TOTAL PLANKTON OF LAKE MENDOTA 101 



such as the algae and flagellates. The Crustacea, in turn, are preyed 

 upon by the larger forms. In addition to these losses, a certain portion 

 of the plankton is constantly dying and sinking to the bottom. Thus, 

 the quantity of plankton material maintained by the lake is the result- 

 ant of the productive processes and of the various destructive proc- 

 esses. In spite of the various changes in quantity in the course of the 

 year, the amount of material found during a particular month does not 

 differ so very greatly from year to year. 



In the spring the water is kept in fairly complete circulation by the 

 wind from the time of the disappearance of the ice, which usually takes 

 place about the first week in April, until early June; the same is true 

 for the autumn and early winter, or from the autumnal overturn about 

 the first week in October until the lake becomes covered with ice, usually 

 in December. As a result of this extensive circulation of the water at 

 these seasons, the plankton becomes fairly uniformly distributed from 

 surface to bottom for about four to five months each year. Since the 

 mean depth of the water within the 20 meter contour is 22 meters, the 

 quantity of plankton in the upper 11 meters would be half as large 

 as that shown in table 26 during these periods of general circulation 

 and one-quarter as great in the upper 5.5 meters. 



In summer and in winter, however, the plankton organisms do not 

 have such a uniform distribution ; the upper strata in summer are more 

 densely populated than the lower, while in winter after the lake be- 

 comes covered with ice there is a tendency for the chlorophyl-bearing 

 organisms to come nearer the surface where light conditions are more 

 favorable and for the Crustacea to be more numerous in the lower strata 

 where the water is somewhat warmer. 



Two special runs made on August 7, 1915, will serve to illustrate the 

 unequal distribution of the plankton at this time of the year. The re- 

 sults for organic matter are shown in table 27. Samples No. 593 and 

 No. 594 represent the organic matter in the nannoplankton and in the 

 net plankton respectively, from the 0-13 meter stratum ; this stratum in- 

 cluded the epilimnion and the mesolimnion at this time. Samples No. 

 591 and No. 592 are the catches of nannoplankton and net plankton 

 from the 14-20 meter stratum and they represent the hypolimnion. 

 The nannoplankton obtained from the 0-13 meter stratum yielded 

 nearly two and a half times as much organic matter per cubic meter 

 of water as that from the 14-20 meter stratum. The difference was 

 still more striking in the net plankton; the upper stratum yielded al- 

 most twelve times as much organic matter per cubic meter as the lower 

 stratum. 



Enumerations were made for the purpose of ascertaining the vertical 

 distribution of the various plankton organisms at the time these catches 



