41 



The relation is here more apparent, but the resulting pulse is no 

 larger than those following upon floods during summer, and but 

 little larger than the ones which precede it in the autumn. The 

 effect of this overturning upon the plankton of the Illinois River 

 may thus be detected, though it is here of less importance than in 

 lakes and reservoirs since it is overshadowed or replaced by other 

 and more potent factors. 



The relation of the seasonal distribution of the diatoms to that 

 of the total plankton is not readily unraveled. The latter is the 

 resultant of a most complex series of factors, whose number and 

 relative potency are subject to constant change and readjustment 

 in the unstable environment of the stream. It is the biological 

 expression of the state of tension among these various factors which 

 for the moment exists. Of these factors the diatoms are but one, 

 though an important one, in the food cycle and ecology of the 

 plankton. The volumetric determinations in. the diagram (p. 

 37) do not give" the true seasonal distribution of the total plankton 

 owing to the escape of an unknown quantity through the meshes of 

 the silk net. They represent more truly that of the animal plank- 

 ton than that of the phytoplankton. A comparison of the seasonal 

 distribution of the diatoms and total plankton may serve, in spite 

 of the errors involved in the volumetric determinations and the 

 disparity of individuals among the diatoms, to throw some light on 

 the effect of the fluctuations of the latter upon the movement in the 

 volume of plankton. A close comparison of the two seasonal curves 

 reveals the fact that the diatom curve is not identical with the vol- 

 umetric curve. It is true that the double vernal (April-May) pulse 

 of diatoms coincides in location with the vernal volumetric pulse. 

 This is also true of the pulses of June 14 and July 19. The crest of 

 the volumetric vernal pulse is, however, lodged between the double 

 apices of the diatom curve, and all the subsequent volumetric 

 pulses from July on lie in depressions of the diatom curve, and vice 

 versa. It is apparent at once on examination of our planktons that 

 the catches of the silk net are from the volumetric standpoint 

 largely, indeed overwhelmingly, of animal origin. These volu- 

 metric pulses are as a rule largely pulses of the zooplankton. It is 

 therefore to be expected that the diatoms would decrease at such 

 times, since they form the food of many Entomostraca and not a few 

 Rotifera. The appearance of the diatom pulses before or after the 



