466 BULLETIN OF THE BUREAU OF FISHERIES 



The obstacle to the advance of knowledge along this line has not been any lack 

 of plausible explanations; on the contrary, various changes take place in spring in 

 the physical medium in which the plankton exists, any or all of which might, a priori, 

 be assumed to control the life history of the planktonic plants. Such, for example, 

 are the seasonal variations in the temperature of the water; in its salinity; in its 

 density, viscosity, and vertical stability; in the activity of its vertical circulation; 

 in its alkalinity ; in the supply of dissolved foodstuffs ; and in the strength of the sun ; 

 every one of which, directly or indirectly, affects the viability and reproduction of 

 the phytoplankton, and which, in unfavorable combination, may make existence 

 impossible for them. 



It has been observed repeatedly, and in widely separated seas, that the vernal 

 augmentation of the diatoms synchronizes with the first vernal warming of the water. 

 But, as Moore, Prideaux, and Herdman (1915, p. 247) have emphasized, "it is to be 

 remembered that the physical cause must have a latent period ahead of the biolog- 

 ical effect." It may be stated as a general rule that the vernal flowerings of diatoms 

 follow so closely after the commencement of vernal warming (if not antedating it) 

 that the latter can not be the cause of the former. This is certainly the case in the 

 waters between Cape Ann and the Isles of Shoals, where diatoms commence to 

 multiply actively in March, the temperature still being at its winter minimum (p. 383) , 

 while in 1925 winter flowering of Ehizosolenia alata commenced in the falling tem- 

 perature of December (p. 396). Furthermore, marine diatoms as a class have been 

 found tolerant of such wide variations of temperature and of salinity, both over the 

 geographic and seasonal ranges in nature, and in cultural experiments (Allen and 

 Nelson, 1910; Fritz, 1921a), as to make it in the highest degree unlikely that slight 

 changes in either of these environmental features, within the limits of both obtaining 

 in the Gulf of Maine, are themselves of prime importance in the economy of these 

 pelagic plants. But temperature and salinity in combination determine the viscosity, 

 the density, and the vertical stability of the water, which in turn tend to control the 

 activity of its vertical circulation and thus indirectly to favor or hinder the flotation 

 and food supply of marine diatoms as the seasons change. 



Herdman (1920) believes the increasing intensity of the sunlight is the chief 

 stimulant for the spring flowering of diatoms, and certainly without sufficient sun- 

 lignt the reproduction and even the continued existence of diatoms — for that matter, 

 of all chloroplvyllous plants — would become impossible. This may well be the 

 case in the higher latitudes of northern Europe, likewise in Canadian waters, during 

 the short winter days. And while terrestrial experience in the latitudes of the Gulf 

 of Maine (40 to 45° N.) shows that the sun rises high enough in the sky for active 

 photosynthesis at all seasons, the increasing percentage of hours of sunlight per day, 

 and its greater intensity consequent on the increasing declination of the sun, no 

 doubt help to make, the spring a more favorable season for the flowering of diatoms 

 than late autumn or winter. But this factor can not by itself explain the seasonal 

 cycle of diatom flowerings as they actually occur, for if increasing light be a factor 

 inducing their commencement it should equally favor their continuance throughout 

 the summer, instead of the culmination and disappearance after a few weeks that 

 characterizes most parts of the Gulf of Maine (p. 396) . 



