390 



BULLETIN OF THE BUREAU OF FISHEKIES 



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 tbe 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 



ipfr 





sssj^k" 



Fig. 107.— Distribution of tbe more characteristic types of phytoplankton, July to August, 1914. 1, Ceratium and diatom; 

 2, diatom; 3, Ceratium; 4, tropical, characterized by Trichodesmium; 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. 



