1920] Allen: Plankton of the San Joaquin River 55 



less nearly the same liere. His observation that silk catches sliowed 

 the same seasonal routine as the filter catclies makes it seem probable 

 that we can use our counts here with some confidence in the conclu- 

 sions they may indicate. M. granulata is recorded in every collection 

 through the whole year at all stations. At all stations the numbers are 

 comparatively small until April, though the million mark was reached 

 a few times before that date at each. The earliest count of this size 

 was at Station I in January. The maximum at all stations falls on 

 September 6, after a considerable period of rather high temperatures. 

 A temperature of 25° C. or higher and stagnation of the water are at 

 least two favorable factors in production. Since Station I has less 

 than 10 per cent of the number at the other stations at that time it 

 seems equally clear that sewage is detrimental. So far as the 1913 

 collections show, there is a rather definite growth period in September, 

 though there are several prominent pulses through the warm season 

 just as there were in Illinois. ]\Iinor pulses at from two to six weeks 

 intervals were quite prominent characteristics of the occurrence of 

 this organism ever.ywliere. The steady decline from the September 

 maximum in spite of occasional minor pulses strongly supports 

 Kofoid's view that temperature is the most potent factor influencing 

 production. 



The time of maximum production corresponds fairly well with the 

 time of greatest production of total plankton mass (plate 1), though 

 the largest number is reached a little later. It is, however, coincident 

 with the maximum of the total count of organisms. To this last total 

 M. granulata contributes largely since it is the most abundant local 

 planktont at Stations II and 111. Owing to its large numbers it also 

 contributes a very considerable mass. 



There was a good deal of variability in this species. Spines varied 

 from very coarse, prominent projections to none. Granulation was 

 very prominent in some cases, absent in others. There was a wide 

 variation in size. This was so great that for a time a small form 

 was listed as BI. granulata A and counted separately (tables 1-3). By 

 so doing the main count was fairly well restricted to the more nearly 

 typical granulata and to the variety spinosa. Sometimes these seemed 

 to be nearly equal in numbers but usually the spinous form was less 

 noticed than the other. 



Various encumbrances of the filaments were common. None of 

 these were certainly identified. The only one counted was a small 

 Rotifer egg listed as Diurella sp., by guess. 



