479 
Lake—where flood factors are largely excluded—and the ther- 
mograph (air) for 1896 will serve to suggest the possibility of a 
causal nexus between the two phenomena of fluctuations in 
heat and some of the movements in plankton production. The 
many exceptions to any close correlation emphasize, however, 
the fact that heat is only one of the many factors involved in 
the problem, and also indicate the necessity for much fuller 
plankton data, with closer interval and the proper quantitative 
representation of the minute forms now lost by leakage through 
the silk, for any adequate discussion of the problem. The 
present data serve only to suggest the problem for investiga- 
tion. 
The effect of the ice-sheet upon the course of plankton pro- 
duction is apparent ina number of instances in our records. 
The most noticeable case was the extermination of the plank- 
tonin the channel in February, 1895, by the ice-sheet of two 
months’ duration; but this catastrophe was not repeated else- 
where in our records in this or other years. Indeed, owing to 
the fact that the period of the ice blockade is usually one of 
lower levels and more stable conditions, we find generally that 
production under the ice, even at minimum temperatures, 
rises above prior or subsequent levels. An inspection of the 
plates, especially those of 1898, will show repeated instances of 
this phenomenon in both channel and backwaters. One of the 
most striking phenomena in all our records is this winter pro- 
duction under the ice-sheet “in 1898-1899, a production which 
in the river attained an amplitude in December (.99) not 
equaled since June, and in February (.81) one surpassed only 
by the August (.91) and December means. In Quiver Lake 
likewise, the December (1.74), January (.77), and February 
(1.05) means are all considerably in excess of the June-Novem- 
ber production, the average of the winter months (1.19) being 
over threefold greater than that of the warmer months (.33) 
named. In Thompson’s Lake also the midwinter production 
in this season was large, reaching an average of 1.94 for the 
winter months above named, and only 1.96 for the five preceding 
