Descriptions Page
Littoral Area:
 

 

 This is defined as that portion of the lake that is less than 15 feet in depth. The littoral area is where the majority of the aquatic plants are found and is a primary area used by young fish. This part of the lake also provides the essential spawning habitat for most warmwater fishes (e.g. bass, walleye, and panfish). 
Secchi disk:

 

 A white 8 inch round disk used to measure water clarity.  The disk is suspended on a length-marked small cord and is lowered into the water until it disappears.  This distance is used as a marker of relative clarity.  The readings are normally taken by the same observer and in the same location in the lake.
TSI Index:      
 

 
Trophic Status Index (TSI)
The Carlson Trophic Status Index (TSI) is a tool used to summarize several measurements of water quality into one index value which can be used to compare a lake to other lakes, or to historic/future data as a measure of degradation or improvement.  In many ways, the index can be viewed as a measure of the potential for algal productivity.  Since most people value lakes with low algal productivity, the lower the TSI value the healthier the lake is.
 TSI Range  Trophic Status  Characteristics
0-40   Oligotrophic   Clean lake
 41-50   Mesotrophic   Temporary algae & aquatic plant problems
 51-70   Eutrophic   Persistent algae & aquatic plant problems
 71-100  Hypereutrophic  Extreme algae & aquatic plant problems

Components of TSI
Total Phosphorus (TP) -- Phosphorus is a major plant nutrient.  When phosphorus is present in high concentrations it stimulates plant (both algae and other plant) growth.  Although other nutrients can also influence water quality, phosphorus is most often the limiting factor.
Chlorophyll a -- All green plants contain chlorophyll a,  which allows limnologists to use the concentration of chlorophyll a  as an estimate of total algal biomass.
Secchi disk -- A Secchi disk is a white disk which is lowered into a lake to measure clarity or transparency of the water.

TSI (S)  is based on Secchi disk readings.
TSI (C) is based on presence of chlorophylla  in water sample.
TSI (P) is based on presence of phosphorus in water sample.

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