154 PLANKTON OF WISCONSIN LAKES 



standing crop of plankton and the degree of increase depends upon the 

 excess of the former over the latter. On the other hand, when the rate 

 of destruction exceeds that of production there is a decrease in the 

 standing crop of plankton which corresponds to the excess of the former 

 over the latter ; when these two processes just about balance each other 

 the quantity remains fairly uniform. The variations in the quantity of 

 organic matter in the standing crop of plankton in Lake Mendota are 

 well illustrated in figures 34 and 35. 



The present data do not enable one to make a definite assessment of 

 the value of the plankton crop in the biologic economy of the lake, 

 nevertheless it is well worth while to consider certain phases of this 

 question. The larger zooplankton forms, such as the Crustacea and the 

 rotifers, and even the minute forms such as algae and protozoa, are 

 eaten more or less extensively by fishes. The plankton Crustacea are 

 of special economic importance in this respect since most freshwater 

 fishes at some period in their lives feed chiefly or exclusively upon these 

 small organisms. Some fishes, in fact, are plankton feeders during the 

 whole period of their existence. Among the Crustacea the Cladocera are 

 more important than the Copepoda because they are used for food more 

 extensively by fishes ; hundreds or even thousands of Daphnias may be 

 found in a single fish stomach, while the smaller Cladocera may be 

 eaten in much larger numbers. Some of the insect larvae prey upon the 

 Cladocera and they, in turn, are fed upon by the fishes. Also midge 

 larvae feed upon plankton algae and they, too, constitute an item in the 

 menu of fishes. 



The bivalve mollusks depend chiefly upon the bacteria, algae, and 

 protozoa of the plankton for their food. Part of the plankton sinks to 

 the bottom of the lake and this constitutes a source of food for the insect 

 larvae, mollusks and worms which dwell upon the bottom ; this material 

 is especially important for the bottom dwellers which are found in the 

 deeper portions of a lake. 



In the assemblage of plankton organisms themselves those forms 

 which do not bear chlorophyl are dependent, either directly or indi- 

 rectly, upon those members which do possess this substance for their 

 food. The Crustacea are rather voracious feeders and their food con- 

 sists chiefly of algae and protozoa; when the former are abundant, 

 therefore, they consume large quantities of the latter organisms and at 

 such times they are very important agents in reducing the stock of the 

 organisms on which they feed. The rotifers also feed upon the algae 

 and protozoa, while the protozoa that do not possess chlorophyl, in 

 turn, feed upon the algae and the bacteria. 



Just how much food a rotifer or a crustacean consumes each day is 

 not known, but the following figures show how much water would have 



