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 subuierged 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 suspensioii 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. 



