390 
BULLETIN OF THE BUREAU OF FISHERIES 
eastern, where Ceratium dominated the plankton as early as the first week of May 
in 1915, leaving diatoms still overwhelmingly dominant in the central deeps of the 
gulf and along its northern coastline. Comparison of the chart for May (fig. 106) 
with that for April (fig. 105) illustrates the encroachment of these two peridinian 
centers — western and eastern — on the areas previously characterized by abundant 
diatoms, the former replacing the latter over the coastal zone from Cape Cod 
northward across Massachusetts Bay and past Cape Ann, on the one side of the gulf, 
southward, too, as far as Georges Bank, and offshore over the eastern side of 
the basin on the other, by the last half of May. 
Probably peridinians would also have been found to dominate the phytoplank- 
tonic community right across the southern part of the deep basin of the gulf at that 
Fig. 107. — Distribution of the more characteristic types of phytoplankton, July to August, 1914. 1, Ceratium and diatom; 
2, diatom; 3, Ceratium; 4, tropical, characterized by Triehodesmium; 5 , Radiolarion. (Reproduced from Bigelow, 
1917, fig. 97) 
time. This is certainly the case by mid-June, when we have found them in consider- 
able abundance at all our stations near the coast as well as offshore (and this covers 
the whole northern half of the gulf) , except in the rich but circumscribed diatom areas 
just described for that month, where peridinians were still extremely rare. 
No doubt variations from this planktonic cycle are to be expected from year to 
year, but it is sufficiently established that the vernal flowerings of the pelagic diatoms, 
followed by their eclipse, with the coincident disappearance and reappearance of 
peridinians, are as characteristic of the spring season in the offshore waters of the 
Gulf of Maine as are the spring freshets from the rivers that discharge along its coast, 
in which, as in so many other ways, the gulf closely parallels other northern seas. 
