538 



midsummer decline and the autumn maximum are evident, but 

 not in all oases well defined. The environmental factors com- 

 mon to both, especially that of temperature, thus tend to pro- 

 duce similar effects. Throughout all these changes, save in the 

 low-water period of 1897, the marked contrast in the total 

 population continues unbroken, a constant witness to the effect 

 of the great points of difference in the two habitats, namely, 

 the age of their respective waters and their relative richness in 

 the immediately available elements for the support of life. It 

 is only when these points of contrast in the environment van- 

 ish, by reason of the slight current, as in the low water period 

 of 1987, and by reason of the increase of the products of or- 

 ganic decay at such times in the water of Spoon River, that the 

 plankton of the two streams approaches equality in production 

 and exhibits a comparable assemblage of constitutent organ- 

 isms. 



The contrast between the autumn planktons of 1896 and 

 1898 and that of 1897 is not only due to the low water of this 

 year, but also to frequent flushings which were caused by the 

 many minor rises during the autumn of the two years first 

 named. (See Plates X., XL) These autumnal rises are due to 

 general rains, which, as a rule, affect both Spoon River and the 

 main stream at about the same time. The difference in their 

 effect upon the plankton of the two streams is very evident in 

 the data of Table XIV. The autumnal plankton of the Illi- 

 nois shows a somewhat similar population in all three seasons. 

 Spoon River, on the other hand, shows a great falling off in 

 numbers — from several million to a few score thousand — in 

 both autumns, in which these rises are frequent. The effect of 

 these minor floods is thus deleterious to the plankton of the 

 tributary in accordance with the general principle that they 

 decrease the time for breeding the plankton in its waters. They 

 also prevent the development of plankton maxima in the 

 tributary by repeated removals of whatever accumulations are 

 produced. Owing to the differences in their volumes and in 

 the slope of their channels, these flushings are more complete 



