289 



The hydrographic conditions and the location of the test 

 in the stream are responsible for a large percentage of this va- 

 riation. Thongh the low- water levels cut off and reduce the 

 diversifying action of impounding backwaters, the slight cur- 

 rent minimizes the equalization due to mingling by the flow 

 of the water in the channel, and, most of all, the location of 

 the test just below the outlets of Quiver Lake and Spoon Riv- 

 er (PI. II.) involves the full effect of the diluent action of 

 their relatively poorer waters. In Spoon River, on the day of 

 the test, 3.12 cm.^ of plankton per m.'' of water was found 

 (Table IV.), while in Quiver Lake on October 1 there was only 

 .07 cm.^ per m.^ (Table V. ). The discharge from Quiver Lake 

 is reinforced by the seepage from springs along the eastern 

 shore, and these diluents are probably the cause, to some extent, 

 of the low plankton content in the two collections nearest 

 the eastern shore — 2.4 and 8.88 cm.'^ to an average of 6. for the 

 ten collections. The effect of Spoon River is seen in the much 

 smaller decline in the inshore collection on that side of the 

 river. Combined with the diluent action of these plankton- 

 poor tributaries may also be the effect of shoal water and the 

 horizontal stratification of the plankton. 



If we eliminate from the test the two collections made in 

 the marginal belt of spring-fed waters, 24 meters wide along 

 the eastern shore, the ± departures from the mean fall from 

 ±22.3 and =33.8 to ±12.1 and ±20.2. These latter figures 

 more truthfully represent the variation in distribution of 

 plankton in channel waters including four fifths of the width of 

 the stream — a lateral extension far beyond the range in that 

 direction of the mid-channel collections of our chronological 

 series which form the basis of the conclusions of the present 

 paper. 



The data concerning the local distribution of the plankton 

 in the Illinois River in longitudinal and transverse directions 

 presented in the preceding pages may be summarized as fol- 

 lows: The average ± departure from the mean longitudinal 

 distribution in consecutive catches at the same point in the 



