NET PLANKTON OF LAKE MEN DOT A 



49 



reach its maximum height until the last of May, while the latter reaches 

 its highest point about three weeks earlier. Between the last week in 

 August and the first of October, there was a thirty-fivefold increase in 

 the ash while the organic matter on the latter date was only a little more 

 then seven and a half times as much as that on the former date. During 

 the rest of October the ash showed a decline ; the organic matter showed 

 a decline at the end of the first week in October and then rose to the 

 highest point for 1913 at the end of the third week in October before it 

 began to decline in amount. Thus, the curve for organic matter shows 

 two peaks for the autumnal period of this year, while that for the ash 

 shows only one. During the latter half of November and the first half 

 of December, the ash showed approximately a fivefold increase while the 

 change in organic matter during this interval was less than twofold. 



400 



40O 



too 



-4-14-. 



_j_.|~f-H-TTfTTt 



Tn. 



Fig. 18. — The quantity of dry organic matter and of ash in the net plankton 

 of Lake Mendota in 1914. Curve A represents the organic matter and curve 

 B the ash. The amounts are indicated in milligrams per cubic meter of 

 water. 



In 1915 (fig. 19) the vernal increase of ash did not begin as early 

 as that of the organic matter and it declined more promptly following 

 the maximum; as a result the curve for ash possesses a sharp-pointed 

 peak and that for the organic matter has a peak with a much broader 

 outline as well as a small secondary peak in the middle of May. Up to 

 the middle of May the increase in organic matter was due largely to 

 copepods and this was followed by a marked rise in the number of dia- 

 toms with a consequent increase in ash. Following this the diatoms 

 declined while the organic matter remained high as a result of the 

 increase in the number of Crustacea. 



During the latter part of April, 1916 (fig. 20), there was a small 

 increase in the organic matter which was accompanied by a decrease 



