PLANKTON OF THE GULF OF MAINE 451 



Thus Fritz (1921) found no Thalassiosira during November, December, or January, 

 but a scattering appeared in her tows in February and early March; it was flowering 

 actively by the end of that month, reaching its plurimum during the last half of April 

 and first half of May. Similarly, McMurrich (1917) did not detect it at St. Andrews 

 until March nor regularly until April in 1916. 



Thalassiosira likewise spreads seaward over the whole western half of the gulf 

 from mid March to mid April (fig. 129). And while we found no Thalassiosira on 

 Georges Bank in February, March, or April of 1920, except for occasional examples 

 at one station on the southeastern slope (station 20109) on the 16th of the latter 

 month (flotsam, perhaps, from the Thalassiosira flowerings then under way from 

 Cape Sable out to the Eastern Channel) , this genus is to be expected to appear over 

 the western half of the bank during the last half of April, Douthart having collected 

 masses of it over the north central part on the 14th of the month in 1913 and in less 

 abundance at various locations in that same general region on the 27th (Bigelow, 

 1914a, p. 415). 



It is not clear whether this Georges Bank flora is primarily driftage from the 

 inner parts of the gulf which multiplies actively in the shoal waters over the bank, 

 or whether it represents the local flowerings of Thalassiosira that have survived there 

 since the last preceding period of multiplication as resting spores on the bottom. 

 In any case the result is that the range of Thalassiosira extends from the north shore 

 of the gulf right out across the western side of the basin to Georges Bank by the last 

 week in April, and Douthart's rich gatherings point to the northwestern part of the 

 latter as the site of very productive flowerings. 



The flowerings of Thalassiosira that take place in the shoal waters off Cape 

 Sable and out to Browns Bank arise entirely independent of those in the western 

 side of the gulf. They do not commence until later in the season, for only an 

 occasional specimen was found off the Cape on March 23 in 1920 (station 20084), 

 and none on Browns Bank or in the northern channel a few days earlier (stations 

 20072 and 20078). However, production must have been under full headway there 

 soon after that, because the genus occurred in abundance at all the stations off Cape 

 Sable, on German Bank, and right out to the Eastern Channel by April 15 and 16 

 (stations 20103 to 20107). 



At this time the Eastern Channel marked the extreme limit of the shoals of 

 Thalassiosira in this direction, there being none in our towings on the neighboring 

 parts of Georges Bank on April 16«and 17 (stations 20108 to 20111), although there 

 was a very abundant community of Chfetoceras over the seaward slope (station 

 20109). But what is known of the expansion of the Nova Scotian current during 

 the later spring makes it probable that Thalassiosira would have been found generally 

 dispersed over the eastern half of Georges Bank a week or two later, thus making 

 its range continuous over the whole of the latter at some time late in April. 



It is at about this date that Thalassiosira attains its widest distribution as an 

 important factor in the plankton of the gulf, as outlined on the chart (fig. 129). 

 It is doubtful whether it ever spreads in any abundance over the western side of 

 the basin, for we found a belt of considerable breadth entirely free from it there 



