72 



DISCOVERY REPORTS 



features of these catches may be briefly summarized for the 

 sake of the light that they throw on conditions in this water 

 of Bellingshausen Sea origin at this time of year. 



At St. WS 471 the dominant species in a fairly rich haul 

 were Nitzschia seriata, Chaetoceros dichaeta and Ch. neglectiis 

 in that order of importance, while at the next station to the 

 south the richest haul of the whole line was obtained, this 

 catch being also characterized by a large variety of species. 

 Here the dominant forms were Thalassiosira antarctica, 

 Fragilaria antorctico and Nitzschia seriata. The next station 

 south also showed a fairly heavy catch, but here the dominant 

 forms were Nitzschia seriata and Chaetoceros neglectiis. At the 

 most southerly station the phytoplankton was relatively thin, 

 and the presence of Rhizosolenia alata f. gracillima points to 

 possible mixing with Bransfield Strait water : other important 

 forms were Nitzschia seriata and Thalassiosira antarctica. 



Summing up this survey of the line as a whole, the main 

 feature was undoubtedly the close general resemblance to the 

 type of phytoplankton one has learnt to expect in late autumn, 

 with small Chaetocerids, Nitzschia seriata and Fragilaria 

 antarctica dominant ; though the presence of the spring form 

 of Thalassiosira in moderate numbers at all the stations south 

 of the convergence, and dominant at St. WS 472, showed 

 that the spring increase had definitely begun. The bulk of the 

 samples was also somewhat greater than that of the autumn 

 catches, but in other respects the resemblance is most marked. 

 This is possibly to be explained by the fact that as the 

 north-easterly flow of the Antarctic surface water is at a 

 minimum (sometimes even apparently at a standstill) in 

 winter, the autumn forms will probably persist in small 

 numbers in the older water just to the south of the con- 

 vergence, and so be the first to multiply in spring. The small 

 catch at the southernmost station may have been due to the 

 spring increase not having started farther south, but con- 

 ditions in the Bransfield Strait only a short time later render 

 this highly improbable. This station (WS 474) was worked 

 just to the north of the submarine ridge connecting King 

 George Island and Elephant Island, and from other work 

 carried out later in the following seasons it would seem that 

 there is usually an area to the east and north of King George 

 Island where phytoplankton is scanty. This may be due 

 to constant vertical mixing over the ridge preventing the 



BO 

 40 

 20- 



[Nor e5timate:d^ 



EXCEEDED OTHER 

 FORMS] 



FRAGILIA ANTARCTICA 



% 



eo-^B 



40-^1 

 20-^1 



NITZSCHW. SERIATA 



60 

 ■40 

 20- 



Fig. 33. Diagram showing the 

 percentages of the principal 

 species in the catches on a line 

 of stations worked southwards 

 across the Scotia Sea, Novem- 

 ber 1929. 



