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. 3 

 is found. This or at least the first two months of it is the period 

 of the ice blockade (Pt. I., PI. IX.-XIIL), 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 is 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 



