Diaptomus was the most abundant adult form in the plankton 

 of 1929. In late May there was an average of h individuals per liter. 

 The number increased slowly in June, but rapidly in early July to 

 the maximum for the season of 19 per liter. There followed a sharp 

 decrease in the last part of the month. In August the abundance was 

 maintained at 12 per liter. After that the number gradually decreased 

 to a minimum of 1 per liter for late October. Thus, Diaptomus presents 

 a remarkably regular curve with low points in spring and fall, and the 

 high point in summer. 



Cyclops was a less important constituent of the plankton than 

 Diaptomus. They were about equally abundant in late May, but Cyclops 

 increased more rapidly, reaching its maximum of 10 per liter in early 

 June. By late June it had decreased to 6 per liter and varied little 

 from this number until October. Like Diaptomus, it reached its 

 minimum at the end of the season, 



Daphnia was less abundant than either Diaptomus or Cyclops, 

 on the average, although it reached a higher maximum than the latter. 

 In late May Daphnia was rare but increased to a high of 12 per liter 

 in late June, The period of abundance was short, however, for by 

 early July it had decreased to 2 per" liter. The count for late July 

 indicates a second period of increase, followed by a gradual decline 

 to the minimum for the season in October, 



Thus, the three important groups of adult Crustacea reached 

 their maxima in rapid succession: Cyclops in early June, Daphnia in 

 late June, and Diaptomus in early July. Diaptomus was so much more 

 niamerous than the other two that it determined the time of maximum for 

 the three combined. This came in July when the combined count was 28 per 

 liter for both periods of the month, 



Diaphanosoma can scarcely be regarded as an important plankter 

 in 1929, for its season was short and it never became niomerous. It 

 appeared in traces in early June but remained below 1 per liter until 

 late July, The maximum of 3 per liter was reached in August, and the 

 form had disappeared by early October, 



The copepod nauplii composed the most important group numer- 

 ically. The average n\jmber for late May was lli per liter. This 

 average figure masks an important point in connection with their 

 probable abundance earlier in the season. Table 69 shows that 11 

 stations were averaged together for late May. Three of the 11 were 

 actually second series taken at 3 of the 8 stations. The first time 

 the 3 stations were sampled they showed an average of I8 nauplii per 

 liter, and the second time an average of 11 per liter. This would 



218 



