1920] Allen: Plankton of the San Joaquin River 107 



that period. In addition, Stations II and III showed occasional ab- 

 sences up to May and June, and in the last two months of the year. 

 The greatest abundance at all stations was in July, August and Sep- 

 tember with the maximum in August or September at all. 



The count of Entomostraca is unsatisfactory for two reasons: first, 

 specific identifications were too difficult for the writer under the con- 

 ditions of work ; second, the method of counting permitted too much 

 error. Special trials showed that even distribution of Entomostraca 

 in the counting cell seldom occurs. As the records stand, the main 

 error was due in most cases to counting only the same fractional field 

 as was counted for smaller organisms. It was the intention to make 

 a special count of Entomostraca later in order to correct this, but an 

 examination of the records showed that it was not probable that the 

 limited increase in accuracy of count would make any material change 

 in the possible conclusions. For that reason the intention of recount- 

 ing was abandoned. So far as can be estimated from several recounts 

 made at random, the variance in the count by the two different meth- 

 ods is mainly from 10 to 25 per cent. 



Cladocera 



Station I Station II Station III Daily Hourly 



Forms recorded 3 4 4 2 2 



Individuals per cu. meter.. 3,836 7,385 9,197 ' 7,432 34,752 



The Cladocera were rarely prominent at any station. There was 

 probably a much larger number of forms present than was recorded, 

 since names were given only to those with conspicuous characters. The 

 averages were distinctly lower at all stations than they were found 

 to be in Illinois, except for one year there of recurrent floods. Since 

 1913 seems to have been an unusually stable year here, there is reason 

 for believing that the Cladocera are naturally fewer here. They were 

 recorded in every month of the year at some station but the catches 

 were few and far between and the numbers small at all stations until 

 August. The maximum came on October I at Station I and in Sep- 

 tember at Station II and III, after which the numbers rapidly de- 

 clined at all stations. Our records therefore agree with those of 

 Illinois in showing the favorable effect of stable (or even stagnating) 

 water and of high temperatures. We have the further indication that 

 sewage is detrimental since Station I had so much fewer numbers than 

 either of the other stations. The explanation of the deleterious effect 

 of flood waters in Illinois (Kofoid 1908) applies equally well here. 

 The evidence for recurrent pulses is not convincing here at any station. 



