570 
5. The mean of the monthly averages of 235 collections 
in the Illinois River is 2.71 cm.* of plankton per m.* of water. 
6. The plankton of the river channel is subject to great 
seasonal and annual variations. The monthly averages of all 
collections indicate a period of minimum production of plank- © 
ton in January-February, of rising production in March, of 
maximum production for the year in April—June, usually cul- 
minating ina vernal maximum about the end of April and 
often declining rapidly to a low level in June. The average 
monthly production declines gradually during the remainder 
of the year to the winter minimum in December. 
7. Individual years vary greatly from these averages as a 
result of hydrographic, climatic, and other environing con- 
ditions in varying combinations. 
8. The waters of Spoon River contain but a very small 
amount of plankton (.465) except at very lowest stages, when 
the flow is at a minimum. Its production at other times (.044) 
is less than one fiftieth of that in channel waters which it joins. 
Chemical conditions in this tributary are apparently such as to 
support a large plankton. The recent origin of the water is 
the cause of the low production. Its diluent effect on the plank- 
ton content of the channel is about 10 per ceut. 
9. Quiver Lake produces less (1.75) than channel waters. 
At high stages its production is relatively larger, and most re- 
sembles that in channel waters, while at low levels, when sub- 
merged vegetation is dominant and access of tributary waters 
of recent origin relatively great, its production is both rela- 
tively and actually low. It is predominantly a diluent of 
channel plankton. 
10. Dogfish Lake produces (3.16) more than channel 
waters, freedom from access of tributary waters permitting a 
higher production than in the contiguous waters of Quiver 
Lake. 
ll. Flag Lake also produces more (9.25) than channel 
waters, the freedom from access of tributary waters, the im- 
pounding function, and decaying vegetation favoring high pro- 
