DISTRIBUTION OF PHYTOPLANKTON 7 , 



Rhizosolenia styliformis almost absent, but it differs from Group 2 markedly in the much 

 larger quantities of Corethron valdiviae, almost complete absence of Chaetoceros socialis 

 and Thalassiothrix antarctica, and absence of Fragilaria antarctica and Nitzschia seriata 

 (the last two were present in small numbers in Group 1 and large numbers in Group 2). 



St. WS 30 combines the most outstanding character of Group 2 with that of Group 3 

 in having both large quantities of Corethron valdiviae and Chaetoceros socialis. It has 

 also a moderate quantity of Ch. criophilum. According to the water movement chart this 

 station should be in Bellingshausen Sea water, but Mr Clowes has indicated that there 

 is a great swirl to the north-west of the island, and on consulting him he agrees that it is 

 possible that some of the water from the area of Group 2 may have been detached and 

 carried round in the swirl to St. WS 30. This is shown in the dotted line in Fig. 35, 

 and would account for the presence of Ch. socialis in large quantities at this station. 



Phytoplankton Group 4. The stations of this group lie in a patch to the south-east of 

 the island in the track of the incoming Weddell Sea water ; it is a patch which sends up 

 arms on each side of the island in horseshoe shaped fashion. It is characterized by a large 

 number of species which are absent or poorly represented in Groups 1, 2 and 3. See 

 Appendix I. It has also large quantities of Chaetoceros socialis, Fragilaria antarctica and 

 Nitzschia seriata and fair quantities of Corethron valdiviae, Chaetoceros criophilum, 

 Thalassiothrix antarctica and Eucampia antarctica. There are in addition in this area 

 small quantities of a number of species not tabulated in Appendix I. The distribution of 

 these is shown in Fig. 36. There can be little doubt that this represents a homogeneous 

 mass of water of Weddell Sea origin which penetrated in this survey farther up the west 

 side of South Georgia than the hydrological data would lead one to believe. St. WS 51 

 appears to belong to, and has been included in, this group. On the eastern side too the 

 water of this group does not fit exactly with the water movements based on hydrological 

 data alone, but nearly so. In comparing Fig. 36 with Fig. 6 it is seen that in general the 

 two sets of observations fit well together, but in detail perhaps the study of the plankton 

 may be of assistance in filling in the picture. 1 



1 Since this section was written Hart's (1934) extensive study of the phytoplankton in these regions has 

 appeared ; it embraces later seasons at South Georgia and the Weddell Sea and Bellingshausen Sea areas. 

 He refers to the recognition by Mr Dilwyn John, during the voyage of the 'Discovery II' in 1932, of two 

 types of Weddell Sea surface water; these he calls "eastern" and "western" Weddell Sea water — see his 

 p. 10 and Fig. 2. The "western" water is that which passes round the Weddell Sea (in a clockwise direction) 

 nearest the continent, and emerges from its north-west corner; the "eastern" water is that which does not 

 penetrate so far, but circulating inside the "western" water emerges to the east of it. Hart finds that these 

 two types of water are characterized by different phytoplankton floras — see his p. 177. The "eastern" water 

 is only moderately rich in species, but is very rich in quantity, the larger forms — Chaetoceros criophilum, 

 Rhizosolenia styliformis and Corethron valdiviae — being strongly dominant. The "western" water is very rich 

 in species with small forms dominant, particularly Chaetoceros socialis, Thalassiosira antarctica, and Chaeto- 

 ceros neglectus, followed by Fragilaria antarctica and Nitzschia seriata. In his South Georgia survey of 

 November 1930, in addition to the Bellingshausen Sea water he recognizes these two types of Weddell Sea 

 water at South Georgia, the "western" Weddell water occurring to the south and south-west of the island, 

 and the "eastern " Weddell water running up the north-east coast. It seems likely that in the phytoplankton 

 of our 1926-7 survey the Groups 3 and 4, both from their position and character, may be identified with 

 Hart's "eastern" and "western" Weddell Sea water respectively. 



