abundant in early June. During the second period of abundance it 

 was less abundant, and no counts of 1 per liter were recorded. 

 Leptodora was present (always less than 1 per liter) from early May 

 to end of the season. 



The Rotifera were fairly abundant from April through June, 

 but were rare during the rest of the season. There were marked 

 fluctuations in abundance, with maxima indicated for early April, 

 early May, and late June. After that time the counts were uniformly 

 low. 



Comparison of seasonal distribution in 1928, 1929» 

 and 1930. 



Fig. 21 was designed to facilitate comparison of seasonal 

 distribution of the four leading crustacean groups in 1929 and 1930. 

 Most of the following discussion will be 'devoted to those groups in 

 the two years, although some attention will be given to other groups 

 and to the data of 1928. 



Diaptomus . Diaptomus was a much more important constituent 

 of the plankton in 1929 than in 1930. For a large part of the 

 period for which comparable data are available (May l6 - October l5) 

 the counts for 1929 were well above those of 1930. The average 

 abundance for this period was 9 per liter in 1929, and h per liter 

 in 1930. It seems probable that there was no early season period of 

 abundance in 1929, corresponding to the one found in May, 1930. 

 The reason for believing so is that the water temperature in May, 

 1930 was almost two weeks in advance of the 1929 temperature (Fig. 8 

 and Fig. 9). Assumi.ng that temperature is an important factor in 

 the control of the increase of the Crustacea, the increase for 1929 

 should come later than the one for 1930. The validity of this 

 assumption is indicated by the fact that Cyclops, Daphnia, and the 

 Nauplii were also more abundant in late May in 1930 than in 1929. It 

 seems likely, then, that Diaptomus had only one period of abundance 

 in the spring and summer of 1929 and this came in July, while it had 

 two such periods in 1930, one in May, the other in late June. The 

 seasonal distribution in 1928 was similar to that in 1930. There were 

 two periods of abundance in each year and the times of these periods 

 were not far different. However, the average abundance for the eight 

 periods represented between late May and early October, 1928, was 8 

 per liter, which is twice that of 1930, and almost the same as that of 

 1929. 



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