PLANKTON OF THE GULF OF MAINE 391 



In midsummer (fig. 107) wo have usually found the entire basin of the gulf 

 occupied by a peridinian (Ceratium) plankton, with only occasional diatoms, and we 

 have never found diatoms in abundance anywhere in the gulf in July or August 

 except close along the coast, on the one hand, and on Georges Bank, on the other. 



We found diatoms flowering in abundance on each of our summer visits to the 

 latter locality, but in different regions in different years. Thus they dominated on 

 the western end of the bank on July 9, 1913 (station 10059) and on July 23, 1916 

 (stations 10347 and 10348), but when we visited that general locality on July 20, 

 1914, the water contained very few diatoms but, instead, a characteristic peridinian 

 plankton. Three days later, however, we encountered a rich flowering of diatoms 

 near the northeastern edge of the bank (station 10224). Furthermore, the Georges 

 Bank flowerings of July, 1913 and 1914 (stations 10059 and 10224; see list, p. 430), 

 though far apart geographically, were both dominated by Guinardia. In July, 1916, 

 however, we found no Guinardia on a traverse of the western part of the bank but 

 swarms of Thalassiothrix (p. 455) and Rhizosolenia (p. 444) in its stead. With so little 

 data available it is not possible to outline the normal summer status of diatoms for 

 the Georges Bank region. Nevertheless, the fact that we have found them in such 

 abundance on some part of the bank on each visit in summer, with the abundant flow- 

 erings encountered there in February and May of 1920 by the Albatross (pp. 383, 387) , 

 and in April, 1913, by Douthart (p. 3S5), shows that swarms of diatoms may be ex- 

 pected somewhere on its extent at any time from late winter until midsummer. It 

 is not unlikely that this applies to Nantucket Shoals also, for Dr. W. C. Kendall writes, 

 in his field notes, that on September 2, 1896, the water "was very full of brown slimy 

 stuff" at latitude 40° 47', longitude 69° 43', which could only have been diatoms. 



It is not yet clear whether any particular region on the banks is more favorable 

 for the multiplication of diatoms than another, except that we have always found 

 these rich flowerings on its shoaler parts and never close enough to the continental 

 slope to be within the influence of the high temperature outside the edge, which, in 

 its own turn, supports various oceanic diatoms in small numbers mingled with 

 peridinians of similarly Tropic origin. 



The fertility in diatoms of the waters over Georges Bank is interesting, not 

 only from the standpoint of the phytoplankton per se, but because of the great im- 

 portance of the bank as a spawning ground for haddock. The prevalence of the 

 genus Guinardia on the bank, contrasted with its absence or rarity in the deeper 

 waters of the gulf to the north, is likewise instructive for its bearing on the circula- 

 tion of the water in this region. 



Turning now to the coastwise belt, diatoms continue a more important factor 

 in the phytoplankton of estuarine situations throughout the summer than they are 

 in the open waters of the deeper parts of the gulf at that season. Owing to the 

 fact that most of our towing has been well out at sea, we have few data to offer on 

 this regional differentiation. It was clearly demonstrable in Massachusetts Bay on 

 August 22 to 24, 1922, however, when several stations close in to the land, following 

 around the coast line from Cape Cod Bay to Cape Ann, 28 were dominated by diatoms 



» Stations 10633, 10634, 10137, 10639, and 10642. 



