2 PLANKTON OF WISCONSIN LAKES 



autumnal overturning of the entire body of water. Complete circula- 

 tion of the water is again established and it continues until the lakes 

 become covered with ice. The ice prevents further circulation of the 

 water and this results in indirect stratification; that is, the coldest 

 water is at the surface and the warmest at the bottom. Direct stratifi- 

 cation is found in the summer when the warmest water is at the sur- 

 face and the coolest is at the bottom. 



During the two periods of complete circulation the substances that 

 are dissolved in the water are uniformly distributed from surface to 

 bottom, but in the intervening periods of stratification the chemical 

 conditions in the lower water become very different from those in the 

 upper stratum. These differences are especially marked in late sum- 

 mer. The hypolimnion, for example, is cut off from contact with the 

 air by the epilimnion and the mesolimnion so that its supply of dis- 

 solved oxygen is limited to the amount which is held in solution by this 

 water at the time stratification takes place. For a certain time this 

 stratum is populated by many animals which use up some of the dis- 

 solved oxygen in their respiratory processes. Decomposition is a still 

 more important factor in exhausting the supply of oxygen in the 

 hypolimnion. Through these two agencies, therefore, practically all 

 of the free oxygen in this lower stratum is used up by the first of 

 August, and a further supply is not obtained by this water until the 

 autumnal overturn takes place. During the winter period of stratifica- 

 tion, also, the oxygen may be exhausted from a bottom stratum of 

 varying thickness. 



Kespiration and decomposition not only make inroads upon the 

 supply of dissolved oxygen, but they also contribute certain products 

 to the water, such as carbon dioxide and nitrogen compounds, for ex- 

 ample; the latter remain in the lower strata until an overturning of 

 the water takes place. These products may then be taken up by chloro- 

 phyl bearing organisms and again play an important role in life proc- 

 esses. 



The following discussion regarding seasonal changes in the crop of 

 plankton refers to the results obtained on Lake Mendota because no 

 observations were made on the other lakes during the winter and early 

 spring. During the vernal period of complete circulation the water 

 becomes well aerated at all depths, the dissolved substances are evenly 

 distributed, and the temperature of the water gradually rises; the 

 plankton organisms respond to these favorable conditions by producing 

 a large crop of material at this time. The vernal crop, in fact, proved 

 to be the largest one of the year. 



When the water is in complete circulation conditions are favorable 

 for the growth of the various organisms at all depths, but as soon as 



