209 



lakes rich in vegetation the grosser forms of aquatic vegetation 

 draw heavily upon these resources. The accumulations of de- 

 cay in winter and the increased products of decomposition in 

 summer are all largely and promptly transformed again into 

 organized matter, leaving only an unutilized residual mini- 

 mum which represents an equilibrium of the processes of 

 growth and decay in progress in summer waters. The seasonal 

 distribution of floods may also enter as a determining factor in 

 the problem. 



The coincidence of the spring plankton maximum and the 

 decline of nitrogenous matters in the river water has its par- 

 allel in the decline of nitrates in soil waters with the pulse of 

 spring vegetation. In both cases the decline in nitrogenous 

 matters seems to be due to utilization by growing vegetation, 

 by chlorophyll-bearing organisms. 



These maximum and minimum pulses of nitrogenous mat- 

 ters may also be traced in the analyses of samples from Spoon 

 River. In 1896-97 (PI. XL VI.) the nitrates exhibit most clearly 

 the fluctuations in question. Traces of their presence can be 

 detected in the plottings of the organic nitrogen, albuminoid 

 and free ammonia, and oxygen consumed, though in all these 

 cases the effect of flood waters is also evident and cannot be 

 eliminated from the problem. Invasion of Illinois River water 

 is also apparent in October of the low-water period of 1897, be- 

 ing shown especially by the chlorine curve. 



In 1898 and the first three months of 1899 both the cold 

 weather maxima and the warm weather minimum are more 

 sharply defined and appear in all the substances above enumer- 

 ated. 



The plankton of Spoon River, with the exception of that 

 of the low-water period of 1897, is too insignificant to make 

 much of a showing even when plotted upon a scale tenfold 

 that used for other stations (see explanation of PI. XLVL); 

 nevertheless we still find here the same midsummer reduction 

 in nitrogenous substances which has just been explained as the 

 result of the utilization of such matters by the phytoplankton. 



