104 DISCOVERY REPORTS 



The last series of observations obtained from the Weddell Sea area during this season 

 consisted of a series of thirteen stations (Sts. 613-26) worked from the neighbourhood 

 of Elephant and Clarence Islands on an approximately east-north-easterly course past 

 the South Orkneys to a point between Saunders Island and the Candlemas Islands in the 

 South Sandwich group, in late February 193 1. The full analyses of the phytoplankton 

 material collected will be found in Table XXXII and the positions of the stations on 

 reference to Fig. 32. The very variable salinities and temperatures recorded at the surface 

 during this series of observations indicate that conditions were greatly influenced by 

 melting of pack-ice at no great distance to the southward. The relative proportions of 

 the more important phytoplankton organisms at these stations are shown in Table 20. 



From the phytoplankton totals it will be seen that to the west the catches were very 

 poor, but that they became slightly richer near the South Orkneys, and after a further 

 falling off, increased steadily as the South Sandwich Islands were approached, in which 

 region some of the catches might fairly be described as rich despite the lateness of the 

 season. 



The three westerly stations first dealt with, at which the phytoplankton was com- 

 paratively poor, showed a phytoplankton indicative of old Bellingshausen Sea water of 

 the type commonly met with in Bransfield Strait, and, at certain times of the year, in 

 the southern half of Drake Passage also. At these stations Corethron valdiviae was the 

 dominant form, with Fragilaria mitarctica the most numerous of the few other species 

 present at St. 613, and Thalassiothrix antarctica at the two following stations. To the 

 west of the South Orkneys, at Sts. 617 and 618, Fragilaria antarctica was of practically 

 equal importance to Corethron, and a relatively large proportion of the poor catches ob- 

 tained was made up of moderate numbers of a variety of small forms which were of 

 little significance elsewhere during this series of observations. This may be seen from 

 the large percentage of "other forms" at these stations in Table 20, and reference to 

 the full analyses (Table XXXII) shows that while individually of small account, such 

 species as Thalassiosira antarctica, Biddiilphia striata, Chaetoceros atlanticus and Ch. 

 schimperianus were of greater importance here than anywhere else on the line. These 

 indications all point to the western Weddell Sea origin of the surface water at these 

 stations, though the distinction was by no means so clear as in the catches to the south- 

 west of South Georgia early in the season. This is probably to be ascribed to the opera- 

 tion of one or all of the following factors : a slackening in the flow of the current out of 

 the Weddell Sea after mid-season, mixing with Bellingshausen Sea surface water from 

 the west, and to a change in the nature of the phytoplankton of western Weddell Sea 

 surface water as the season advanced, the large forms characteristic of eastern Weddell 

 Sea conditions earlier in the year attaining greater prominence. This last factor may help 

 to explain the character of the phytoplankton observed at Sts. 619 and 620, which was 

 of the type we have come to associate with eastern Weddell Sea conditions, although 

 from the positions one would have expected some sign of mixing with water of the 

 western Weddell Sea type. The haul from St. 619 was rather poor, that at St. 620 

 slightly richer, and from St. 621 onwards a fairly rich phytoplankton of definitely 



