233 



gives in general a suggestive index to the extent to which 

 oxidation of the sewage has proceeded. 



When the facts of both tables are taken into consideration 

 it becomes evident that the sew^age of Chicago has been thor- 

 oughly decayed, and its fertilizing capacity presumably to some 

 considerable extent utilized in the development of the plank- 

 ton, before the water reaches Havana. The sewage of Peoria 

 likewise, during the summer months, is well oxidized by the 

 time the sluggish current of low water brings it to Havana, 

 thus adding new^ resources for the increase or rehabilitation of 

 the plankton. During the colder months the process of decay 

 is not so fully completed owing to the lowering of the temper- 

 ature and the increased current attending the higher water 

 which often prevails at that season. In the winter the initial 

 effect of the sewage upon the plankton may be witnessed, par- 

 tially at least, at Havana. At all seasons the plankton of the 

 channel waters passing Havana is the resultant of two succeed- 

 ing pulses of fertilizing additions to the normal constituents of 

 stream w^aters. It represents during the warmer months pre- 

 dominantly the later phases of the cycles of organisms, which 

 multiply and succeed each other with considerable rapidity 

 after the enrichment of the water. In this important particu- 

 lar the plankton at this point in the stream differs from that 

 of the lake, where the whole sequence of changes may be 

 accomplished in one locality. The fact that a relatively small 

 proportion of the tributary waters enters the stream between 

 La Salle and Havana makes it possible for these chemical 

 changes to take place, and for the plankton cycles to run their 

 courses with less interruption and disturbance than in other 

 parts of the stream. 



The enrichment of the Illinois River and its backwaters 

 by the sewage of Chicago and Peoria has been utilized thus to 

 some considerable extent before the waters reach Havana. 

 The chemical products of its oxidation have been converted 

 into aquatic vegetation and phytoplankton, and some of the 

 latter in turn into zooplankton. The development of new^ 



