207 
of other factors is evident, chemical conditions alone offering 
no satisfactory clue to causes of many of the fluctuations in the 
amount of plankton. 
The cycle of seasonal fluctuations in chemical conditions 
is best seen in years of more normal hydrograph, such as that 
of 1898, and it is more regular in the backwaters, such as Quiy- 
er and Thompson’s lakes, than it is in tributaries such as 
Spoon River, or in the Illinois itself. In the streams the floods 
produce irregularities which either do not enter the reservoir 
backwaters or reach them only in diminished volume. ‘The 
varying degree of contamination by sewage in the different lo- 
calities and in different seasons in the same locality adds an- 
other element which diversifies the seasonal changes and makes 
it more difficult to detect the common features which the fluc- 
tuations exhibit in all the localities. 
The cyele of seasonal fluctuations (see Pl. XLIII.—L.) in 
the chemical conditions is, in the most general terms, an in- 
crease in the nitrogenous compounds during the colder months 
and a decrease during the warmer ones. The maximum period 
usually appears in October and continues until the following 
summer, declining in May and June to the summer minimum, 
which in the following October and November rises again to 
the winter maximum. This fluctuation is somewhat similar to 
that found in soil waters. This coincidence suggests the oper- 
ation of fundamentally similar causes back of the common phe- 
nomenon. 
These maximum and minimum pulses in the Illinois River 
in 1896 (Pl. XLIII.) are most evident in the nitrates and free 
ammonia, though traces of their influence can be detected in 
the curve of the albuminoid ammonia. The suppression of this 
spring flood and the recurrence of four minor but unusual 
floods during the summer and fall are probably the cause of 
the nonconformity of some of the substances to these pulses 
and of the irregularity which they all exhibit in this year. 
In 1897 (Pl. XLIV.) the curve of the nitrates again exhib- 
its these pulses, but they are not apparent elsewhere unless it 
