22S 
early spring of 1896, and 2 in the delayed high temperature of 
October, 1897. 
The minimum records (less than 500 per m.*) are found during 
minimum temperatures. The numbers increase slightly (generally 
less than 2,000) as temperatures rise in March-April, rise abruptly, 
as they approach or pass 70°, to a vernal maximum in May-June, 
and decline during midsummer excepting when unusual pulses of 
Mona or Diaphanosoma raise the level of the pulse maxima above 
25,000. This decline continues in channel plankton through the 
autumn until the low level of approximately 2,000 per m.?, at the 
most, 1s again attained in October, and falls irregularly to 500, or 
less, aS minimum winter temperatures arrive in December. Ex- 
ceptions appear in 1897, when a well-defined autumnal pulse of 
large amplitude (193,500) is found on September 14, and is followed 
by others of declining amplitudes (137,600, October 5; 5,520, No- 
vember 15; 4,240, December 14) during stable autumnal conditions. 
All of the records above 4,000 per m., with one exception, are 
found at temperatures above 45°, and all in excess of 8,000, with 4 
exceptions, after the vernal rise in temperature passes 70° in April— 
May, and before the autumnal decline reaches this point in Septem- 
ber. The Cladocera are thus planktonts of the warmer channel- 
waters. 
The relation which hydrographic conditions bear to the seasonal 
occurrences of Cladocera is apparent in the yearly averages above 
quoted, and appears still more clearly in a comparison of the 
cladoceran population and movement in river levels in July— 
December, 1897 and 1898, as given below. 
July August Sept. Oct. Nov. Dec. 
Average No. | | } | 
Cladocera 1897 |1898) 1897 1898 1897 1898) 1897 1898/1897|1898)/1897/1898 
per m.$ | | | 
127203050) 139603756 706751700 40350 1615|2532| 620|1945 236 
Total movement 
in river levels, S32 tate) AGN Si) O20 WOLF. GNSS) So Oa e! 
rit dike 
