Trophic State Index - Trophic Classifications

Trophic Classifications

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.

Relationships between Trophic Index (TI), chlorophyll (Chl), phosphorus (P, both micrograms per litre), Secchi depth (SD, metres), and Trophic Class (after Carlson 1996)

TI Chl P SD Trophic Class
<30—40 0—2.6 0—12 >8—4 Oligotrophic
40—50 2.6—20 12—24 4—2 Mesotrophic
50—70 20—56 24—96 2—0.5 Eutrophic
70—100+ 56—155+ 96—384+ 0.5—<0.25 Hypereutrophic

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 TI), massive fish die-offs may occur as decomposing biomass deoxygenates the water.

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