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 (Huglena), 
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 [linois River (cf. Pl. XLV.and XLVIL.). 
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 1897 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 sufficient 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 Pl. II. and hydro- 
