Birge. — Plankton of the Lakes. 



125 



numbers on the left side of the diagram indicate the depth in 

 meters. The figures at the top indicate the dry weight of the 

 organic matter of the plankton in milligrams per cubic meter of 

 water. The live weight is about ten times as great. The total 

 quantity of fresh plankton in a column of water one meter square 

 and 65 m. high is about 455 grams, a little over 16 ounces. 





Diagram showing temperature of Green Lake, Wis., and quantity of 

 organic matter in its plankton on Aug. 17, 1922. Temperature in centi- 

 grade degrees; depth in meters; weight of plankton in milligrams of 

 dry organic matter per cubic meter of water. 



A — A, total plankton; B — B, net plankton. For explanation see text. 



The two lines, A-A and B-B, indicate the dry organic matter 

 of the plankton at the various depths; B-B shows the larger 

 planktonts collected by the net to which A-A adds the material 

 extracted by the centrifuge and therefore shows the total plank- 

 tons. You will see that the smaller creatures constitute by far 

 the greater part of the total. It is also noteworthy that the 

 quantity of plankton is greatest in the warm surface stratum, 

 that it diminishes rapidly in the thermocline stratum of tem- 

 perature and then decreases slowly, reaching a minimum at about 

 40 m. and increasing again toward the bottom. 



This is a typical picture of the summer plankton of Green 

 Lake. It represents a total of nearly 4,000 pounds of fresh ma- 

 terial per acre of water at the depth indicated, of which the net 



