523 



ment-basin of the Illinois. It also receives a moderate amount 

 of sewage from the cities of Lewistown and Canton, and the 

 drainage from a considerable number of towns. Its diluent 

 effect upon the plankton of the Illinois is thus not due to the 

 poverty of its own waters but rather to the excessive fertility 

 of the main stream, a fertility resulting from the sewage and 

 industrial wastes received by that stream from the cities of 

 Chicago and Peoria. 



The contrast in fertility as indicated by the analyses tabu- 

 lated above is not as great as the differences in the plankton 

 production of the two streams. The ratio of nitrates which 

 perhaps most fully expresses their relative fertility is 1.01 to 

 1.58, while the ratio of the plankton production as expressed 

 in the average of the monthly means of the catches by the 

 silk-net method is 0.256 to 2.71. The failure of Spoon River to 

 develop a more abundant plankton is thus apparently due to 

 some other cause than the lack of nutritive elements in the 

 water for the support of the plankton. The development of a 

 considerable volume of plankton in it at times of low water 

 and slack current makes patent the probability that the lack of 

 time for breeding is at least one of the important factors in the 

 relative paucity of the plankton of this tributary stream. 



QUANTITATIVE COMPARISON. 



A comparison of the quantities of plankton taken by means 

 of the silk net in the two streams aft"ords a fair contrast of 

 their relative productivity. Certain sources of error are, how- 

 ever, present in the data of comparison, and as they are not 

 equally distributed in the case of both streams they invalidate 

 to some undetermined extent precise comparisons. These 

 sources are the leakage of the plankton through the silk and 

 the presence of silt. The plankton escaping through the silk 

 is largely made up of the M((sti(jophor(( and small diatoms and 

 alga3, and they are found alike in both streams. The presence 

 of a more abundant plankton in the Illinois and the somewhat 

 flocculent nature of much of its silt tend to induce more per- 



