PLANKTON OP THE GULF OF MAINE 467 



On the whole, with successive observations and experiments it grows more and 

 more probable from year to year that, given temperatures, salinities, and alkalinities 

 (p. 486) in which diatoms can exist, with sunlight sufficient for active photosynthesis, 

 their regional and seasonal abundance depends chiefly on the richness of the water 

 in dissolved food substances, organic and inorganic, and to a less extent on the 

 activity of vertical circulation of the water and its viscosity. 78 



The suddenness with which diatoms commence flowering in spring tends to 

 corroborate this generalization, for if the gulf were abundantly supplied with nutrients 

 the year round we might expect to find their numbers steadily augmenting through- 

 out the coastal waters of the gulf during the late winter, as vertical circulation 

 grows more and more active and as the sun rises higher and higher; but as a matter 

 of fact (and this is true not only of the Gulf of Maine but of other northern coastal 

 waters) the tremendous flowerings of diatoms so characteristic of early spring culmi- 

 nate almost between one week and the next. 



The most reasonable explanation for this is that at least one of the nutritive 

 substances on which they depend, whether it be nitrogen, phosphoric acid, silica, or 

 some other, occurs in less than the minimum required for their active growth and 

 reproduction during the winter and until the first days of spring, when the increasing 

 outflow from the rivers, combined with an increasingly active vertical circulation of 

 the sea water, raises the supply above this critical point, whereupon a rapid multiplica- 

 tion of diatoms at once ensues. Conversely, an exhaustion of one or other foodstuff 

 is now generally accepted as the cause of the sudden disappearance of diatoms after 

 their vernal flowering period. The diminishing viscosity, also, and the increasing 

 vertical stability of the water, which characterize the advancing summer owing to 

 the rising temperature, likewise militate against the continued multiplication of 

 diatoms. The former renders flotation difficult, as explained below (p. 482), and 

 the latter so effectively isolates the surface stratum of water (where diatoms find 

 their optimum light conditions) from the underlying layers that replenishment with 

 nutrients from below is effectively hindered. 



Although our Gulf of Maine studies touch only the outer edge of this very 

 complex subject, it is of such fundamental importance in the economy of the sea 

 that a brief discussion here needs no apology. 



Diatoms being producers, not consumers, it is, of course, from what Johnstone 

 (1908, p. 212) has called the "ultimate foodstuffs in the sea" that they derive their 

 nourishment, chief of which are carbonic acid, the nitrogen compounds, phosphoric 

 acid, silica (because of their habit of secreting silicious skeletons), and various other 

 mineral salts in minimal quantities; also oxygen (not, of course, a food substance 

 but necessary for life). Except under very special circumstances it is hardly con- 

 ceivable that the phytoplankton of the open sea ever suffers a shortage of oxygen 

 or of the available sources of carbonic acid. But as all the other nutrients occur only 

 in minute quantities in sea water we can readily understand that the supply of one 

 or the other might fall temporarily below the minimal amount T0 required for diatom 



" See Johnstone (1908), Herdman (1923), and Johnstone, Scott, and Chadwick (1924) for general discussions of the nutrition 

 of the phytoplankton. 



"For discussions of Liebig's "Law of the Minimum" in its relation to marine plants, see Johnstone, 1908, p. 234; Gran, 1912, 

 p. 367. 



