1920] Allen: Plankton of the San Joaquin River 51 



ant planktont. Occurs five times at Station I, thirteen times at Station 

 II with a maximum of 105,792 at the end of May, and four times at 

 Station III. 



Cyclotella spp. 



station I Station II Station III Daily Hourly 



Average 15,052,042 6,335,253 6,865,673 13,191,571 5,415,717 



Genus identification certain. Probably includes three or more 

 species, certainly two. Impossible to distinguish species completely 

 while counting tliough the attempt was made throughout the whole 

 series for 1913. It was finally decided that more accurate conclusions 

 w'ere probable if all counts were thrown together. So few individuals 

 are retained by the net that any conclusion must be taken with cau- 

 tion even tjien. Present through the year in all collections at all sta- 

 tions. Wide range in size. Most of them nearer the smaller limit as 

 noted by Kofoid (1908). 



Maximum numbers at Station I in June, at both of the other sta- 

 tions in October. One minor pulse is of unusual interest because it 

 falls on January 19 at all three stations (tables 1-3). There is no 

 great change in temperature to account for it but it came after a week 

 of cloudy weather with more or less rain on the five days immediately 

 preceding. It would seem that the condition of falling flood is res])on- 

 sible in this case. Possibly tlie June maximum at Station I can be 

 explained in the same way. No such local conditions apply in case of 

 the October maximum at Stations II and III. There was, however, 

 about that time higher water than there had been for several weeks 

 previous, due to the inflow from mountain tributaries enlarged by the 

 early mountain rains. 



So far as our present records show, the optimum temperature seems 

 to be nearer 20° C. than 15° C. as found by Kofoid (1908) in Illinois. 

 Sewage contamination, stagnation and flood waters all appear to be 

 factors of marked importance in determining maximal production of 

 this diatom. There is no indication of a maximum corresponding to 

 the volumetric maximum (plate 1) such as was observed iu Illinois, 

 but this may be due to the small numbers caught as compared with 

 those escaping. 



Cymatopleura solea Breb. 



station I Station TI Station III Daily Hourly 

 Average 219 11,043 3,796 



Identification satisfactory. Recorded only six times at Station I, 



but found at intervals through whole year at other stations. Numbers 



