210 



In spite of this seeming contradiction, I believe the explana- 

 tion still holds in the case of Spoon River. The minimum peri- 

 od occurs during the time of low water, when the principal 

 source of the flow in the stream is ground water which has 

 already been robbed of its nitrates to some extent by terrestrial 

 vegetation. Again, the plankton production of Spoon River, 

 judging from the development of the water-bloom {Euylena), 

 consists largely of chlorophyll-bearing organisms, which also 

 rob the water of its nitrogenous substances. The period of de- 

 velopment of the water-bloom covers the months of summer 

 and early autumn, thus coinciding with the period of depressed 

 nitrates. It is quite certain that the collections of the silk net 

 fail completely to represent the quantity of those minute or- 

 ganisms which compose the water-bloom, and thus give no 

 adequate clue to the amount of nitrogen-consuming organisms 

 present in these or other waters. The reduction in nitrates in this 

 stream during summer months is not, however, as great in 

 quantity as it is in the Illinois River (cf. PL XLV. and XLVII.). 

 The excess of sewage in the latter creates a greater winter 

 maximum, thus permitting a greater range in reduction to the 

 residual minimum of midsummer, which is about the same 

 in both streams. But little correlation between the chemical 

 conditions of Spoon River and its plankton production can be 

 established beyond the reduction in nitrates in the plank- 

 ton maximum of the autumn of 1S97 at a time of abnor- 

 mal low-water. Under normal conditions the plankton curve 

 (silk-net catches) exhibits no movement correlated with or 

 commensurate with the changes in chemical conditions. Flood 

 and current afford here no time sufticient for the expression of 

 the chemical factors. 



In Quiver Lake the maximum and minimum periods ap- 

 pear with distinctness and affect all of the substances in ques- 

 tion. This is partly the result of the diminished effect of 

 floods in this reservoir area, and also of the delimitation of the 

 lake as a separate unit of environment with the cessation of 

 overflow. During the flood period (see PI. III. and hydro- 



