223 



early spring of 1896, and 2 in the delayed high temperature of 

 October, 1897. 



The minimum records (less than 500 per m. 3 ) 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 

 Moina 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. 3 , at the 

 most, is 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. 3 , 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. 



