PLANKTON OF 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 79 required for diatom 
78 See Johnstone (1908), Herdman (1923), and Johnstone, Scott, and Chadwick (1924) for general discussions of the nutrition 
of the phytoplankton. 
78 For discussions of Liebig’s “Law of the Minimum” in its relation to marine plants, see Johnstone, 1908, p. 234; Gran, 1912, 
p. 367. 
