32 DISCOVERY REPORTS 



a position just north of the ice-edge but west of 8o° W, the surface content of only 

 iooo mg. appears to confirm the low values referred to above. 



In general our knowledge of the surface value of silicate content at the ice-edge is 

 restricted to the Scotia Sea, io° W to 44 E and 8o° to 170 W. The maximum 

 amount of silicate present in the surface at the ice-edge appears to be of the order 

 3000-3500 mg. The minimum net withdrawal of silicate in this position varies with the 

 season of the year and is between o and 1000 mg., being greatest of course in the season 

 of maximum phytoplankton concentration. 



North of the ice-edge but still in the arbitrarily defined southern region of the 

 Antarctic zone greater amounts of silicate are withdrawn from the surface by 

 phytoplankton requirements. An average value of 50 per cent of the available 

 silicate is withdrawn, and exceptionally on occasion as much as 75 per cent has been 

 observed. 



The silicate content in the southern region of the South Pacific sector of the 

 Southern Ocean is much lower than in the South Atlantic-South Indian sector. The 

 silicate content in the South Pacific sector falls from west to east at the ice-edge, and 

 in the southern region for both the surface and 0-100 m. layer values. This fall in 

 silicate content from west to east is connected with a difference in the level of the warm 

 deep water on the two sides of the ocean. A large proportion of the warm deep water 

 in the South Pacific belongs to a current of North Atlantic and North Indian deep 

 water which enters the Pacific south of Australia. Silicate is lost to this current by 

 mixing during its passage eastwards, and this is reflected in the fall of the silicate in the 

 surface water from west to east. 



We must now pass on to consider the silicate content of the surface in the northern 

 part of the Antarctic zone. 



In the north of the Antarctic zone, just south of the Antarctic convergence, con- 

 siderably less silicate exists at the surface than in the south of the zone at the ice-edge. 

 At the northern boundary of the Scotia Sea between South Georgia and the Falklands 

 an observation was made in mid-October 1934 just on the southern side of the con- 

 vergence. At this position, St. 1439, the surface silicate content was 750 mg. at a time 

 when the phytoplankton concentration was not great. 



At the western boundary of the Scotia Sea the surface value in late December 1933, 

 at St. 1234, was 450 mg., but there are indications of the presence of a mixture with sub- 

 Antarctic water in the 0-20 m. layer at this station. Just south of St. 1234 a value of 

 950 mg. was recorded in typical Antarctic surface water at a time when the phyto- 

 plankton concentration was fairly high. 



South of the Antarctic convergence in the Scotia Sea between the Falklands and 

 Elephant Island two observations were made in late March 1934 and late September of 

 the same year; these were 850 and 900 mg. respectively at Sts. 1326 and 1425. At both 

 stations a very small concentration of phytoplankton was present. The March value is 

 undoubtedly showing the effect of regeneration, but the September figure is rather low 

 for such a position at a time previous to the main outburst. North of the South Orkneys 



