoff values

for the 4 components and the overall TSI are 0-60 for good quality, 60 to less than 70 for fair quality and

70 to 100 for poor quality. The index was also applied to Florida estuaries to describe estuarine water

quality. The estuarine criteria cutoff values are 10 points less than the lake criteria (i.e., good is 0 to <50,

fair is 50 to <60, poor is 60- 100).

Source

The basic water quality monitoring data is gathered by Florida Department of Environmental Protection,

the water management districts and various federal, state, and local agencies. These agencies then

enter the data into the STORET system for storage and subsequent analysis. This information can be

obtained from Joe Hand at the Florida Department of Environmental Protection, Bureau of Surface Water

Management, 2600 Blair Stone Road, Tallahassee, Florida 32399-2400, or at (904) 487-0505.

Data Characteristics

Samples may be gathered monthly, semi-monthly, quarterly, annually, or on a one-time grab sample

basis. The frequency is determined by the site and the needs of the sampling agency. The 2059 miles of

lakes are 94 percent assessed, with 75 percent subject to quantitative assessment between 1989-1993

and 19 percent under qualitative assessment. The 3,925 square miles of estuaries are 94 percent

assessed, with 62 percent subject to quantitative assessment between 1989-1993 and 33 percent under

qualitative assessment. The STORET data is the source of the raw data that generates the Water

Quality Index. This data is then converted into statewide trend data and presented in graph format with

data points produced every two years. This information is available in hard copy and computer disk copy

formats. There are no additional costs involved in obtaining this information which is regularly compiled

from surface water monitoring activities.

Overall Assessment
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The Florida Trophic State Index is one of the most reliable data sets currently available because it is

based on the largest and most frequent sampling available and individual quality assurance steps taken

during sampling and analysis. The data will continue to be entered into STORET and with more agencies

participating, the potential data base should increase in the future. Trend analysis is a means of

identifying changes in water quality. By using this technique it is possible to determine whether efforts to

maintain or improve Florida's waters have been successful.

Data may not be gathered with sufficient frequency to provide a valid information base in all areas of the

state. Much of the data currently used is actually an averaging of reach values gathered at irregular

intervals rather than individual sites being sampled at a regular comparable time interval for specific

parameters. When insufficient information is available to generate a valid Water Quality Index or where

calculated Water Quality Index values conflict with local knowledge of the waterbody, professional

judgment is used to adjust the assessment of this waterbody.

Analysis of Indicator

The Trophic State Index for the number of square miles of lake and estuaries that exhibited good, fair, or

poor water quality is indicated below for two-year periods corresponding to the periods covered by the

Florida Water Quality Assessment prepared to meet the requirements of Section 305(b) of the federal

Clean Water Act. More detailed information on trophic state index values for individual waterbodies is

included in the 305(b) report and its technical appendix which can be obtained from the Department.

The comparable data for lakes shows a general decline in water quality in the good category from 1980 to

1986, with a recovery in 1988 to 1992. In 1988-92, the large increase in square miles of lakes with good

water quality was accompanied by a substantial drop in the square miles of lakes with waters in the fair

category. The comparable data for estuaries shows a drop in the quality of waters in the good category

for the period 1980-1982, with a general improvement from 1982-1992.

Florida Lakes Trophic State Index (TSI) Statewide

Summary

Square Miles Sampled are Comparable

1980 1982 1984 1986 1988 1990 1992

Year Sampled

0

100

200

300

400

500

Numbers of Square Miles

Good Fair Poor

Florida Estuaries Trophic State Index (TSI)

Statewide Summary
Strategic Assessment of Florida's Environment (SAFE)

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Square Miles Sampled are Comparable

1980 1982 1984 1986 1988 1990 1992

Year Sampled

0

100

200

300

400

500

600

Numbers of Square Miles

Good Fair Poor