ii6 DISCOVERY REPORTS 



while the Corethron entering farther to the south was probably the vanguard of an in- 

 vasion due to the increase off Adelaide Island which has been observed in February. 

 It is fairly certain, and is consistent with this view, that the north-easterly set from the 

 vicinity of Adelaide Island immediately outside the Biscoe and Palmer Archipelagos, 

 is stronger than that over the whole of the Bellingshausen Sea. 



Compared with Corethron the species totalling over one hundred thousand are in- 

 significant ; but those most widely distributed may be considered further, as they serve 

 to show the probable sources of the surface water. These were Thalassiosira antarctica, 

 Chaetoceros criophilmn, Rhizosolenia gracilUma and Nitzschia seriata. 



The distribution of Thalassiosira antarctica is shown in Fig. 52. It will be seen that it 

 occurred in small numbers at all the stations on the eastern line, and in moderate plenty 

 at two stations well to the north just inside the South Shetland Islands. This is in fair 

 agreement with the view that the almost pure Corethron plankton found over most of 

 the Bransfield Strait at this time, was the result of recent incursion from the Bellings- 

 hausen Sea, the hauls at which Thalassiosira was found in some quantity including the 

 last traces of the slightly more varied phytoplankton which had been found earlier in 

 the year. 



Fig. 53 shows the distribution of Chaetoceros criophilmn. The species was fairly 

 generally distributed in small numbers, with a slight indication of greater abundance to 

 the south of the eastern line, correlated with the Weddell Sea influence. Its maximum, 

 however, was reached at St. WS 399 to the north-west. This species is widely distri- 

 buted in the Bellingshausen Sea, but does not appear to be of much importance there. 

 It would therefore seem that conditions at St. WS 399 point to an influx of com- 

 paratively old surface water from the north of the Bellingshausen Sea area. 



The only other species of much significance on this particular survey were Rhizo- 

 solenia gracilUma and Nitzschia seriata. Their closely parallel distribution is shown in 

 Figs. 54 and 55. It will be seen that both showed a maximum at St. 399, where it is 

 thought that the bulk of the more northerly Bellingshausen Sea water was entering. 

 Otherwise they were present in small numbers at all the stations at the western end of 

 the strait, but were confined to the northern ends of the more easterly lines. 



THE SURVEY OF NOVEMBER 1929 



The positions of the stations worked on this survey were almost exactly the same as 

 on that described above, so that in order to avoid confusion of the numbers they have 

 been plotted on a separate reference chart. Fig. 56. The first and most easterly line was 

 worked from a point off Admiralty Bay southwards, as before. The full analyses of the 

 phytoplankton hauls obtained on this line (Sts. WS 476-81) are given in Table XXXVI 

 together with that from one isolated station, St. WS 475, still farther to the north-east 

 (at a point north-east of Bridgman Island). It will be seen that at this station a rich 

 haul was obtained, again with Corethron valdiviae strongly dominant, but with Fragilaria 

 antarctica, Thalassiosira antarctica and Rhizosolenia gracilUma also present in con- 



