NET PLANKTON OF LAKE MENDOTA 



33 



quantity of crude protein nitrogen varied from a minimum of 3.4 

 milligrams per cubic meter of water in one sample collected in 1911 to a 

 maximum of 93.0 milligrams in one of the 1916 samples; a minimum of 

 5.2 milligrams was noted in one of the 1916 catches, thus giving an 

 eighteenf old variation in amount during that year. In 1911 the quan- 

 tity varied from 3.4 milligrams to 40.8 milligrams, which represents a 

 twelvefold variation in amount. 



The quantity of nitrogen, like that of organic matter, showed vernal 

 and autumnal maxima separated by summer and winter minima. The 

 autumnal maximum always exceeded the vernal maximum of the same 

 year ; the greatest difference between the two was noted in 1912 when the 

 vernal nitrogen rose to 49.3 milligrams per cubic meter of water and the 

 autumnal to 85.0 milligrams, the difference being 35.7 milligrams. In 

 1911 the difference between these two maxima was 22.7 milligrams and' 

 in 1913 it was 19.4 milligrams, while in 1915 it was only 13.0 milligrams. 



Figures 9 and 10 are graphical representations of the quantitative 

 results obtained for nitrogen; they show the number of milligrams of 

 crude protein nitrogen per cubic meter of water. These curves serve 



70 



Fig. 9. — The quantity of nitrogen in the net plankton of Lake Mendota in 1911 

 and 1912. The vertical spaces show the number of milligrams of nitrogen 

 per cubic meter of water. 



