1920] Allen: Plankton of the San Joaquin River 113 



Text Table 6. — Numbers of Kinds op Planktonts by Stations and by Months 



IN 1913 — Continued 



September October November December 



Stations " I II III I II III" I II III" I II~III 



Baoteriaceae 10 1110 11110 1 



Schizophyceae 12 12 11 11 10 6 9 12 11 12 5 3 



Chlorophvceae 11 10 11 11 11 11 10 11 10 11 6 7 



Bacillariaceae 25 28 25 28 25 31 26 29 35 32 29 36 



Conjugatae 565554343454 



Mastigophora 15 12 16 13 9 10 14 10 12 13 8 9 



Rhizopoda 676645756543 



Heliozoa 3 3 4 5 3 5 3 2 2 10 1 



Ciliata 5 3 6 12 6 5 15 8 11 20 6 10 



Suctoria 11112 12 2 3 3 1 



Rhizota 111112011000 



Bdelloida 332332332332 



Ploima 13 17 16 16 15 16 11 11 11 8 12 4 



Cladocera 2 3 12 2 12 12 12 1 



Copepoda 112121110101 



Miscellaneous 010011100021 



Total 104 108 108 116 100 101 108 101 107 115 85 83 



This table shows some points quite well. The number of forms 

 was lowest at all stations in January. There was then steady increase 

 to May, when flood waters were highest and the number of forms re- 

 corded slightly less. Increase in number of forms began again in June, 

 becoming greatest at Stations II and III in August and at Station I 

 in October. The numbers were well sustained at the two stations 

 until December, when there was a marked decrease. Station I not 

 only showed increase to October, but almost equaled it in December. 

 While it is probable that more accurate species determination might 

 change the detail of this showing, it is not probable that the general 

 differences would be affected. 



One legitimate inference from this table is that Station I is less 

 subject to seasonal fluctuations than either of the other stations. At 

 present the only reasonable explanation of this fact seems to be that 

 the dilute sewage of Station I is the prime factor, for the superficial 

 resemblances would lead one to expect tliat Stations I and III would 

 most closely approach each other in character and distribution of 

 populations, rather than Stations II and III. But the reverse is true, 

 and sewage is the only known factor of sufficient moment to account 

 for it. It is true that temperatures run slightly higher at Station T, 

 but is it not possible that this higher temperature is partly due to the 

 rapid turnover of the great quantities of organic matter. 



Aside from the question of temperature there is ample reason for 

 assigning main influence to sewage because of the great food supply, 

 a supply superabundant and hence essentially constant through the 



