38 
approximately that of the average thermal exposure. The vernal 
pulse may, in part at least, be the result of a process of natural 
acclimatization. The fact that a similar development does not 
recur when this temperature is repassed in the autumnal decline 
militates, it is true, against the potency of this temperature as a 
factor in the vernal pulse. This temperature is passed in October 
(Pt. I., Pl. VIII.—XIII.), but October pulses are rarely so pronounced 
as those of adjacent months. Other factors more potent than tem- 
perature are operative at that season of the year. 
As will be seen in the diagram, the most pronounced and pro- 
longed minimum appears in January, February, and March. In 
these months but a single record in excess of 100,000,000 per m.* 
is found. This—or at least the first two months of it—is the period 
of the ice blockade (Pt. I., Pl. [X.—XIII.), during which the aeration 
of the water by the wind is prevented, and the customary equilib- 
rium in gaseous contents may be disturbed. It is the time when 
stagnation most threatens disaster to the plankton. The earlier 
stages of this blockade in December do not seem to be deleterious 
to the growth of diatoms, since at such times the blockade 1s less 
complete, the exclusion of light by the ice less effective, and the 
accumulation of the products of decay less pronounced. The data 
at hand do not suffice to elucidate the matter further. 
The position of the diatom pulses with respect to the movement 
of the hydrograph is suggestive—though not conclusive—of a pos- 
sible correlation between the two phenomena. The double vernal 
pulse of April-May appears in the declining waters of the major 
spring flood. The diatom pulse of June 14 is found in the decline 
of the May—June flood. The pulse of August 9 is caught on the ris- 
ing waters of a slight flush of the river, and that of August 30 on its 
decline. That of September 27 appears after a series of slight rises, 
and those of both October and November attend rising water, but 
the well-developed pulse of December appears with its decline. 
There are, counting the double vernal pulse, ten pulses in 1898, 
from March to January. Of these, seven are found on declining 
floods, and but three on rising water, and two of these three appear 
during the slow rise of October-November. Furthermore, the 
magnitude of the flood is correlated with that of the diatom pulse. 
The vernal pulses of 3,453,778,080 and 3,865,257,360 attend the 
major spring flood, culminating April 2 at 18 feet; the pulse next in 
