1920] Allen: Plankton of the San Joaquin River 43 



small numbers in Stockton Channel indicate that much sewage is 

 detrimental. By similar reasoning it might be concluded that Smith 

 Canal (Station III) supported a larger number than the river (Sta- 

 tion II) because more organic matter is beneficial up to a certain 

 point. Such a, conclusion is not entirely warranted, however, be- 

 cause the river has an open channel with some flow while the other is 

 closed at one end. This difference alone might amount to more than 

 the organic content. 



The indication of recurrent cycles due to lunar influence is more 

 distinct with this form than the preceding though they are not at all 

 regular even here (tables 1 to 3). The daily series shows an interesting 

 suggestion of recurrent pulses at intervals of four to six days (table 

 4). In the hourly series (table 5), it appears that both P. horyanum 

 and P. duplex reach maxima in the afternoon, suggesting diurnal in- 

 fluence of light and temperature. The data, however, do not war- 

 rant a conclusion. The hourly series needs extension. 



There is no very definite relation of Pediastrum pulses to the 

 volumetric pulses. 



Pediastrum simplex Meyen. 



station I Station II Station III Daily Hourly 



Average 7,422 82,333 36,044 16,796 49,189 



Identification fairly satisfactory. Coenobia having marginal 

 cells with one median spine were counted under this head. Probably 

 only one species included. Fewer than either of the foregoing fonns 

 at all stations, except that it exceeds P. horyanum in both the daily 

 and hourly series. Rare at Station I through the first six months, 

 considerable numbers there in July, very few in August, maximum 

 in September, rare thereafter. Occurrence at the other two stations 

 similar, except that the numbers were larger. So far as the records 

 go they indicate the same general characteristics of distribution as 

 mentioned for the foregoing species. 



It is evident that Pediastrum is a very important genus, both 

 numerically and volumetrically in the local plankton. The greater 

 numbers here, as compared with Illinois, indicate that local condi- 

 tions are better suited to this genus, but it is also true that this may 

 have been an exceptionally favorable year. It is worth noting in 

 this connection that the representation of Pediastrum is proportion- 

 ally greater here as compared vsdth other forms found in both sec- 

 tions and this fact favors the view that it is really more characteristic 

 of our plankton. 



