1920] Allen: Plankton of the San Joaquin River 39 



ring in other months were very small. It is quite difficult to de- 

 termine the cause of the two pulses at Station I as distinguished from 

 the other two stations. It could hardly be temperature since that 

 factor remains too nearly constant during the period involved. The 

 earlier increase in May might be ascribed to the more quiet water rich 

 in organic matter, and the continuance through November might be 

 aided by the sewage. The most probable explanations of the inter- 

 mediate fall in numbers seems to be that predatory organisms may 

 have been most prominent at that period or that stagnation of the 

 sewage laden water hindered growth and multiplication. The possi- 

 bility of the last named factor being the more important is supported 

 by the fact that increase comes in September when there begins to 

 be some relief from stagnation by increase of supply from the moun- 

 tain streams. This relief was not very great in 1913, however, nor is 

 it very well marked in any year. The possibility of interference by 

 other organisms is supported by the fact that the numbers present 

 are mainly influenced by the numbers of Scenedesmus, an organism 

 very likely to be extensively used for food by some of the organisms 

 of the zooplankton. Amongst the Ciliata, Vorticella seems most likely 

 to be responsible while Asplanchna is the most prominent of the 

 Rotifera. But the Copepoda are still more characteristic of this 

 period and the summer decline of Scenedesmus may be due mainly 

 to their activity. Chlorophyceae were never very conspicuous in 1913 

 and they were outnumbered by diatoms 14 to 1 and by Mastigophora 

 about 2 to 1. Uncertainty as to the percentages of losses of various 

 forms through the net makes definite conclusion impossible. 



The very interesting question concerning recurrent pulses and 

 their relation to lunar cycles, discussed by Kofoid (1908), cannot be 

 answered any more definitely here. It is clear from these net catches 

 that there are recurrent pulses (plates 1-5) at about three to six 

 weeks intervals but there is nothing which warrants more than an 

 indorsement of his provisional conclusion that there may be an in- 

 crease in number of chlorophyll bearing organisms to correspond 

 with each recurrent increase of light from the moon. It is alto- 

 gether probable that this problem cannot be solved until some one is 

 able to carry a long series of daily catches, carefully timed, and by 

 more accurate methods than those of the silk net. Whether any filter 

 method would suffice is hard to say. It was hoped that the daily 

 series carried for thirty-one days in 1913 would help to solve this 

 problem but it presented no conclusive evidence. (PL 6.) 



