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 (PI. 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. 

 XLIII.-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 



