114 PLANKTON OF WISCONSIN LAKES 



Nitrogen. The nitrogen content of all of the nannoplankton samples 

 obtained on Lake Monona in 1915 and in 1916 was determined. A sum- 

 mary of the results is given in table 34. On an ash free basis the nitro- 

 gen ranged from a maximum of 11.28 per cent to a minimum of 5.92 

 per cent in the former year and from 10.08 per cent to 3.58 per cent 

 in the latter year. The difference for 1915 was about twofold and for 



1916 almost threefold. Both maximum and minimum were higher in 

 1915 than in 1916, and the mean percentage, likewise, was almost two 

 per cent higher in the former year. 



The maximum percentage of nitrogen in the net plankton was some- 

 what higher in 1915 than in the corresponding nannoplankton catches, 

 but it was lower in the net plankton in 1916 than in the nannoplankton. 

 (Compare tables 30 and 34.) The minimum for nannoplankton was 

 below that of the net plankton both in 1915 and in 1916 ; the mean 

 percentages also show small differences in favor of the former. 



A rise in the percentage of nitrogen in the nannoplankton of Lake 

 Monona accompanied the increase in the quantity of organic matter in 

 1915 as well as in 1916. (See general table, No. 46.) In the former 

 year the largest percentage was found in sample No. 5144, collected on 

 September 24, in which the quantity of organic matter was within 15.0 

 per cent of the maximum for the year, and in 1916 it was noted in sam- 

 ple No. 6158, October 12, which yielded the largest quantity of organic 

 matter in that year. In Lake Mendota, on the other hand, the maxi- 

 mum percentage of nitrogen in the nannoplankton was found during 

 the third week in June both in 1915 and in 1916 and at this time the 

 quantity of organic matter was distinctly below the annual mean. In 



1917 the maximum was noted on May 23, which was about two weeks 

 after the organic matter had reached its highest point ; the quantity of 

 organic matter at this time was 80.0 per cent of the amount obtained 

 two weeks earlier. 



When expressed in terms of milligrams per cubic meter of water the 

 largest quantity of nitrogen was found in sample No. 6158 obtained 

 from Lake Monona on October 12, 1916 ; but the minimum for 1916 as 

 well as the mean for this year fell below those of the year 1915. 



The ratio of the organic matter to the total nitrogen in the nanno- 

 plankton of Lake Monona varied from 8.1 to 27.9, more than a threefold 

 difference. (See table 46.) This is a much greater variation than in 

 the net plankton ; in the latter three quarters of the ratios fall between 

 9 and 12, while in the nannoplankton material only half of them are 

 within these limits. In eight of the nannoplankton samples, also, the 

 proportion of nitrogen falls below the minimum of the net samples. 



Crude Protein. The nitrogen results have been expressed in terms 

 of crude protein in table 35 ; the first part of the table shows the per- 



