569 



lation which should efficiently prevent pollution of the stream 

 by deleterious industrial wastes, protect the most desirable 

 food and game fishes from depletion, and, at the same time, per- 

 mit the full utilization of the annual crop of matured and mar- 

 ketable fish, there is no apparent reason why the Illinois River 

 and its backwaters should not become an increasing source of 

 wealth to the state, and the great waste which now occurs be 

 utilized to a considerable extent in future development. 



Conclusions. 

 The following are the more important conclusions arrived 

 at from this examination of the plankton and its environment 

 in the Illinois River and its backwaters, based upon the study 

 of 645 collections made in 7 localities in 1894-1899. 



1. There is little'correlation between the seasonal flux in 

 chemical conditions (as shown in data of sanitary analyses) 

 and the seasonal course of plankton production (as shown in 

 the catches of the silk net). The nitrogenous matters are in- 

 fluenced by the plankton pulses, especially when diatoms are 

 multiplying rapidly, but the changes are not uniform or pro- 

 portional. 



2. The plankton in the Illinois River is distributed with 

 a uniformity approximately equal to that found in German 

 lakes and in Lake St. Clair. The average departure from the 

 mean in short distances (3 miles) probably falls within ± 10 

 per cent. Chronological catches in periods of 2 to 15 days in 

 14 series yield an average departure of ± 14.1 per cent. In the 

 river, in 205 miles of the course the average departure in flood 

 conditions was ± 51 per cent., or ± 43 per cent, if the river is 

 divided into four sections, or ± 29.7 per cent, if computations 

 are based on total catch of the net. 



3. The average departure from the mean plankton con- 

 tent in two tests in a cross-section of the river is ± 27.2 or 

 ± 23.2, or, omitting marginal collections, ± 21.9 or ± 12.1 per 

 cent. 



4. The plankton method can be applied to a stream as 

 legitimately as to a lake. 



