215 



correlation between the respective fluctuations is apparent in 

 the data except in the fall of 1897, when an unusual minimum 

 of nitrates appeared in conjunction with an unusual develop- 

 ment of plankton (PI. XLVL). Decrease in nitrates often at- 

 tends the initial stages of flood independently of plankton 

 development, as in December, 1896 (PI. XLVL). Some nitrate 

 increases, as in the autumn of 1896 (PI. XLVL), appear with 

 the crests of floods, especially those of the gradual type. Other 

 fluctuations in the nitrates — and they are often considerable — 

 show no correlation with available data 



In Quiver Lake in 1898-99 (PI. XLIX.) the plankton maxima 

 of April-May, June, and December all occur when nitrates de- 

 crease. The tendency of nitrates to increase and then fall 

 again with the crest of the flood is apparent in January, March, 

 May, November, January, and March. 



In Thompson's Lake in 1897-99 (PI. L.) practically all of 

 the maxima are attended by a greater or less diminution of the 

 nitrates. This appears in October, November, December, April- 

 May, June, July, August, and December. The effect of floods 

 in decreasing the nitrates in their initial stages and subse- 

 quently increasing them is slightly indicated in January, Feb- 

 ruary, November, and February. 



The nitrites exhibit a tendency in the Illinois River to ex- 

 cess during the low-water period of midsummer (PI. XLIIL- 

 XLV.), averaging about .3 to .4 parts per million to .1 during 

 the remainder of the year. This excess was prolonged into 

 November in 1897 with the low-water period of that year. It 

 seems thus to attend the concentration of sewage in the river. 

 No constant correlation of movement between the nitrites and 

 plankton can be detected. In a few instances, however, plank- 

 ton maxima coincide with marked decrease in nitrites, as, for 

 example, in the river in September and October, 1897 (PI. 

 XLIV.), and the spring maximum precedes the rise in nitrites 

 in each year. The changes in the nitrites show no constant 

 correlation with those of other forms of nitrogen, though at 



