454 
the organic substances in solution and suspension in its waters, 
substances which in the other localities run off with the de- 
clining flood. During last stages of low water in Phelps Lake 
there is usually some dying off of the fish and other aquatic 
animals, possibly as a result of extreme temperatures, and the 
lake becomes a favorite resort for fish-feeding water-fowl. The 
organic substances thus released for immediate solution in the 
water and utilization by the phytoplankton may be of sufficient 
quantity to materially increase production at these low levels. 
It may be that the more complete access of light in these very 
shoal waters and the condensation caused by evaporation and 
seepage as the lake dries up are contributory to the increased 
plankton content, but none of these factors seems adequate to 
explain the excessively large production found in the summer 
and autumn in the shallow pools which form the remnant of 
the lake. 
RELATION OF ENVIRONMENTAL FACTORS TO PLANKTON PRODUCTION. 
The detailed discussion of the course of plankton produc- 
tion as defined by volumetric data found in the preceding sec- 
tion of this paper, has afforded many specific instances of the 
relationship existing between the movement in production and 
a number of factors in the environment. Prominent among 
these are hydrographic conditions, temperature, light, chemic- 
al conditions, vegetation, and the reproductive cycles of the 
constituent organisms of the plankton. 
It is my purpose in the following pages to summarize, with 
a few references to specific illustrations, the conclusions as to 
general tendencies and the effect of these various factors upon 
the course of production in the river and its backwaters. 
HYDROGRAPHIC CONDITIONS AND PLANKTON PRODUCTION, 
This is a comprehensive designation for a great variety 
of major and minor influences which continually impinge upon 
the plankton as a result of its environment in water. In the 
case of the plankton of the Illinois River and its backwaters it 
