NANNOPLANKTON OF LAKE MENDOTA 75 



The crude protein constituted 45.0 per cent of the organic matter, or 

 more, in three-quarters of the net plankton samples, but in only 30 of 

 the 87 centrifuge samples, or about a third of them, did it reach this 

 large a proportion ; in only 50 samples did the crude protein equal 40.0 

 per cent or more of the organic matter in the nannoplankton. 



The details of the relation of the crude protein to the organic mat- 

 ter are shown in figures 30 and 31. In these diagrams the quantities 

 are indicated in milligrams per cubic meter of water. In 1915 the 

 curve representing the crude protein, marked B in the diagram, pos- 

 sesses an outline similar to that representing the organic matter, curve 

 A, but attention may be called to two minor differences. The small peak 

 in the former which appears in the last week of July has a valley cor- 

 responding to it in the latter, the organic matter having reached a small 

 maximum during the previous week. Also during the latter half of 

 November and the first week in December the changes in the quantity 

 of crude protein were not closly correlated in time with similar changes 

 in the organic matter. 



In 1916 the vernal maximum for crude protein came a few days later 

 than that of the organic matter and a rise of the latter during the first 

 part of July was correlated in time with a decrease in the former. The 

 curve for crude protein in November and December, 1916, presents a 

 rounded form while that for the organic matter shows two peaks at 

 this time. In 1917 the second peak in the curve for organic matter 

 during April is much lower than the first, while the two peaks in the 

 curve for crude protein at this time are substantially equal in altitude. 



Column 5 under nitrogen in the general table (No. 44) gives the ratio 

 of the organic matter to the total nitrogen in the various samples. In 

 60 of the 87 samples of nannoplankton this ratio varies between 12 and 

 18 ; that is, the nitrogen constituted from one-twelfth to one-eighteenth 

 of the organic matter in these samples. The ratio fell below 12 in 10 

 samples and above 18 in 18 samples. The sample containing the largest 

 proportion of nitrogen gave a ratio of 9.1 and the one having the small- 

 est proportion 23.5. These figures again serve to show that the pro- 

 portion of nitrogen in the nannoplankton, in general, was distinctly 

 smaller than in the net plankton. In a little more than two-thirds of 

 the net plankton samples, for example, the ratio fell between 9 and 12, 

 while in the nannoplankton samples only about one-ninth of them gave 

 a ratio as low as 12 or less. 



Ether Extract 



A summary of the results of the determinations of the ether extract 

 in the nannoplankton is given in table 20, p. 192 ; the figures indicate 

 the percentage of the organic matter and the number of milligrams per 



