PLANKTON OF LAKES WAUBESA AND KEGONSA 131 



Curves A and B in figure 39 represent the organic matter in the net 

 plankton and in the nannoplankton, respectively, of Lake Waubesa; 

 they show that the former was always smaller than the latter in 1916. 

 In 1915, on the other hand, the net plankton gave a larger yield of dry 

 organic matter than the corresponding nannoplankton catches in two 

 instances. (See samples No. 597-598 and No. 5128-5129, tables 47 and 

 48.) Thus, in half a dozen catches out of 37 from Lakes Monona and 

 Waubesa, the net plankton yielded a larger amount of organic matter 

 than the nannoplankton, but in all of the samples from Lake Mendota 

 the net plankton gave a smaller amount of organic matter than the 

 nannoplankton. 



These two curves (A and B in figure 39) do not indicate any definite 

 correlation between the quantity of organic matter in the net plankton 

 and that in the nannoplankton. There was a moderate rise in the 

 former in late June and early July corresponding to a rise in the 

 latter, but beyond this point the net plankton changed independently 

 of the nannoplankton. In fact, the former showed a decline at the time 

 that the latter reached its maximum point and the maximum of the net 

 plankton was not reached for fully three weeks after the nannoplankton 

 had passed its maximum. 



Curve C in figure 39 shows the amount of dry organic matter in the 

 total plankton of Lake Waubesa, that is, the net plankton plus the 

 nannoplankton. The autumnal peak of the total organic matter pre- 

 sents a different outline from that of either curve A or curve B. The 

 maximum point in the total organic matter is correlated in time, how- 

 ever, with the maximum of the net plankton instead of the nannoplank- 

 ton and the small secondary peak is found on the September side of the 

 main peak as in the net plankton instead of on the October side as in 

 the nannoplankton. These two facts show clearly that the quantity of 

 net plankton in this lake is large enough to affect materially the form 

 of the curve representing the organic matter of the total plankton. 

 The dry organic matter in the total plankton amounted to 6,378.3 milli- 

 grams per cubic meter on October 2, 1916, as compared with 1,271.7 

 milligrams on June 15 ; the latter was the minimum for the year and 

 the October maximum was five times as large as this minimum. 



Nitrogen. Quantitative determinations of the nitrogen were made on 

 all of the samples of nannoplankton taken from Lake Waubesa. A sum- 

 mary of these results is given in table 40, p. 201, and all of the de- 

 terminations are shown in table 48. The material obtained in 1915 

 contained a higher percentage of nitrogen than that secured in 1916. 

 The mean for the former year was 9.03 per cent, while that of the latter 

 was 7.41 per cent. Both the maximum and the minimum were higher 

 in 1915 than in 1916. The difference between the maximum and the 



