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.

Because they are of public concern, the Carlson index uses the algal biomass as an objective classifier of a lake or other water body's trophic status.

[2] Because they tend to correlate, three independent variables can be used to calculate the Carlson Index: chlorophyll pigments, total phosphorus and Secchi depth.

Phosphorus may be a more accurate estimation of a water body's summer trophic status than chlorophyll if the measurements are made during the winter.

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.

(This contrasts against eutrophic lakes, which are highly productive due to an ample supply of nutrients, as can arise from human activities such as agriculture in the watershed.)

Occasionally, an excessive algal bloom will occur and can ultimately result in fish death, due to respiration by algae and bottom-living bacteria.

The excessive algal blooms can also significantly reduce oxygen levels and prevent life from functioning at lower depths creating dead zones beneath the surface.

Nutrients carried into water bodies from non-point sources such as agricultural runoff, residential fertilisers, and sewage will all increase the algal biomass, and can easily cause an oligotrophic lake to become hypereutrophic.

[11][12][13] Although there is no absolute consensus as to which nutrients contribute the most to increasing primary productivity, phosphorus concentration is thought to be the main limiting factor in freshwater lakes.

[16] In some coastal marine ecosystems, research has found nitrogen to be the key limiting nutrient, driving primary production independently of phosphorus.

The limiting nutrient may vary in different marine environments according to a variety of factors like depth, distance from shore, or availability of organic matter.

Natural resource agencies are generally responsible for reconciling these conflicting uses and determining what a water body's trophic index should be.