456 



our environment the submergence of all the localities, large 

 and small alike, in the major floods, tends to obliterate areal 

 differences, and to unify their plankton and to give them all 

 much the same initial start in the season's course of production. 

 This is noticeable in the marked similarity in the vernal plank- 

 tographs of the several localities in years of high water. The 

 areal differences in later months and lower river stages are, 

 however, very considerable (cf. PI. XX. and XXI.). Thus in 

 Thompson's Lake there is a broad expanse of open water sev- 

 eral square miles in extent, while in the last stages of Phelps 

 Lake there are only a few pools, a few acres in extent (PI. XXL). 

 Under similar climatic conditions production in both runs 

 high, higher in fact than in other localities, and the same plank- 

 tonts are dominant, though in varying proportions in the two. 

 In general the plankton content per cubic meter runs very much 

 higher in Phelps Lake, the smaller body of water. There is no 

 evidence that the smaller size has anything at all to do with 

 this larger production, but this instance suffices to show that a 

 typical plankton with large production may be found in small 

 areas. On the other hand, Quiver and Dogfish lakes, next in 

 size to Phelps Lake, are the least productive of the backwaters. 

 Thus, area in itself tends apparently neither to deter nor to pro- 

 mote production. 



The effect of depth, in all of its relations, upon plankton 

 production is manifestly not demonstrated in our data, since all 

 of our collections have been made in shoal water of less than 

 10 meters in depth. Our deepest waters are Spoon and Illinois 

 rivers, where spring and flood water of recent origin tend to 

 depress production, and the shoaler waters are found in the 

 backwater reservoirs where impounding favors larger plankton 

 content. The significance attaching to the fact that our lar- 

 gest plankton production (224 cm. 3 per m. 3 on August 24, 1898) 

 is found in Phelps Lake, the shoalest of all our localities, and in 

 depths no greater than 20 centimeters, is a matter of conjec- 

 ture, since many other factors are also involved. The highest 

 production in this lake was, as a rule, found during the periods 



