Toe i a 
230 
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.? 
| 
Year Jan. Feb. | March | April May June 
ROG 2 es ceen one a oe RE ECE —— ——— ————— 234 
OST. See ee ae ere —— LAL a 2,044 | ———— O 
SOOM ee eada MeN: 304 167) |11.682 . | 10;070)|- 5: 704 448 
MO enn Mega eres teat Sass ie —_ 20 540 320 | 32,800 900 
HBOS teats Ses coerced errereere 160 0 256 300 3,364 356 
TES OOS E Ree el rs eC, Wr 36 65 193 — | ——— — 
PAS ETEALS Co swwepataiate Recess 2 167 53 668 S235 0 132955 388 
No. of occurrences...... 9 6 15 9 9 10 
Percentage of occur- | 
TENCES SEM wise ves 75 40 100 82 90 UP 





