235 



78, and a secondary, hiemal rise becomes apparent in December. 

 This December pulse of Chydorus is one of the elements in the 

 upward movement of production in this month (see Part I.), and 

 fuller data may serve to connect it fully with the September-October 

 pulse, especially in more stable conditions. Both of these autumnal- 

 hiemal movements have less than one tenth of the development that 

 the vernal pulse exhibits. 



The number and percentage of occurrences also confirm the 

 conclusions drawn from the numbers per m. 3 Percentages run 

 higher in the spring, in March-May, and in September-October 

 and in December, and lower in June- August, November, and 

 January-February. Chydorus occurred in all March collections, 

 and in only one third of the August collections. 



The analysis of the data in this table indicates the presence of 

 Chydorus in the plankton practically throughout the whole year in 

 the whole seasonal range in temperatures, with the larger develop- 

 ments following shortly after the thermograph passes the yearly 

 mean (57 average of monthly means of surface waters) in vernal 

 rise and autumnal decline, the maximum development in April-May 



SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M. 3 



