Trophic State Index
is a measurement of how much algae is in the water. It is a more
direct measurement than secchi depth. Every algal cell contains
chlorophyll-a, the pigment that captures sunlight for photosynthesis.
Measuring chlorophyll usually requires collecting a water sample
and taking it to a laboratory for analysis. There are also probes
or instruments now available that can measure chlorophyll directly
in the water and eventually we hope to purchase one for the RUSS
units. Right now we do it the old fashioned way in the lab. The
graphs are average values for each year we have summer chlorophyll
data (summer = May through September).
Much more information can be found in the Limnology
Primer and the Trophic
State Index section to find out
more about chlorophyll and its typical range in Minnesota lakes.