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 

 w^ith 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 w^hen 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. 



