1920] Allen: Plankton of the San Joaquin River 119 



question as to the sufficiency of the above evidence, but it will be 

 noted that after 12 m. there was a steady increase broken by only one 

 fall below the last preceding number and this break occurred at 6 p.m. 

 A more detailed analysis of tlie chlorophyll bearing group very forci- 

 bly emphasizes the reliability of the record covering this point. In 

 plate 11 it will be seen that Schizophyceae and Bacillariaceae have 

 very pronounced increase after 12 o'clock, while Chlorophyceae and 

 Conjugatae have moderate increase, somewhat wavering, and that 

 Mastigophora have a strong but erratic rise from greater numbers at 

 1 P.M. a low minimum, to 7 p.m., almost equal to the forenoon maxi- 

 mum. It is also clear that all these except the Chlorophyceae show 

 the sharp temporary decline at 6 p.m. Looking up species records 

 in table 5, we find that the 6 p.m. decline is mainly due to deficiencies 

 in numbers of Nostoc and Cyclotella. Also that the erratic record of 

 Mastigophora through the day is due principally to Chromiilina, 

 Heniidinium and Traclidomonas, all very difficult to identify or to 

 count. Taken as a whole, the evidence indicates that the records 

 give a fairly dependable idea of the history of the chlorophyll bearers 

 through the twelve-hour period. From this history the tentative con- 

 clusion may be drawn that temperature is a major if not the deter- 

 mining factor in daily fluctuations of numbers. It may be, however, 

 that wind and waves do not exclude enough sunlight in shallow water 

 to make any great difference and that the greatest influence in this 

 case was due to sunlight. This last view is supported by the history 

 of the other gi'oups. 



In case of the Protozoa, the Rhizopoda (pi. 11, and table 5) and 

 the Heliozoa (table 5~) give responses similar to chlorophyll bearers, 

 i.e., an afternoon rise ; but their numbers are relatively small, for most 

 of the catches and their afternoon prominence might easily be due 

 to the stirring of bottom waters by the strong currents caused by 

 combined wind and tide. This leaves, then, the Ciliata as the only 

 typical protozoan group with a reasonably clear record. Reference 

 to table 5 shows the ciliate assemblage to consist almost entirely of 

 Holophrya, Tinlinnidium and two Vorticella and that they all agree 

 in a strong forenoon representation with an afternoon decline, well 

 pronounced for three of them. Since temperature would be expected 

 to affect these as markedly as it did the chlorophyll bearers, while 

 light probably would not, we have a valid conclusion indicated that 

 light is the major factor in the afternoon rise in numbers of chloro- 

 phyll bearers. 



