156 PLANKTON OF WISCONSIN LAKES 



Owing to the absence of free oxygen in the hypolimnion of Lake Men- 

 dota in July and August, the rotifers and Crustacea are limited to the 

 epilimnion and the mesolimnion during this time and the food supply 

 in these two strata is about 12.5 per cent larger than the mean on which 

 these computations are based. 



The data presented in this report do not indicate the quantity of 

 plankton material produced annually, but by far the greater part of the 

 standing crop of plankton is contributed by organisms that multiply 

 rather rapidly under favorable conditions of temperature and food; 

 thus the turnover in this stock of material is proportionally rapid. As 

 indicated in a previous paragraph only about 115 milligrams of organic 

 matter out of a mean of 1,974 milligrams (table 25) per cubic meter of 

 water in the total plankton is derived from the rotifers and Crustacea, 

 and these two forms have the slowest rate of reproduction among the 

 various fresh-water planktonts. Substantially all of the other material 

 is derived from organisms that reproduce much more rapidly ; that is, 

 from unicellular forms or from colonies consisting of groups of cells. 

 The single cells of the various forms are capable of dividing once a day 

 or oftener when the temperature of the water is 20° C. or higher; the 

 bacteria, in fact, may divide oftener than once an hour. At a rate of 

 one division per day the possible progeny of a single cell would amount 

 to more than one billion in a period of thirty days, while one division 

 every three days would result in the production of 1,024 descendants in 

 the same length of time. 



The epilimnion of Lake Mendota has a temperature of 20° or higher 

 from about the middle of June until the middle of September and dur- 

 ing this period the turnover in the stock of plankton will be rapid in 

 this stratum. It will not be so rapid in the mesolimnion and in the 

 hypolimnion because the temperature of the water is lower in these 

 strata. The cooling of the water in the autumn and early winter will 

 tend to decrease the rate of production. The temperature of the water 

 rises slowly during the winter, but it does not reach a mean of 4° until 

 after the ice disappears, or about the middle of April. This is followed 

 by a period of vigorous production which culminates in the vernal 

 maximum. 



The foregoing discussion clearly brings out the fact that the problem 

 of ascertaining the quantity of plankton that a body of water produces 

 annually is a very complex one and its solution will require very exten- 

 sive and detailed investigations ; the present data show only the stand- 

 ing crop of plankton and its quantitative variations. A more complete 

 knowledge of (1) the rate of reproduction of the various planktonts 

 under natural conditions of light, temperature, and food, (2) the 

 length of their life in the natural environment, and (3) the average 



