1920] Allen: Plankton of the San Joaquin River 13 



basis for a new plankton crop. The combined flood periods usually 

 extend from late December to about July, after which there is grad- 

 ual subsidence to the low-water conditions of sluggish flow and par- 

 tial stagnation. It should be said that the term "flood" is here 

 used to include all stages of water at Stockton which are high enough 

 to keep a distinct current in the river. 



The mean annual temperature of the valley, stated as 15.5° C, 

 does not give a very good idea of the real conditions at Stockton. For 

 one thing, the range in temperature in every twenty-four hours is 

 considerable throughout the year. The nights are almost invariably 

 cool even in late summer and by far the larger number of days 

 become quite warm. The average range for the year is about 8° C and 

 is about 3° C in winter and 12° C in summer. On the other hand, 

 the seasonal range is not so very great. Very rarely a high tempera- 

 ture near 40° C is reached in summer and a low temperature of about 

 — 10° C in winter. On the whole, there is good reason for thinking 

 that temperature fluctuations in the San Joaquin River Basin have 

 less influence than some other conditions on the general plankton 

 production. 



Light 



Of course the fluctuations in available light are of great import- 

 ance. These fluctuations are dependent on a number of conditions, 

 such as the seasonal changes in length of days, turbidity of the water, 

 cloudiness, and agitation by wind. All these influences are most ad- 

 verse during the winter months, coincident with adverse temperature, 

 so that it is almost impossible to prove which is most responsible for 

 scarcity of plankton at that time. 



As already stated, the turbidity of the waters in the Stockton region 

 is very great and fairly constant. During the greater part of the first 

 six months of the year this is evidently due mainly to very fine silt. 

 During the rest of the year the high organic content seems to have 

 some influence. Although the net with its brass parts was a rather 

 conspicuous object in clear water, it was never visible one meter be- 

 low the surface at any station. There are, however, no data available 

 for accurate determination or comparison of turbidity. 



Air Currents 



Since Stockton is in a low region adjacent to extensive swamp areas 

 and waterways, it is considerably affected by fog and clouds. The 

 exclusion of light from this cause is quite important in the course of 



