453 
Flag Lake. This, like Quiver Lake, is an area of great con- 
trasts in production, resulting from its varying occupancy by 
vegetation as levels rise and fall and from the changes in nutri- 
tion available for the plankton attendant upon its growth and 
decay. In so far as the data go they indicate a production sey- 
eral fold greater than that attained in Quiver and Dogfish lakes. 
Thompson’s Lake. In this area, by virtue of its considera- 
ble size and freedom from immediate access of tributary water 
of much volume, the fluctuations of the hydrographic, and to 
some extent of the other, factors of the environment are ina 
measure equalized. Production is therefore less disturbed than 
in the backwaters previously named, pursuing what may be 
called a more normal course, and is accordingly greater than 
theirs,—excepting perhaps that of Flag Lake in seasons of high 
water,—being three-fold that in the channel, from which its 
main water supply is directly drawn. Owing to this intimate 
connection with the river and freedom from dominating local 
influences the planktograph of this lake is more like that of the 
river than are those of other localities. Comparative freedom 
from vegetation, highly developed impounding function, free- 
dom from access of tributary water of recent origin, and close 
connection with channel waters rich in organic matter, all com- 
bine to cause the high production found in this area. 
Phelps Lake. Yn this area production reaches the highest 
level found in any of our localities. It exceeds that in the chan- 
nel eightfold, and that in the most productive backwaters else- 
where examined by us by two and one-half- fold to threefold. 
Ingress of flood water from Spoon River depresses production 
at high levels. At lower levels the freedom from vegetation 
of the coarser, more cumulative, and permanent sort, the abun- 
dance of alge whose decay releases nutrition for the summer 
plankton, and the absence of tributary creek or spring wa- 
ters, all favor the high production found in this area. Per- 
haps most potent of all factors is the isolation of the lake at 
high levels and consequent retention within its boundaries of 
