Birge. — Plankton of the Lakes. 119 



For practical purposes the plankton may be divided into 

 two groups, separated chiefly by the size of the organisms: (1) 

 that which can be strained from the water by a net of fine silk 

 bolting cloth, and (2) that whose individuals are so small that 

 they must be extracted from the water by a centrifuge or simi- 

 lar divice. 



To the first group belong all of the larger animals of the 

 plankton, such as the water-fleas (Cladocera and Copepoda) 

 which are of special interest to us here, because they constitute 

 an important item in the food of the fish. It also includes the 

 larger algae — those, for instance, which give rise to the "green 

 scum" in lakes — and the larger diatoms. The second group in- 

 cludes the very minute animals (protozoa) and algse, together 

 with the bacteria of the water. 



In the investigation that I am reporting the Survey both 

 strained and centrifuged large quantities of water, doing this 

 work in a special laboratory on the shore of Lake Mendota. 

 Altogether, in 481 catches made from 1911 to 1917, we strained 

 out the plankton from about 4,750,000 pounds of water (2,157,000 

 liters), securing some 45 ounces (1,292 grams) of dry material 

 for chemical and food analysis. The water centrifuged to secure 

 the minuter organisms during the years 1915-1917 was neces- 

 sarily much less in quantity; but it aggregated in 184 catches 

 about 482,000 pounds, from which we obtained somewhat less 

 than 24 ounces (752 grams) of dry material, nearly half of which 

 was fine silt derived from the water and less than half was 

 organic material. These facts look as though the food material 

 of the open water of the lakes is very small in amount, but it is 

 really quite considerable. If the ash is taken out the average 

 amount of dry organic material yielded by the plankton of Lake 

 Mendota is about two pounds in a million pounds of water. In 

 the living condition about nine-tenths of the weight of these 

 plants and animals consist of water, so that in the lake there 

 are by weight about 20 parts of live plankton to a million of water. 



This quantity in the area occupied by the deeper water of 

 Lake Mendota (depth 20 m. or more) would give a standing 

 crop of more than 3,800 pounds per acre, or nearly two tons of 

 fresh material. February yielded the smallest amount — about 

 2,300 pounds per acre — and December the largest — about 4,500 



