254 
from the plankton in September—October. In 1898 (Table I.) the 
last record was made October 11—the latest in any year with the 
exception of an isolated record October 26, 1897. Moina muicrura 
is thus distinctly a summer planktont. 
It appears in the plankton only after maximum summer tem- 
peratures of approximately 80° have been reached, and decreases 
rapidly as soon as the autumnal decline passes this point, and soon 
thereafter vanishes from the plankton. Its optimum temperature 
in channel waters is thus near 80°. 
The relation which hydrographic conditions bear to the ap- 
pearance of Moina in channel plankton appears upon a comparison 
of the Moina population and the movement in river levels in differ- 
ent years, as shown in the following table. 
Mo1Ina AND HyDROGRAPHIC CHANGES. 
June July August September October 
Se g Soo g Se g Sica g Sc g 
oo ee ea Vora ye ap er te Po celine ee | 
° fe) o e) (e) ° 
Zo 5 Zo = 29 E Zo 5 25 E 
areata or alae ele ee a i a a ele 
e ° e ° r [e) > ° a ° 
< a < a < a < = < a 
1894 124! Se | AO 415 2d 20 880M) 2 OR eae onset ay 0 Sed 
USOS SAO RAAS 2-7 | Ot STS a 7 ese) e250 Me Sic 87 | 8.8 10:, RZ 
1896 Oy .3.4 |) SS 2a ees 220) |e Ol Seg @, |, 4x6 
1897 Os “6e3"|" 41,3730), 5.2) |" Ws280) | 2.6n| 70,0407 Ora Gos 0.6 
1898 75 | 4.0 660) 7A) 4063), 725 TUOR|eO2 40 3.9 
While the correlation is not proportionate between the extent of 
movement in levels and the Moina per m.%,it is still very evident 
that in years of continued and more stable low water Movzna is 
found in much greater numbers, as appears on a comparison of 1897 
and 1898. It is also confined largely to the more stable part of the 
year, appearing in 1895 in June—July in large numbers, but falling 
off when the minor floods of August-September occur, while in 1897 
the large numbers are found in the stable levels of August. 
