TROPHIC STATE INDEX (TSI):
Trophic State is a measure of the degree of plant material in of a body of water.  It is usually measured using one of several indices (TSI) of algal weight (biomass):  water transparency (Secchi Depth), algal chlorophyll, and total phosphorus .

The TSI values of Carlson (1977) range from less than 0 to greater than 100, where each 10 units represents a doubling in Transparency or a halving of Total Phosphorus .  Suggested TSI limits to classical trophic state terminology are: 
   Ultra-Oligotrophy     (0-20), 
   Oligotrophy              (20-40), 
   Mesotrophy             (40-50), 
   Eutrophy                  (50-70), 
   Hypereutrophy           (>70).  

Caution must be used in interpreting algal trophic state from transparency values alsone because of the potential interference of non-algal turbidity and dissolved water color. 

For a more complete description of the Trophic State Index see: http://dipin.kent.edu/tsi.htm and http://www.pca.state.mn.us/water/lakeacro.html

Drawn by Minnesota Pollution Control Agency (http://www.pca.state.mn.us/water/lakeacro.html. After Moore, L. and K. Thornton, [Ed].  1988.  Lake and Reservoir Restoration Guidance Manual.  USEPA.  EPA 440/5-88-002.