PLANKTON OF THE GULF OF MAINE 



457 



our other towings on the bank irrespective of season, is best explained as due to a 

 drift of the Nova Scotian current moving southwestward in spring from the Scotian 

 banks across Browns Bank and the eastern channel and along the outer part of Georges 

 Bank. This is corroborated by sundry other lines of evidence, planktonic as well 

 as hydrographic. 



As there is some confusion between this species and the closely related Th. 

 frauenfeldi in the European lists published by the International Committee for the 

 Exploration of the Sea (Ostenfeld, 1913), I may note that only such cells as were 

 attached to one another in their characteristic zigzag chains are recorded here as 

 nitschioides, these being quite different in appearance from the chains of frauevfeldi. 

 The latter species has not been identified in any of the Gulf of Maine tow nettings. 



Other diatoms 



The genera so far discussed include all that we have found important in the 

 plankton of the outer waters of the Gulf of Maine, and while the station lists (p. 423) 

 include various others, none of them occur regularly or abundantly enough to color 

 the plankton. I may emphasize especially the universal rarity of brackish-water, 

 littoral, and bottom-dwelling diatoms out at sea. Pleurosigma, for example, is never 

 represented by more than occasional examples, though detected at many localities 

 far and wide. Under estuarine conditions, however, as in the tributaries of the Bay 

 of Fundy, littoral diatoms of many genera are much more abundant (Bailey, 1917; 

 Fritz, 1921; Bailey and Mackay, 1921). 



Finally, I may emphasize our failure to find any diatoms in the gulf to which 

 it is safe to ascribe either a Tropic or an Arctic origin, except, perhaps, for Fragilaria 

 oceanica, occasional examples of which were detected in the tows in the Eastern 

 Channel and over the southeast slope of Georges Bank on April 16, 1920 (stations 

 20107 and 20109). The absence of other arctic diatoms in the Gulf of Maine is 

 the more striking if contrasted with their abundance and frequent dominance in 

 the Gulf of St. Lawrence in spring, as is illustrated by the following table based on 

 Gran's (1919) list for May 11, 1915. This Arctic community proved so shortlived 

 there, however, that it had entirely disappeared in June, to be replaced by a typically 

 boreal assemblage, most of whose members — RMzosolenia setigera, NitscMa seriata, 

 Coscinodiscus, and Chsetoceras laciniosurn — are equally characteristic of the spring 

 plankton of the Gulf of Maine. 



St. Lawrence diatoms, May 11, 1915 



Acnanthes taniiata 



Ampin pnjr:i hyperborea., 



Bacteriosira fragilis 



Biddulphia aurita 



Chaetoceras atlanticum... 

 Chtetoceras eompressum. 

 Chaetoceras eriophilum... 



Chaetoceras debile... 



Cbfetoceraa decipiens 



ii tdema 



Chaetoceras scolopendra.. 



Chietoceras teres 



ula conferva 

 Bucampla grcenlandica... 



X 



X 



X 



X 



X 



X 



Gulf of 

 Maine 



X 



x 



X 



x 



x 

 x 

 x 

 x 



X 



St. Lawrence diatoms, May 11, 1915 



Fragilaria cyclindrus X 



Fragilaria oceanica X 



Navicula pelagica -- I X 



Navicula septcntrionalis - I X 



Navicula vanhotTeni --- X 



Nitschia closterium ..- 



Nttschia frigida X 



Pleurosigma sluxbergi _ 



Rhizosolenia hebetata - - 



Thalassiosira bioculata 



Thalassiosira gravida : 



Tlutlissiosira hyalina : X 



Thalassiosira nordenskioldi — 



Thalassiothrix longissima 



Oulfof 



Maine 



X 

 X 



X 

 X 



x 

 x 



1 Species that are endemic in the Polar seas, where ice forms in winter, and in the Oulf of St. Lawrence, but which occur only 

 as immigrants farther south. 



