533 



The Rhizopoda are twelve times as abundant in the Illinois 

 as in Spoon River. During the winter and spring flood of 1897 

 the tributary waters contained more Rhizopoda than the main 

 stream, but throughout the remaining period of our collections 

 the tributary waters would serve, almost as a rule, as diluents of 

 the rhizopodan fauna of the Illinois. This group of planktonts 

 did not show an increase in the low-water period of 1897 simi- 

 lar to that of many other groups — a phenomenon to be ex- 

 plained by the fact that the diatoms, their main food supply, 

 made no unusual growth during that period. 



The species of Rhizopoda are about half as many in Spoon 

 River as in the Illinois (17 to 30 — see table on page 518), and 

 all of the species from Spoon River are present in much the 

 same relative proportions in the Illinois. There do not appear 

 to be any which find their centers of distribution in the tribu- 

 tary. Its contributions of rhizopods thus neither diversify nor 

 relatively increase the plankton in the main stream. 



The data concerning Mastigophora are to a large extent 

 vitiated by the error of leakage. The tabulations of the silk- 

 net catches indicate their great predominance in the main 

 stream, 1 to 24 as an average, though other seasons than the 

 low water of 1897 exhibit a much greater contrast, that for the 

 whole year 1898, for example, being 1 to 351. In the period of 

 low water of 1897 the tributary stream showed a greater devel- 

 opment of these organisms than the main stream, especially of 

 the green forms. 



The presence of these organisms in Spoon River in consid- 

 erable numbers during the heated term of every year is indi- 

 cated by the remarkable water-bloom which appears on clear 

 still days about the middle of the afternoon. This is similar 

 to that of the Illinois, but seems to be better developed, taking 

 the form of a decided green scum on the surface. How far 

 this indicates a quantitative predominance of the organisms of 

 the water-bloom can only be determined when our filter 

 catches shall have been examined. So, far as the silk catches 

 go they do not show the quantitative predominance of the 



