RHINCALANUS GIGAS 297 



northwards between the South Orkneys and South Georgia, the Hnes of equal numerical 

 distribution are widely spread out (Fig. 8). 



In the South Atlantic, where Weddell Sea water comes into contact with water of 

 Drake Passage and Bellingshausen Sea origin roughly along the i-o° C. line, the dis- 

 tribution lines are crowded together. They approximate to the position of the o and 

 i-o° isotherms, and where, in about longitude 23° W, the 0° isotherm takes a south- 

 ward bend, the 500 and 1000 and the 25 and 50 per cent equi-distributional lines take 

 a southward bend also. In this position the surface water appears to have a southward 

 movement (Deacon, 1936, in press), and the course of the lines of equal distribution of 

 R. gigas shows that this southward movement of warmer water has its effect upon the 

 copepod fauna. 



There is thus a sharp boundary between the copepod fauna of the South Atlantic 

 water of Bellingshausen Sea origin and that of Weddell Sea origin. In the fauna of the 

 former kind of water Rhincolainis predominates, while in the fauna of the latter type of 

 water other species predominate, and the boundary between the two faunas lies along 

 the o and i-o° isotherm lines, which, east of South Georgia, are close to each other. 

 Immediately north of the island, however, and in the Scotia Sea, a spreading out of the 

 lines of equal distribution indicates a much greater degree of mingling of the influences 

 of the two types of water. North of the island Weddell Sea influences appear to pre- 

 dominate in the fauna (Figs. 8, 9), while south of the island in the Scotia Sea the in- 

 fluence of water from the Bellingshausen Sea appears to predominate. 



In general it may be said that in the Scotia Sea and in the South Atlantic east and north 

 of South Georgia (Nov. 1931-Jan. 1932) the southern limit of distribution of R. gigas 

 corresponded in position with the 0° isotherm (average of o-ioo m.). South of this line 

 the catches amounted to less than 500 individuals and usually constituted less than 1 5 per 

 cent of the copepod plankton. The 0° isotherm is also the boundary of Weddell Sea 

 water carrying melting pack-ice or in which pack-ice has recently melted. In the 

 southern Drake Passage, however, there is an abundance of Rhincalanus in water con- 

 siderably colder than this, carrying melting pack-ice, which has been ascribed to the 

 upwelling of warm deep water along the Antarctic continental shelf. 



Falkla?id Sector, mid-Janiiory to mid-February (Table III 6) 

 Not many observations were made at the end of the season 1931-2 by which changes 

 in the distribution of the population can be judged. At the end of January a line of 

 stations was run from the farthest point south in the Weddell Sea, at the pack-ice edge 

 in 69° 59' S and 23° 53' W to South Georgia (Fig. i b). This line traversed water 

 flowing out of the Weddell Sea on the west along the coast of Graham Land. At the four 

 most westerly stations (822-5 — ^^^ Table III a and Fig. 10) the catches oi Rhincalanus 

 amounted to between 400 and 700 individuals. Sts. 824 and 825 show a diminution in 

 numbers compared with St. 768 in the middle of December but no marked change com- 

 pared with St. 766 (Figs. I a, 8). No observations were made farther south in this water 

 during the early part of the season with which to compare Sts. 822 and 823. In the 



