Trophic state index

The Trophic State Index (TSI) is a classification system designed to rate water bodies based on the amount of biological productivity they sustain.^[1] Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.

The TSI of a water body is rated on a scale from zero to one hundred.^[1] Under the TSI scale, water bodies may be defined as:^[1]

• oligotrophic (TSI 0–40, having the least amount of biological productivity, "good" water quality);
• mesotrophic (TSI 40–60, having a moderate level of biological productivity, "fair" water quality); or
• eutrophic to hypereutrophic (TSI 60–100, having the highest amount of biological productivity, "poor" water quality).

The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a water body's TSI. Nutrients such as nitrogen and phosphorus tend to be limiting resources in standing water bodies, so increased concentrations tend to result in increased plant growth, followed by corollary increases in subsequent trophic levels.^[a] Consequently, trophic index may sometimes be used to make a rough estimate of biological condition of water bodies.^[2]

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