379 
by from 25 to 100 per cent. This is apparently due to the pre- 
ponderance of the impounding factor in the former. 
Thus in this season also the similarity in the movement of 
production noted in the previous year can be traced, and the 
excess of production in Dogfish over Quiver continues in the 
main. Its run-off therefore serves in this period to enrich the 
plankton alike of Quiver Lake and the river. This is correlated 
with the high levels and consequent increase in the impounding 
factor and the relative diminution of vegetation in this area. 
GENERAL SUMMARY, 
RELATIONS OF PRODUCTION IN DOGFISH AND QUIVER LAKES TO THAT IN THE ILLINOIS RIVER. 
The analysis of the data of production in these two lakes 
leads to the following conclusions. 
Plankton production is in a large degree a function of the 
time allowed for the breeding of the plankton. Thus at times 
of high water, when both lakes are filled principally with the 
impounded backwaters of overflow, production is greater, 
other things being equal, than at low water, when a greater 
proportion of water of the lake (Quiver) is of recent origin 
from tributary creeks and springs. So, also, areas such as Dog- 
fish Lake, in which by reason of absence of tributaries and 
springy shores the impounding function is greater, show a great- 
er plankton content than similar areas (Quiver) where by reason 
of access of tributary water the impounding function-is de- 
creased. 
Vegetation of the character of that found in these lakes 
seems to exercise an inimical effect upon plankton production. 
Thus the season of dominant vegetation is generally one of low 
production in these lakes. Also during this period, as levels fall 
and occupation of the lake by vegetation becomes relatively 
greater, production generally declines, and, conversely, produc- 
tion rises when levels rise. Years of greater dominance of veg- 
etation, other things being equal, are wont to exhibit a decline 
in production, and, conversely, with lessened vegetation pro- 
duction rises. 
