460 



days suffice in most cases for production to recover from the dis- 

 aster of, or respond to the stimulus of, the flood. It is evident, how- 

 ever, that this matter is greatly complicated with the rythmic, 

 pulse-like character of the movement in plankton production . 



In passing, attention should be called to the fact that re- 

 covery from the flood is not merely replacement by up-stream 

 waters rich in plankton, as might be concluded regarding chan- 

 nel waters. It takes place also in the impounded backwaters 

 under conditions of absolute independence of channel, as in 

 Phelps, Dogfish, and Flag lakes. In these cases it is an indig- 

 enous development in impounded flood waters, and, by infer- 

 ence, there must be a process of like import in channel waters 

 and areas more or less intimately connected therewith. 



This necessity of some lapse of time before accessions of 

 tributary flood and spring waters can produce a plankton of 

 any considerable volume makes all important the impounding 

 function of the backwaters of the Illinois River. They act not 

 only as storage reservoirs whence the floods are drawn off in 

 reduced and equalized volume, but they serve also as nurseries, 

 where under favorable conditions the flood waters are seeded 

 with planktonts whose progeny utilize the organic materials 

 in suspension and solution in the invading flood or derived from 

 the invaded territory. The plankton thus developed, or the 

 product of its decay, is carried away by the run-off of the flood 

 into channel waters unless utilized by the larger and more per- 

 manent residents of the backwaters as food, or sequestrated in 

 some land-locked pool or lake as levels fall. 



Intimately connected with the age of the water are the as- 

 sociated factors of current and rate of renewal. In and of itself 

 alone, current has little demonstrable influence upon plankton 

 production, but conjoined with other factors it becomes potent 

 for ill. Thus, in narrow confines, as within the banks of a 

 stream, by its mechanical action it fills the water with silt, 

 which diminishes access of light and thus tends to decrease 

 photosynthesis by the phytoplankton. The silt likewise, as 

 has been pointed out on page 185, seems by adherence to im- 



