PLANKTON OF LAKE MONONA 



105 



average of 85.4 per cent for the whole series of 47 samples. The 

 variations in the amount are summarized in table 29, in which the 

 quantities are given in milligrams per cubic meter of water. Details 

 for the organic matter and the chemical analyses are given in the gen- 

 eral table, No. 45, p. 210. The maximum quantities in the different 

 years ranged from 519.3 milligrams of organic matter in 1911 to 3,303.6 

 milligrams per cubic meter of water in 1912. The smallest minimum 

 was found in 1916 and the largest in 1915. The smallest mean was 

 noted in 1911 and the largest in 1915. The average amount of organic 

 matter yielded by the 47 samples of net plankton was 850.2 milligrams 

 per cubic meter of water as compared with 332.5 milligrams in Lake 

 Mendota. 



Curves indicating the number of milligrams of dry organic matter 

 per cubic meter of water are shown in figure 37 for the summer and 

 autumn of 1913 and 1916. The curve marked A represents the results 

 for the net plankton in 1913 and the one marked B those of 1916. In 

 the former there was a comparatively small rise in the quantity of or- 

 ganic matter during the month of June but this was followed by a 

 marked rise in July. The amount remained high during the early 

 part of August so that the curve shows a conspicuous peak covering 

 this period of time. This was followed by a minimum during the 

 early part of September ; following this came the autumnal rise which 

 reached its maximum point about the middle of October. 



In 1916 there was a slight decrease in the amount of organic matter 

 in the net plankton between the middle of May and the middle of June, 

 but there was a marked increase during the third week in June, fol- 

 lowed by a gradual decline during the first half of July. This forms 

 a prominent peak in the curve extending from the middle of June to 

 about the middle of the third week in July. The quantity then re- 

 mained fairly uniform until the middle of August, after which time 

 it declined to a minimum covering the latter part of August and the 

 first half of September. There was a gradual rise after this time but 

 there was no autumn maximum this year comparable to that of 1913. 



Nitrogen. Quantitative determinations of the nitrogen were made 

 on 41 samples of the net plankton from Lake Monona; the results of 

 these analyses are summarized in table 30. The highest percentage of 

 nitrogen was noted in material obtained in 1915 and the lowest in 1911. 

 The mean percentage was lowest in 1916, namely, 8.37 per cent, and 

 highest in 1915, namely, 10.71 per cent, thus showing a difference of 

 2.34 per cent. 



When stated in milligrams per cubic meter of water, the largest 

 amount of nitrogen was found in 1912, namely, 302.2 milligrams, and 

 the smallest in 1911, that is, 13.2 milligrams. The mean quantity was 

 highest in 1915 and lowest in 1911. 



