1920] Allen: Plankton of the San Joaquin River 101 



Keratella quadrata Miill. (Anuraea aculeata.) 



station I Station II Station III Daily Hourly 



Average 1,276,350 42,919 57,432 1,467,437 219,396 



Identification usually certain. Contrary to Illinois records this 

 was numerically our most important rotifer, all stations considered. 

 It was one of the most important planktonts. Furthermore our rec- 

 ords show it to be a distinctly perennial planktont here, since consid- 

 erable numbers were found at all stations in every month of the year. 

 At Station I there M'as only one lapse in the record. The increase was 

 unusually uniform from January 5 to the maximum on June 28, fol- 

 lowed by a similar decrease to December 31. At Station II the occur- 

 rence resembled that of K. cochlearlis in that there was a decrease in 

 numbers with a few absences in the summer months, thus making the 

 records for spring and autumn more prominent. The maximum came 

 on October 18 not far from the K. cochlearis maximum. At Station 

 III the records for the summer were not materially different from 

 tliose for spring and fall except that the maximum came on October 

 11 in a much larger pulse than at other times. 



Not only do our records fail to correspond with those of Illinois, 

 they also fail to agree at our three stations. Consider, for example, 

 the maximum in relation to temperature. Station I shows a maximum 

 in a strongly developed pulse at a temperature of 23° C, Stations II 

 and III at 17° C. Station I has its largest numbers in summer. Sta- 

 tion II its smallest and Station III much the same as in other seasons. 

 In spite of these differences some definite conclusions are possible. 

 The vastly larger numbers at Station I at all seasons indi- 

 cate not only a beneficial effect of sewage but the importance of the 

 food factor. The smaller numbers at all stations in January and 

 December show that temperatures may get low enough to be injurious, 

 though it is not clear that this may not be due to reduced food supply 

 incident to low temperature. In fact, when one considers that the 

 numbers appearing at Station I at 7° C. or 9° C. were as large as 

 those found in the culminations of ordinary pulses at the other sta- 

 tions in any season, it seems that the range of temperatures in ordi- 

 nary fresh waters has little direct bearing on production. The un- 

 usually strong pulses in October at Stations II and III indicate a 

 beneficial effect due to relief from Stagnation by the incoming 

 autumnal freshet waters from the mountains. 



