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.* 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 
Year Jan. Feb. March | April May | June 
SOA ae ctet craves Sites « a —— —_——— —_ 234 
iLO) Se eee RO aan tee ees —— 11 eee 2,044 | ———— O 
COO SEs SSO Ce eae 304 167 1,682 O27 5,701 448 
US OMPIN i Pine ice, eee se 20 540 320 | 32,800 900 
MNS SiS eres nictia te eco tsieere ss 160 (0) 256 300 3,364 356 
GO OF re ter bere On be Ie 36 65 193 ——_—— 1 | ——— —— 
FRVIGGAS eri eyoe nes tak aT. 167 53 668 8), 235) || 13955) 388 
No. of occurrences...... 9 6 15 9 9 10 
Percentage of occur- | 
TOMCESE once 5 eh iS 40 100 82 90 | 72 
