1920] Allen: Plankton of the San Joaquin River 47 



resembles the river in temperature and is more like Stockton Channel 

 in stability suggests a stronger influence of temperature. A glance 

 at plates 7, 8 and 9 shows a marked resemblance of all three stations 

 in low production of diatoms through the first twenty and the last six 

 weeks of the year. All show comparatively heavy production through 

 all the intervening period though the culmination is more nearly 

 median in this time at Stations II and III. This characteristic of dis- 

 tribution is common to all the algae. These major pulses are evidently 

 composite though the exact location of their maxima may be due to 

 single species. The diatoms as a group show more marked indication 

 of pulses recurrent at approximately four weeks intervals than do any 

 other algae. 



One notable difference in the three stations is that the total pro- 

 duction is less at Station I than at either of the others. Except for 

 the difference in the location of the maxima this was shown through- 

 out the year. The production in winter and spring is continuously 

 less at Station I. Although the maximum came earlier it did not 

 appear so abruptly nor decline so quickly as at both Station II and 

 Station III. Sewage seems to be detrimental, as does a temperature 

 above 23° C. However, it is not certain that temperature is the deter- 

 mining factor, for stagnation of the water probably has a deleterious 

 effect more quickly in sewage water than in water comparatively clean. 

 It seems quite possible that simultaneous strong flood currents through 

 Stockton Channel and the river would hold the maximum back to a 

 similar date. Some light on this question may be expected from the 

 1914 series which covers a time of heavy flood. Again there is the 

 possibility that predatory organisms, notably Entomostraca, cut down 

 the supply of diatoms in spite of favorable conditions for develop- 

 ment. The maximum for Entomostraca comes at the time of decline 

 of production of diatoms in the summer ( plates 3,9). 



The fact noted by Kofoid (1908) that the volumetric measure 

 shows mainly the zooplankton is especially important here because 

 there is not sufficient check by other methods to give any idea of the 

 relative loss of phytoplankton through the net. The diatom count cor- 

 responds pretty closely, in its rise and decline, to the volumetric rec- 

 ord (plates 1, 7, 8, 9) at Station II and III but no very definite rela- 

 tion appears at Station I. This might be expected from the fact that 

 a moment's examination gives one the impression that the Stockton 

 Channel plankton is distinctly animal and the river plankton mainly 

 plant. 



