TRANSITIONAL PHYTOPLANKTON ^l 



The stations worked on these hnes were necessarily far apart, and it would therefore 

 be inadvisable to attempt any such detailed account as has been given of the close lines 

 of stations worked in the South Georgia area. For purposes of comparison, the material 

 may be divided naturally into the collections from the two seasons 1929-30 and 1930-1, 

 though it must be noted that the line worked early in the 1929-30 season shows little 

 relation to the others worked later in the year. All these lines, with the possible exception 

 of those worked between the Falkland Islands and South Georgia, extend much farther 

 to the south and east of the convergence than to the north of it, traversing old Antarctic 

 surface water both from the Weddell and the Bellingshausen Seas. 



THE SEASON 1929-30 



The first piece of work to be considered, was a line of stations (WS 468-74) worked 

 by the R.R.S. 'William Scoresby ' in November, from 55° 52' S, 56° 53' W to 61° 03' S, 

 56° 42' W, that is from a point south of the Burdwood Bank to a point west of Elephant 

 Island. Seven stations were worked on this line ; their positions are shown in Fig. 32. The 

 full analyses will be found in Table XVII, and the percentages of the leading forms in 

 each catch are shown in Fig. 33, together with the temperature mean for the upper 

 20 m., and the total estimated numbers of phytoplankton organisms. From these it will 

 be seen that the phytoplankton was very poor to the north of the convergence, and 

 showed a marked increase with the decreasing temperature gradient as one proceeded 

 southwards. The first station, definitely to the north of the convergence, was St. WS 

 468, and it furnished the poorest haul on the line. Fragilaria antarctica was dominant, 

 and other Antarctic stragglers were present in very small numbers. Only two species 

 were recorded that were not also found farther to the southward, Chaetoceros decipiens 

 and Ceratiumfiisus. At the next station to the south, worked in the immediate vicinity of 

 the convergence itself, the phytoplankton was appreciably richer and several common 

 Antarctic forms made their appearance, Corethron valdiviae and Nitzschia seriata being 

 the most important. The spineless chain form of Corethron, which has been found to be 

 an important member of the phytoplankton both in sub-Antarctic and Antarctic surface 

 waters (in abnormally warm seasons) reached its highest relative proportion here, 

 though larger numbers were actually encountered farther south on this line. This station, 

 WS 469, was also noteworthy for the only occurrence of Rhizosoleiiia polydactyla on this 

 line, a species which has been found as a dominant form of the sub-Antarctic phyto- 

 plankton later in the season, and only occasionally farther south in very small numbers. 



St. WS 470, with a mean temperature of 1*59° C. in the upper layers, and nearly 17 

 million estimated phytoplankton organisms besides uncountable colonies of Chaetoceros 

 neglectiis, brings us fairly into the Antarctic Zone. Other important species at this station 

 were Nitzschia seriata, Chaetoceros atlanticus, and a comparatively scarce form whose 

 occurrence has been found to be very sporadic throughout all the material examined — 

 Ch. castracanei. Farther south on this line minus temperatures were recorded, and 

 a very rich phytoplankton was present at all but the southernmost station. The main 



