124 University of California Publications in Zoology [Vol.22 



One point of general interest may be noted in plate 1, where it 

 appears from the volumetric graph that the total volume decreased 

 gradually to midday and increased again through the afternoon, but 

 that the net volume of plankton increased fairly steadily throughout 

 the twelve hours. This is not what might be expected because the 

 wind and tide of the afternoon should, theoretically, stir the water 

 enough to increase the relative amount of silt. The unexpected silt 

 diminution might be due to collecting conditions, the stronger com- 

 bined current from wind and tide in the afternoon carrying away the 

 silt stirred up by impact of the drain cup cylinder upon the bottom. 



CONCLUSION 



In conclusion it may be said that the following points appear 

 distinctly from the present study : 



1. San Joaquin waters are capable of supporting abundant plank- 

 ton, and they do so in the vicinity of Stockton. 



2. The plankton of the sewage-laden Stockton Channel is distinctly 

 different from that of the river, the number and volume of its animal 

 forms being especially conspicuous as distinguished from the algal 

 dominants of the latter. 



3. Temperature is, within certain limits, the determining factor 

 in seasonal distribution. This may be by direct retardation of growth 

 and reproduction in organisms, or by indirect influence through food 

 supply and gaseous content of water. 



4. Water currents above a very moderate speed are distinctly in- 

 imical to plankton development. 



5. The peculiar succession of rainy season and dry season has 

 resulted in an autumnal maximum of plankton about Stockton, a 

 condition directly contrary to that of vernal maxima recorded by 

 various observers in other localities. 



6. Collections taken at intervals of one week or more do not furnish 

 a basis for accurate determination of plankton distribution through 

 the year. Daily collections properly taken would probably do so. 



7. There is some evidence in favor of the idea that increase of 

 lunar light tends to the increase of plankton, especially chlorophyll 

 bearers. 



8. There is evidence to show that fluctuations in amount of plank- 

 ton occur at various hours of the day. 



