208 
be in the free ammonia. The prolonged and unbroken low 
water from August to the end of the year, and the consequent 
concentration of the sewage in the river and the marked de- 
velopment of the water-bloom during this period, seem to have 
obliterated the minimum pulse in all but the nitrates. The 
marked rise in chlorine and free ammonia gives some idea of 
the unusual degree of concentration of the sewage. 
In 1898 and the first three months of 1899 (Pl. XLV.) these 
pulses are much more evident, being traceable in the nitrates, 
albuminoid ammonia, organic nitrogen, and oxygen consumed. 
The marked depression of the free ammonia during the flood 
season in a measure modifies its conformity to these pulses. 
A relation of these maximum and minimum pulses to the 
growth of the plankton is suggested by the chronology of the 
chemical (especially that of nitrates) and the plankton curves. 
The spring maximum of plankton production, which normally 
occurs in the last of April and the first of May, comes toward 
the close of a long period of high content of nitrogenous mat- 
ters. It is followed by or is coincident with the decline in 
these substances. With the decline in plankton production in 
late autumn the nitrogenous substances again increase (PI. 
XLUI-XLV.). During the low water of 1897, when the mid- 
summer minimum of nitrogenous substances was overshadowed 
by the concentration of the sewage, we also find a marked in- 
crease in plankton production as contrasted with that of cor- 
responding seasons of 1896 and 1898. The warm season is pre- 
sumably one of more rapid nitrification, the heat favoring the 
more rapid decomposition of the organic matter in water, but 
excepting instances of great sewage concentration, as in the 
late summer of 1897, we do not find an increase or an accumu- 
lation of the products of such decay in the water during the 
warm season. Indeed, the opposite seems to be the tendency. 
The explanation of this phenomenon lies, it seems, in the rapid 
utilization of the nitrogenous products of decay by the nitro- 
gen-consuming organisms of the water. In open water these 
are the chlorophyll-bearing organisms of the plankton. In 
