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 sev- 

 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 in a 

 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. In 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 abim- 

 dance of algae 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 



