228 



by a recovery upon the decline of the plankton. Illustrations 

 of this may be seen in the April-May pulse in Thompson's Lake 

 ( PI. L.), where a decline of fifty per cent, accompanies a nine- 

 fold increase in the plankton. The April pulse of 1896 in the 

 Illinois (PL XLIII.) coincides with a still more pronounced 

 decline in the free ammonia. Upward movement of both 

 plankton and free ammonia appears occasionally, as in the 

 Illinois in September, 1897 (PI. XLIV.), though a downward 

 movement of the free ammonia attends the plankton pulse of 

 the subsequent month. The free ammonia thus exhibits some 

 evidence that it enters into the flux of nitrogenous matter 

 involved in the rise and fall of the plankton. It decreases 

 when the synthetic activities predominate in the plankton, 

 and some, at least, of its increases coincide with periods of 

 predominantly animal (analytic) plankton. 



The changes in the oxygen consumed coincide very nearly 

 with those in the organic nitrogen and albuminoid ammonia 

 both in direction and amount, and thus bear much the same 

 relation to the plankton changes. 



The changes in the chlorine are of especial interest, not be- 

 cause of their direct relation to the plankton, but on account of 

 the fact that they indicate, perhaps better than any other ele- 

 ment in the analysis, the relative contamination by and con- 

 centration of the sewage in the different localities at different 

 seasons of the year. 



In the Illinois River (PL XLIII.-XLV.) the chlorine usually 

 fluctuates in a direction opposite to that of the hydrograph, 

 running low during high water and rising with the return of 

 low water. Some exceptions appear, as in the rising flood of 

 December, 1895, and the declining flood of June, 1896, the 

 former apparently due to the flushing of sewers by initial flood 

 water, and the latter to an irregularity for which no natural 

 cause appears. The marked irregularity of the chlorine in the 

 Illinois, indicating a corresponding instability in access of sew- 

 age, with its additions of matter helpful or deleterious to the 

 plankton, adds to the environment of the potamoplankton a 



