227 
occur most frequently when the diatoms are most rapidly mul- 
tiplying. As will be shown later, these seasons occur in the 
colder months, and often precede the summer pulses of plank- 
ton whose crests are predominantly of the animal plankton. 
The upward movement of the organic nitrogen and the down- 
ward movement of nitrates is thus due in large part to the 
synthetic action of these organisms. The major plankton 
pulses, which are as a rule predominantly animal in their com- 
position, usually occur in the warmer months With their cul- 
mination there is always a great decrease in their food supply 
(the phytoplankton) and analytic processes thus predominate, 
and the decay of the products of animal metabolism results in 
a decrease in the total organic nitrogen and leads to a recovery 
of the nitrates. This interplay of the synthetic and analytic 
processes of the phyto- and zodplankton, is, I believe, the basis 
of the coincidence in the fluctuations of the plankton and of 
the nitrogenous contents of the water. Further reference will 
be made to the subject, and data illustrating it will be cited in 
connection with the discussion of the seasonal changes of the 
plankton. 
The seasonal changes in free ammonia seem to be due to 
the effect of floods and temperature upon the processes of 
decay, and reveal but minor correlations with plankton 
changes. A marked increase with rising flood waters is appar- 
ent in Spoon River (Pl. XLVI. and XLVII.) and occasionally 
in the Illinois, as, for example, in February, 1896 (Pl. XLIIL.). 
Prolonged high water, on the other hand, tends to lower the 
free ammonia (Pl. XLIV.). The stagnation in the sewage- 
laden river when it is covered with ice at low-water stages 
appears in the elevenfold increase in free ammonia under the 
ice in December, 1897 (Pl. XLIV.). The fluctuations are also 
much more marked in the rivers (Pl. XLIII.—XLVII.) than in 
the lakes (Pl. XLVIII—L.), owing to the diminished and 
equalized effects of flood and sewage in the reservoir back- 
waters. There are repeated instances where the plankton 
pulses coincide with decreases in the free ammonia followed 
