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 (Pl. XLVI.) 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 1597, is too insignificant to make 
much of a showing even when plotted upon a scale tenfold 
that used for other stations (see explanation of Pl. XLVI.); 
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. 
