Trophic State Index

The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a body of water's trophic state index (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. Consequently, a body of water's trophic index may sometimes be used to make a rough estimate of its biological condition. Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.