PLANKTON OF THE GULF OF MAINE 



393 



The record of towings is now sufficient to show that this peridinian community, 

 with only an occasional diatom, normally dominates and usually monopolizes the 

 phytoplankton of the whole of the central part of the gulf outside the 100-meter 

 contour during the late summer and early autumn, from off Cape Elizabeth and 

 Cape Cod, on the one side of the gulf, to German Bank and Cape Sable, on the 

 other, and from about the 100-meter contour on the north, southward across the 

 whole breadth of the basin to include the Eastern Channel, though with an admixture 

 of diatoms in the northeastern part, as just noted. 



A typical Ccratium plankton, or at least a predominance of Ceratium mingled 

 with the diatoms, has likewise characterized all our summer tows on Georges Bank 

 except for the local diatom flowerings just described. But, judging from St. Andrews 

 and from conditions in north European seas, it is not likely that Ceratium, the peri- 

 dinian genus that is predominant out at sea in the gulf, ever attains abundance in its 

 estuarine waters, for according to McMurrich (1917, p. 3) none of the dinoflagellates 

 were sufficiently numerous to be an important quantitative constituent of the plank- 

 ton at St. Andrews at any season, " C. tripos only on one occasion being in sufficient 

 quantity to be regarded as frequent." Nevertheless, Ceratium follows essentially 

 the same seasonal pulse there as at our stations out at sea, reaching its plurimum in 

 autumn and practically vanishing from the tows in April and May. 



It is impossible to prepare a chart of the mutual limits of the chief classes of 

 phytoplankton in the gulf for midsummer, which shall be as true for one year as for 

 another, because of the yearly fluctuations in the abundance of and area occupied by 

 diatom plankton near its northern coast and of the variable midsummer flowerings 

 of diatoms on Georges Bank. On the whole, however, the state obtaining during 

 July and August of 1914 (fig. 107) seems fairly representative of the offshore waters 

 of the gulf in the summer season, bearing in mind the different locations of the diatom 

 swarms on Georges Bank of July, 1913, and July, 1916. A corresponding chart of 

 the northern part of the gulf for 1912, published in an earlier report (Bigelow, 1914, 

 pi. 8), illustrates a summer more productive of diatoms. 



The sporadic occurrence of swarms of acantharian radiolarians in the western 

 part of the gulf in some summers, though perhaps not annually, a conspicuous feature 

 of the chart for 1914 (fig. 107), need be mentioned but briefly here, being discussed 

 below (p. 460). 



It is in July and August, if ever, that tropical phytoplanktonic communities may 

 be expected to drift northward from the Gulf Stream across Georges Bank and thus to 

 penetrate the inner parts of the Gulf of Maine. But if our hauls are to be trusted 

 as fairly representative, this rarely takes place, the only positive records of this sort 

 which have yet been obtained for the inner parts of the gulf or even for the shoaler 

 parts of Georges Bank itself being a fragment of gulf weed (Sargassum) picked up 

 on German Bank on September 2, 1915 (station 10311; Bigelow, 1917, p. 246), and 

 an occasional Ceratium macroceras detected among other boreal species of the genus 

 off the Merrimac Biver on December 30, 1920 (station 10492). 



Planktonic forms of tropic origin, plant as well as animal, are, of course, more 

 important along the slope south of Georges Bank (p. 54) , thanks to the close proximity 

 of the tropic water. Thus gulf weed is often seen floating there in some quantity, 



