573 



ities but one, but falls much below the summer production in 

 Phelps Lake. The ice-sheet is not inimical to a considerable 

 plankton production unless stagnation conditions occur. 



19. Light affects plankton production. The half year with 

 more illumination and fewer cloudy days produces from 1.6 to 

 7 times as much plankton as that with less illumination and 

 more cloudy days. Seasons of unusual cloudiness are accom- 

 panied by depression in production. 



20. Lakes rich in submerged vegetation produce less plank- 

 ton than those relatively free from it, in an annual ratio of 1 to 

 6 and a monthly ratio varying from 1.5 to 20 to that of 1 to 20. 

 The higher ratios generally prevail in periods of dominance of 

 vegetation. Quiver Lake produces more plankton when free 

 from vegetation than when it abounds in it. The emergent 

 and rooted vegetation of Flag Lake conduces by its autumnal 

 and vernal decay to large plankton production, but tends to de- 

 press production in summer. 



21. The normal regimen of the course of plankton pro- 

 duction in the Illinois River and its backwaters does not form 

 a definite seasonal planktograph, but consists rather of a series 

 of recurrent plankton pulses, whose varying amplitudes are 

 largely determined by the fluctuating environmental factors of 

 the unstable fiuviatile environment. Hence planktographs of 

 the same locality in different years and of the different sta- 

 tions in the same year show resemblances only in such funda- 

 mental features as the winter minimum and the vernal pulse. 

 The relative productive rank of the several localities is gener- 

 ally maintained, and more completely in the more stable en- 

 vironments. 



22. The plankton of the Illinois River is largely autono- 

 mous. Seepage and creek waters are diluents of its plankton 

 and add little to its diversity. Even Spoon River is generally 

 a diluent, reducing the plankton content 10 per cent, and add- 

 ing but few diversifying species to its population. The reser- 

 voir backwaters, on the other hand, generally contain a more 

 abundant plankton than the channel, the amount, on the bases 



