303 



forms (cf. Plates III. and IV. with I. and II.) coincide with or 

 follow shortly after those of the synthetic planktonts on which they 

 feed, we may conclude that the cause of the periodic movement of 

 these animal groups lies in the periodic fluctuations of their food 

 supply. In the causes which control this periodic growth of the 

 chlorophyll-bearing organisms will be found the solution of the 

 general periodic phenomenon in plankton. 



This rhythm is primarily one of growth and reproduction, and 

 its solution must be sought in the forms of matter anctenergy which 

 affect these processes. The nutrition of the chlorophyll-bearing 

 organisms is drawn from matter in the river water. The analyses 

 contained in Part I., Table X., and graphically presented on Plates 

 XLIII. to XLV. trace the seasonal fluctuations in the nitrates -one 

 of the important constituents of plant food. Neither in the seasonal 

 curves of this or other forms of nitrogen delineated in the plates is 

 there any such rhythm of occurrence, though, as has been pointed 

 out in the discussion of the chemical conditions, there are instances 

 of apparent correlation of plankton and nitrate pulses. They occur 

 at irregular intervals, and do not form a continuous series. That 

 there might be a rhythm in the utilized nitrates (the analysis repre- 

 sents only the unused residuum) is of course possible, or that it 

 might occur in some other constituent of the food not determined 

 in the analysis is not impossible, but we have no evidence of its 

 existence. 



The chlorine in our river waters is a fair index of the amount of 

 sewage or pollution by animal wastes. It is subject to considerable 

 fluctuations, resulting in part from dilution by floods or concentra- 

 tion in low waters, and there are other pulses not traceable to 

 hydrographic conditions, which perhaps result from industrial 

 wastes. These fluctuations in some instances coincide with those 

 of the phytoplankton in question, but the instances are few and 

 the correlation is incomplete. Upon investigation I find that 

 sewage pumpage at Bridgeport, which discharged the sewage of 

 Chicago River into the Illinois and Michigan Canal and thence into 

 the Illinois River, was practically continuous, and could not produce 

 the rhythm 'in question. The sewage of Peoria has a much more 

 immediate effect upon the chemical conditions in the river at 

 Havana than has that of Chicago. The sewers of this city, I am 

 informed by Mr. H. E. Beasley, City Engineer, are flushed as 



