Trophic state index

Trophic State Index (TSI) is a classification system designed to rate bodies of water based on the amount of biological activity they sustain. Although the term 'trophic index' is commonly applied to lakes, any surface body of water may be indexed. Trophic State Index (TSI) is a classification system designed to rate bodies of water based on the amount of biological activity they sustain. Although the term 'trophic index' is commonly applied to lakes, any surface body of water may be indexed. The TSI of a body of water is rated on a scale from zero to one hundred. Under the TSI scale, bodies of water may be defined as: The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a body of water'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. Consequently, a body of water's trophic index may sometimes be used to make a rough estimate of its biological condition. Carlson's index was proposed by Robert Carlson in his 1977 seminal paper, 'A trophic state index for lakes'. It is one of the more commonly used trophic indices and is the trophic index used by the United States Environmental Protection Agency. The trophic state is defined as the total weight of biomass in a given water body at the time of measurement. Because they are of public concern, the Carlson index uses the algal biomass asan objective classifier of a lake or other water body's trophic status. According to the US EPA, the Carlson Index should only be used with lakes that have relatively few rooted plants and non-algal turbidity sources. Because they tend to correlate, three independent variables can be used to calculate the Carlson Index: chlorophyll pigments, total phosphorus and Secchi depth. Of these three, chlorophyll will probably yield the most accurate measures, as it is the most accurate predictor of biomass. Phosphorus may be a more accurate estimation of a water body's summer trophic status thanchlorophyll if the measurements are made during the winter. Finally, the Secchi depth is probably the least accurate measure, but also the most affordable and expedient one. Consequently, citizen monitoring programs and other volunteer or large-scale surveys will often use the Secchi depth. By translating the Secchi transparency values to a log base 2 scale, each successive doubling of biomass is represented as a whole integer index number. The Secchi depth, which measures water transparency, indicates the concentration of dissolved and particulate material in the water, which in turn can be used to derive the biomass. This relationship is expressed in the following equation: A lake is usually classified as being in one of three possible classes: oligotrophic, mesotrophic or eutrophic. Lakes with extreme trophic indices may also be considered hyperoligotrophic or hypereutrophic. The table below demonstrates how the index values translate into trophic classes. Oligotrophic lakes generally host very little or no aquatic vegetation and are relatively clear, while eutrophic lakes tend to host large quantities of organisms, including algal blooms. Each trophic class supports different types of fish and other organisms, as well. If the algal biomass in a lake or other water body reaches too high a concentration (say >80 TSI), massive fish die-offs may occur as decomposing biomass deoxygenates the water. An oligotrophic lake is a lake with low primary productivity, as a result of low nutrient content. These lakes have low algal production, and consequently, often have very clear waters, with high drinking-water quality. The bottom waters of such lakes typically have ample oxygen; thus, such lakes often support many fish species such as lake trout, which require cold, well-oxygenated waters. The oxygen content is likely to be higher in deep lakes, owing to their larger hypolimnetic volume. Ecologists use the term oligotrophic to distinguish unproductive lakes, characterised by nutrient deficiency, from productive, eutrophic lakes, with an ample or excessive nutrient supply. Oligotrophic lakes are most common in cold regions underlain by resistant igneous rocks (especially granitic bedrock).