3o6 DISCOVERY REPORTS 



at St. 761, near the South Orkneys, at least a part of the population will have been carried 

 southward in warm deep water from the Scotia Sea. Since the population at these 

 stations occurred almost entirely in the lower nets we may assume that it originated 

 outside the Weddell Sea in the Scotia Sea or South Atlantic. At nearly all the other 

 stations in water flowing out of the Weddell Sea the population must belong to the 

 stock of Rhincalaniis in the Weddell Sea itself (Sts. 765, 766, 768, 780, 809, 812, 822-5). 

 At Sts. 765, 766 and 768, however, it may be expected that a large proportion of the 

 population sampled by the upper nets originated in the Drake Passage, since in this area 

 there is a large degree of mixing of the waters of the Bellingshausen Sea and Weddell Sea 

 currents. At all these stations the catch appeared in the upper nets. Sts. 815, 816 and 

 817 were taken in the current that flows westwards into the Weddell Sea along the coast 

 of Coats Land south of 66° S. At these stations the catch was in the lower nets. At 

 St. 813, at which there was a small majority in the lower nets, part at least of the 

 population belongs to the westward-flowing current, since this station lay on the 

 boundary between water flowing westwards into the Weddell Sea and that flowing east- 

 wards out of it. The deeper layers of this water, as already explained (p. 282), originate 

 in warm deep water in the South Indian Ocean, so that it seems that Rhincalanus enters 

 the Weddell Sea either from the South Atlantic in warm deep water or from the Indian 

 Ocean in the warm deep current that flows westwards into the Weddell Sea south of 

 66° S. Deacon (1936) found Atlantic water as far south as Sts. 806 and 808, but at 

 St. 807 and the other stations farther south the deep water was found to originate mainly 

 from the current flowing westward into the Weddell Sea south of 66° S. In water 

 flowing out of the Weddell Sea, however, Rhincalanus apparently rises to the surface, 

 since at nearly all the stations in the Weddell Sea north-easterly current, except those in 

 the very " oldest " water, the catches were in the upper nets. A hydrological explanation 

 of this is available in the case of certain stations in the centre of the Weddell Sea 

 cyclonic system (Sts. 809, 812, 813, 822 and 823). These stations lay in an area inter- 

 mediate between the westward current flowing into the Weddell Sea and the water 

 flowing north-eastwards out of it. In this area, as Deacon shows in work now in the 

 press (1936), there is an upwelling of warm deep water towards the surface in the deeper 

 layers. Although the isotherms within the surface 250 m. give no clear indication of 

 upwelling it may exist to a small extent in the upper layers and may still influence the 

 plankton above the 250-m. level. At the stations in water flowing out of the Weddell Sea 

 to the west near South Georgia (Sts. 765, 766, 768, 780 and 825) a large part of the 

 population very probably belongs to the Bellingshausen Sea water, which here mingles 

 with water from the Weddell Sea. 



It seems necessary to bear in mind, when considering these questions of the vertical 

 distribution of the plankton, that variations in the level of concentration are not brought 

 about so much through the conveyance of plankton from one level to another by the 

 movements of masses of water as through the active migration of the plankton along 

 temperature gradients to levels where an optimum temperature occurs. For instance, 

 the upwelling of warm water in the centre of the Weddell Sea, above mentioned, in- 



