336 



changes are also relatively gradual with the exception only of 

 the rise on the 17th. There is under these conditions a gradual, 

 though slight, rise in production, which with the establishment 

 of winter temperatures and decline in levels culminates at 1.26 

 cm. 3 on Dec. 6. Increase in stability even in winter conditions 

 thus tends to increase production. 



This November pulse drops suddenly in the silt-laden 

 waters of the slight rise in river level on Dec. 11 to a minimum 

 of .01 on the 13th. The completeness of this decline is doubt- 

 less due to the fact that this collection was made in storm 

 waters of recent local origin due to local rains. Flood waters 

 of slight extent were thus intercalated in the stream, and if 

 there was a normal decline in production accelerated it to 

 this extent ; or it may be that the flood is solely responsible for 

 the separation of the November and December pulses. The 

 cyclic movement elsewhere renders this also a matter of con- 

 jecture. 



The December pulse has a duration of 28 days, — from Dec. 13 

 to Jan. 10, — with a maximum of 1.98 cm. 3 per m. 3 on Dec. 20. Its 

 mean falls on the 22d, 20 days after that of the preceding pulse. 

 This is a month of falling river levels with the exception of the 

 rise of .4 ft. on the 11th. The total movement is 3.4 ft. Since, 

 however, all but 0.4 ft. of this is downward movement, the en- 

 vironmental stability is greater than the extent of the move- 

 ment indicates. Temperatures under the thin ice-sheet that 

 formed in the first week change less than 2°, and throughout 

 the period of the pulse the several forms of nitrogen (PL XLV.) 

 vary but little. The chlorine, free ammonia, and oxygen con- 

 sumed, however, rise steadily as levels fall, to fall again as the 

 river levels rise at the close of the month. This increase is again 

 an index of the approach of stagnation under the cover of the 

 ice-sheet and with the advance of the Peoria sewage pulse down 

 stream as winter comes on. Stagnation is not reached, how- 

 ever, and in the relatively stable conditions of this period of 

 the plankton reaches a level of production (1.98) not before 

 attained since the close of the June pulse (PI. XII. ). It may be 



