70 PLANKTON OF WISCONSIN LAKES 



The curves for 1916 and 1917 show that there are considerable vari- 

 ations in the extent and duration of the vernal pulse of nannoplankton. 

 The highest point of the former, for example, is almost 20.0 per cent 

 above that of the latter. In the former year also a marked increase of 

 organic matter took place during the latter part of February and early 

 in March, while in 1917 it was decreasing at this time; the late April 

 samples of 1917, in fact, contained a smaller amount than that of 

 March 8, 1916. Thus, the vernal peak of 1916 represents very much 

 more organic matter than that of 1917. Following the vernal maximum 

 the curves for both 1915 and 1916 show two secondary peaks during 

 the summer period with an extended minimal region in August and 

 September. 



In the autumn the organic matter rose to a decided maximum in 

 1916, but in 1915 it rose only to a median level and then remained 

 fairly uniform in amount during this season. The winter season was 

 characterized by a decline in the amount of organic matter. The rela- 

 tion of the various organisms to the changes in the quantity of organic 

 matter is discussed on a subsequent page of this chapter. 



On August 7, 1915, two samples were obtained for the purpose of 

 studying the vertical distribution of the nannoplankton material. One 

 sample was taken from the lower water of Lake Mendota, or from the 

 14-20 meter stratum, and the other from the upper water, or the 0-13 

 meter stratum. The sample of nannoplankton from the upper stratum 

 yielded 1,837.0 milligrams of dry organic matter per cubic meter of 

 water, while the one from the lower stratum contained only 854.0 

 milligrams ; that is, the former yielded a little more than twice as much 

 organic matter per cubic meter of water as the latter. 



CHEMICAL ANALYSES OF THE NANNOPLANKTON 

 Nitrogen and Crude Protein 



Owing to the presence of a large amount of inorganic matter in the 

 centrifuge material, the percentage of nitrogen in the dry sample was 

 relatively low. It varied from a minimum of 1.27 per cent in one 

 sample obtained in 1916 to a maximum of 5.18 per cent in one sample 

 secured in 1915. (See general table. No. 44, p. 207). The mean per- 

 centages ranged from 2.37 per cent in 1916 to 3.63 per cent in 1917. 



Calculated on an ash free basis, the maximum percentages of the dif- 

 ferent years vary from 8.77 per cent of nitrogen to 10.66 per cent, 

 while the minima are about half as much, namely, from 4.24 per cent 

 to 5.91 per cent. The results for the different years are summarized in 

 table 18, p. 192. The mean percentage was lowest in 1916, namely, 

 6.27 per cent and highest in 1917, or 8.27 per cent, the difference being 



