4 PLANKTON OF WISCONSIN LAKES 



the smallest winter minimum to 110 kilograms per hectare (98 pounds 

 per acre). The average amount of organic matter yielded by the 

 entire series of plankton catches from Lake Mendota was 240 kilo- 

 grams per hectare (214 pounds per acre) and those from Monona gave 

 267 kilograms per hectare (238 pounds per acre). Lake Waubesa 

 yielded substantially the same average as Lake Mendota. These figures 

 represent the weight of the dry organic matter in the plankton; the 

 weight of the organic matter in the living state would be approximately 

 ten times as large because water constitutes about 90 per cent of the live 

 weight of these organisms, excluding the ash. 



The quantities indicated above represent only the standing crop of 

 plankton and not the amount of this material that is produced an- 

 nually. A quantitative determination of the annual production of 

 plankton is a very complex problem because the factors involved are 

 so numerous and so diverse. The plankton itself is an assemblage of 

 many different kinds of organisms, ranging from the more simple forms 

 such as bacteria, protophyta, and protozoa to the more complex in- 

 dividuals such as Crustacea and insect larvae; a single catch, for ex- 

 ample, may contain as many as fifty or more different kinds of organ- 

 isms. It is a very difficult matter, therefore, to determine the relative 

 importance of the different forms in this plankton complex. 



The various organisms multiply at very different rates also ; under 

 favorable conditions the bacteria may pass through several generations 

 in the course of a single day, the protophyta and protozoa perhaps not 

 more than one or two, while the Crustacea may require two weeks or 

 longer to pass from one generation to the next. These organisms differ 

 enormously in size, ranging from the minute bacteria to forms that are 

 approximately two centimeters in length. 



Another complexity is introduced into the problem through the food 

 relations of the various plankton constituents. The greater part of 

 these organisms possess chlorophyl so that, with the aid of sunlight, 

 they are able to manufacture their own food materials out of the sub- 

 stances dissolved in the water. Therefore these forms constitute, either 

 directly or indirectly, the main source of the food for the other plank- 

 ton constituents; the chlorophyl bearing forms also constitute a large 

 part of the food of some non-plankton forms, such as the bivalve mol- 

 lusks. The rotifers and the Crustacea are the chief plankton forms 

 which feed upon the chlorophyllaceous constituents and they, in turn, 

 are fed upon by fishes ; also certain Crustacea prey upon others. Com- 

 putations based upon the numerical results and upon the average 

 weights of the various forms of rotifers and Crustacea show that the 

 other plankton constituents furnish an abundant supply of food for 

 these two groups of organisms ; that is, the material which can be used 



