TRANSITIONAL PHYTOPLANKTON 



8S 



been found in very small numbers in the cold water much farther south, in both the 

 Weddell and Bellingshausen Seas, but only late in the season, and never forming any 

 large proportion of the total phytoplankton. 



In Fig. 41 the distribution of Nitzschia seriata has been plotted. It will be seen that 

 this form, perhaps the most abundant of all those encountered during this piece of work, 

 was most numerous in the old BelHngshausen Sea water approaching from the west- 

 ward, immediately to the south of the Antarctic convergence. It was also exceedingly 

 abundant at one station (633) worked immediately to the north of the convergence, 

 and was the most numerous and widespread of the Antarctic forms found in the scanty 

 phytoplankton in the sub-Antarctic water on this occasion, so much so that it might 

 almost be described as an invader, rather than a straggler. 



Fig. 41. The distribution of Nitzschia seriata in the Scotia Sea, February-March 193 1. 



Fig. 42 shows the distribution of Choetoceros tieglectus, the most numerous of the 

 small species of Chaetoceros which seem to have been percolating through the southern 

 half of Drake Passage at this time. The definitely Antarctic character of this species is 

 well evidenced by the fact that it was only observed to the south of the Antarctic 

 convergence. 



Ch. atlanticus, on the other hand, the distribution of which is shown in Fig. 43, was 

 almost as frequent in the sub-Antarctic Zone as Nitzschia seriata. Like the preceding 

 species, however, it reached its greatest abundance in the old Bellingshausen Sea water 

 approaching from the west. There is considerable evidence that Chaetoceros atlanticus 

 may be regarded as one of the principal forms of the autumnal increase in both Weddell 

 and Bellingshausen Sea water, and since it has about the most northerly distribution of 



