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 
(Pl. 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 (Pl. 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 
Ihnois, 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 
