NANNOPLANKTON OF LAKE MENDOTA 67 



former reached a height of about 3 centimeters and in the latter about 

 6 centimeters. The maximum height, fully 6.5 centimeters, was at- 

 tained in vial No. 6 (sample No. 714) ; this was succeeded by a rapid 

 decline to a height of only about 3.5 centimeters in vial No. 7 (sample 

 No. 716) during the following week. By the first of June (vial No. 10, 

 sample No. 722) the amount of material was substantially the same as 

 that obtained on April 18 (vial No. 3, sample No. 708). 



Organic Matter in the Nannoplankton 



In the regular observations on Lake Mendota the material obtained 

 from the two runs of the same week were usually combined into one 

 sample for the chemical analyses. In all there were 35 samples of cen- 

 trifuge material in 1915 and 41 in 1916; only one run per week was 

 made in 1917, or 11 in all, and these were not combined so that the 

 total number of centrifuge samples amounted to 87. The results of the 

 chemical analyses of the various samples are shown in detail in table 

 44, p. 207. 



The variations in the amount of dry organic matter in the centrifuge 

 plankton are shown in table 17, p. 192. The largest amount, 3,151.5 

 milligrams of dry organic matter per cubic meter of water, was ob- 

 tained in sample No. 604 on April 15, 1916, and the smallest amount, 

 795.2 milligrams, in No. 706 on March 9, 1917. It will be noted that 

 this minimum is distinctly below the minima of the other two years, but 

 the mean amount for the three years shows a striking uniformity, the 

 differences being only a little more than one per cent. 



The results for the dry organic matter are shown graphically in 

 figure 28 in which the curves represent the number of milligrams per 

 cubic meter of water that were obtained from the various samples. The 

 first sample obtained in 1915 contained the largest amount of organic 

 matter for that year. It is uncertain whether a larger amount was 

 present before this date or not, but the first sample consisted of two 

 runs, one made on April 21 and the other on April 23, 1915, and the 

 latter produced a larger amount than the former, which indicates that 

 the first sample covered the highest point attained by the nannoplank- 

 ton in 1915. 



A sharp decline in the amount of organic matter was noted during 

 the last week in April and the first week in May, 1915, and the quantity 

 thereafter remained fairly uniform until the middle of June. During 

 the last half of June there was a marked decrease, which was followed 

 by a distinct rise in July. The increase culminated in a peak about the 

 end of the first week in August after which the amount fell rapidly, 

 reaching the lowest point of the year about the middle of August. Dur- 

 ing the succeeding two months there was a gradual increase and there- 



