439 
production for those months in the several localities. Out of the 
51 monthly averages in this period (see table on p. 436) 37 are 
below the mean, and of the 14 above, 9 occur in Dogfish and 
Quiver lakes, where reduction in vegetation increases the pro- 
duction. The cause of this sharp contrast in the relative pro- 
duction in the two parts of the year, is to be found in the hy- 
drographic conditions which affect the nutrition of the plank- 
ton. The rank vegetation which filled the forests, marshes, 
and margins of the lakes during the two years of low water 
was submerged by December flood, and by this early submer- 
gence and subsequent decay increased the production in winter 
months. This early consumption of the products of decay and 
the relatively early run-off of the spring flood combined to 
make vernal production relatively low in 1896. A comparison 
of the planktographs of the 6 localities (see plates named at 
the head of this section) will indicate the suppression of the 
April-May, or vernal, pulse in every locality but Phelps Lake. 
No plausible explanation for its occurrence here when it is not 
found elsewhere is apparent. Subsequent floods by their brief 
duration and frequent repetition tend to impoverish the back- 
waters by the removal of vegetation and organic debris, and by 
the run-off of nutrition in solution or suspension and of the de- 
veloping plankton. In more stable conditions or floods of longer 
duration, when the backwaters are impounded for longer times, 
—largely by the restraining action of high water in the Missis- 
sippi,—decay is longer continued, and there is more opportunity 
for the utilization of its products by a plankton not removed 
quickly by the rapid run-off of the flood. 
A comparison of the different regions even in this one year 
bears out this inference. Spoon River, scoured by repeated 
floods and swept by. constant and relatively rapid current, con- 
tains only an insignificant amount (.007 cm.* per m.*) of plank- 
ton. Quiver and Dogfish lakes, rid to some extent of the accu- 
mulated growing vegetation and enriched by dead vegetation 
in their submerged borders, yield in this year the largest an- 
nual mean Of monthly averages (2.19 and 3.99) in our records. 
