io6 DISCOVERY REPORTS 



eastern Weddell Sea type was encountered. This is shown by the greatly increased 

 proportions of Ch. criophihim and Rhizosolenia styliformis in the catches. The other 

 important species at all these stations was Corethron valdiviae. This last was of less im- 

 portance at Sts. 622 and 624 than at the others, Chaetoceros criophihim being very strongly 

 dominant. The catch at St. 624 was a particularly large one for the time of year. At the 

 two most easterly stations the proportion of Corethro?t valdiviae again increased, this 

 species being completely dominant at St. 626, worked close to the positions at which a 

 similar plankton had been observed during the ' William Scoresby's ' cruise a fortnight 

 earlier. From these stations it becomes very obvious that a fairly rich phytoplankton 

 of the eastern Weddell Sea type persisted in the older surface water until well into the 

 second half of the cold 1930-1 season. 



CONCLUSIONS 



In attempting to gain a picture of the phytoplankton production of the Weddell Sea 

 as a whole, a few stations worked within the eddy of western Weddell Sea water at the 

 northern end of Bransfield Strait in December 1930, should be considered along with 

 those described in this section. A full description of the phytoplankton at these stations 

 is given on p. 126, for the present purpose it is sufficient to state that they showed a 

 phytoplankton of similar type to that found to the south-west of South Georgia earlier 

 in the season, but by no means so rich. This would seem to indicate that the very rich 

 phytoplankton with small forms dominant, notably Chaetoceros socialis and Ch. neglectus, 

 is only found in this type of water during the late spring increase, when the northerly 

 flow of Antarctic surface water is at its strongest. This is in good agreement with the 

 more extensive observations to the north of the Weddell Sea, mostly obtained towards 

 the middle and end of summer. As we have seen, these indicate that as the season ad- 

 vanced and the currents slackened, the distinction between the two types of surface 

 water so sharply defined earlier in the season was very largely lost as far as the outflow 

 from the north-western corner of the Weddell Sea was concerned. On the other hand 

 the changes in the phytoplankton in the water flowing into the Sea, far to the south-east, 

 were still well marked as one proceeded southwards in February. 



One of the most obvious facts brought out in the preceding description was the com- 

 parative poverty of the phytoplankton in the South Georgia-South Sandwich Islands 

 area during the mild open season 1929-30, and its great abundance during the cold 

 1 930-1 season, the latter a heavy ice year. Despite the large numbers of the spineless 

 chain form of Corethron valdiviae recorded at a few stations during the earlier of these 

 two years, the total numbers of phytoplankton organisms estimated in the catches were 

 of the order of one-tenth of those found in the same locality during the following season. 

 This poverty of the 1929-30 season appeared to be a very general phenomenon, but the 

 material obtained was too scanty to permit of any statement regarding the Weddell Sea 

 as a whole. However, it remains fairly certain that the phytoplankton of the surface 

 water flowing out of it to the north-east was unusually poor. 



It seems, then, that while the phytoplankton of the two types of surface water known 



