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 sewage 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 isthe resultant of two succeed- 
ing pulses of fertilizing additions to the normal constituents of 
stream waters. 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 [lhinois 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 zoéplankton. The development of new 
