227 



occur most frequently when the diatoms are most I'apidly 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 zooplankton, 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 (PI. XLVI. and XLVII.) and occasionally 

 in the Illinois, as, for example, in February, 1896 (PI. XLIIL). 

 Prolonged high water, on the other hand, tends to lower the 

 free ammonia (PI. 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 (PI. XLIV.). The fluctuations are also 

 much more marked in the rivers (PI. XLIII.-XLVII.) than in 

 the lakes (PI. 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 



