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 Quiv- 

 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 cycle of seasonal fluctuations (see PI. XLffl.-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 (PI. 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 (PI. XLIV. ) the curve of the nitrates again exhib- 

 its these pulses, but they are not apparent elsewhere unless it 



