INTRODUCTION 



of algae. There are certain factors modifying the growth of algae in 

 hard water lakes ; in those deep ones which become thermally stratified 

 in the summer most of the algae are restricted to the epilimnion. The 

 thickness of the epilimnion is in part dependent upon the size and shape 

 of the lake, since the wind is more effective in keeping water in circula- 

 tion on large surfaces. Algae are continually sinking to the bottom and 

 giving rise to carbon dioxide and nitrogenous products. In the deep 



TABLE 1. Results of mineral analyses stated in parts per million. These show 

 average content of various depths and at different dates. 



lakes these products of decay are chiefly in the hypolimnion and are 

 not immediately available for reutilization by other algae. In shal- 

 low lakes without thermal stratification these products are immediately 

 available for the vegetative activities of algae. Shallow lakes, there- 

 fore, support proportionally larger algal floras. The restriction of 

 the desmids to the soft water lakes was first pointed out by the Wests 

 and their observations have been confirmed in Wisconsin (Birge and 

 Juday, I. c. p. 138). An examination of the chemical analyses of such 

 soft water lakes as Devils, Trout and Kawaguesaga (Table 1) shows 

 that in these lakes which contain numerous desmids the calcium and 

 magnesium content is relatively small. The influence of calcium and 

 magnesium on the distribution of desmids may be that of a direct in- 

 hibition of growth or the absence of these elements may permit an 

 acid condition which is favorable for the growth of desmids. Data on 

 organic matter and distribution of nitrogen in lakes of the state are 

 lacking. The scarcity or abundance of available nitrogen is doubtless 

 an important factor in the volume of algae produced in a lake, and it 

 is a well known fact that as regions become more thickly settled and 

 drainage entering the lake becomes richer in nitrogenous material the 

 growth of plankton algae becomes more luxuriant. 



