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.* 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 Dee. 13 
to Jan. 10,—with a maximum of 1.98 ¢m.’ per m.* on Dee. 20. Its 
mean falls on the 22d, 20 days after that of the preceding pulse. 
This isa month of falling river levels with the exception of the 
rise of .4 ft. on the llth. The total movement is 3.4ft. 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, andin 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 (Pl. XII.). It may be 
