PLANKTON OF THE GULF OF MAINE 
389 
fallen to a low ebb everywhere in the offshore waters of the northern hah of the 
gulf by June 10 to 19, when they mingled with a scanty community of peridinians. 
Diatoms, however, were still flowering abundantly in the coastal zone east of 
Penobscot Bay at that time, for we found them in swarms off Petit Passage on the 
southern side of the Bay of Fundy on June 10 and again near Mount Desert Island 
(stations 10285 to 10287) on June 14 in 1915. Fritz (1921) also records diatoms in 
comparatively large numbers at St. Andrews in June, though not as abundantly 
as in May, on the one hand, or in July, on the other. It is probable that in June 
these three localities are local centers of production and not parts of a continuous 
coastwise belt of rich diatom plankton for two reasons — first, because Fritz found 
very few diatoms in the open Bay of Fundy on June 15, 1917, and, second, because 
they were but sparsely represented in our tow in the Grand Manan Channel on 
June 4, 1915 (station 10281). 
Thus, a general and very pronounced diminution in the number of diatoms 
takes place over the offshore waters of the gulf as a whole and all along its western 
shore during May and June; but in the year 1915 diatoms reappeared, though not 
in great numbers, and mingled with peridinians, over the shoal coastal bank off 
western Nova Scotia during the last half of June (station 10290, June 19). The 
scarcity of diatoms in that region in May of that year may be assumed to have 
followed rich April flowerings and coincides with the greatest expansion of the 
Nova Scotian current in that region. Unfortunately we have made no hauls close 
in to this part of the Nova Scotian coast during June and have no data on the 
phytoplankton of the eastern hah of Georges Bank, of the southeastern part of the 
basin of the gulf, or of Browns Bank for May. 
As I have pointed out in an earlier report (Bigelow 1917, p. 326) — indeed, the 
facts outlined above would suggest it — the seasonal history of peridinians in the Gulf 
of Maine is just the reverse of that of the diatoms. In late February and during 
March they join with the latter to characterize the sparse plankton of the whole 
basin of the gulf, this "mixed” zone extending into its northeastern corner, on the 
one hand, and over most of Georges Bank, on the other, likewise over the shelf abreast 
of Shelburne, Nova Scotia. But even this early in the season they are entirely dom- 
inated in the several centers where diatoms have commenced flowering actively, and 
by April they are so wholly overshadowed in the regions where the diatom flora is at 
its climax that only an odd ceratium or peridinian is to be found among the masses of 
diatoms that clog the nets. Over most of the central and southern parts of the gulf, 
where diatoms are not yet very plentiful, they are sufficiently so to make the few 
peridinians a minor element in the tows (though these never wholly disappear from 
any part of the gulf at any season), leaving only a small area in the southeastern part 
of the gulf where there are so few diatoms that the few Ceratium still color the 
plankton of April. 
As the flowering of diatoms reaches its climax and then diminishes in its regular 
seasonal progression, the peridinians (chiefly Ceratium) take their place in constantly 
augmenting abundance. This happens earliest in the season in the Massachusetts 
Bay region in the western side of the gulf and off southwestern Nova Scotia in the 
8951—28 26 
