WARM DEEP WATER 83 



Also north of 20 S the actual phosphate maximum increases towards the north, and 

 the North Indian deep water has a greater content than the water of North Atlantic 

 origin south of 20 S. As the section shows, the phosphate of the warm deep water 

 increases from 139 mg. at a depth of 1500 m. in 18J S to 161 mg. at 800 m. at 7°N. 

 The level of the warm deep current rises towards the north, the depth of the salinity 

 maximum rising from 3000 m. in 25 S to 1000 m. in 13 J° S and 400 m. in 7 N, whilst 

 the depth of the maximum content of phosphate rises correspondingly from 1500 m. 

 in 25 S to 900 m. in 7 N. 



The western part of the South Indian Ocean differs from the western half of the 

 South Atlantic Ocean where north of the Rio Grande ridge in 35° S the phosphate 

 content of the warm deep water is less than that of the intermediate current. In the 

 western part of the South Indian Ocean in the tropical zone the phosphate content is 

 greatest in the warm deep water ; the source of the very high content in this water is at 

 present unknown. It is possible that the plankton carried northwards in the inter- 

 mediate current decomposes at a lower latitude in the Indian Ocean than in the Atlantic, 

 and that this decomposition takes place in the warm deep water which is travelling 

 southwards. However, some further work is needed north and east of our section to help 

 decide this question. 



The phosphate content of the South Indian Ocean along section 7 between the ice- 

 edge north of Enderby Land and Fremantle is shown in Plate XII. Throughout this 

 section, which was made in April-May 1932, the phosphate content of the warm deep 

 water was large, being greatest in the upper layers at about 150-600 m. in the Antarctic 

 zone and progressively deeper in the sub-Antarctic and subtropical zones. At the 

 southern stations in the deep basin west of the Kerguelen-Gaussberg ridge, the 

 maximum phosphate occurred at a depth of about 150-300 m. and was of the order 

 of 1 15-130 mg. At stations near and on the ridge the maximum was found closer to the 

 surface at about 150 m. and was considerably greater, at about 140-160 mg. North-east 

 of the ridge in the sub-Antarctic zone maximum phosphate was found at 800 m. in 

 the south and as deep as 2000 m. to the north but always in the warm deep water ; the 

 maximum was never as great as that near the ridge but was of the order 145-155 mg. 

 in the south and 130-140 mg. in the north. In the subtropical zone the phosphate 

 content of the warm deep water was slightly greater than that in the northern part of the 

 sub-Antarctic zone and the maximum was usually found at a depth of 1 500-2000 m. 

 except in the area of the west Australian current where it was deeper at 2000-2500 m. 



In the southern half of the Antarctic zone south of Australia, along section 9 (Plate 

 XIV), maximum phosphate occurred in the upper layers of the warm deep water where 

 140 150 mg. were found. North of about 56 S the maximum appeared to be less and 

 was of the order of 130 mg., whilst in the sub- Antarctic zone it was found at a depth 

 of 800 m. in the south and 1500-2000 m. in the north. Just south of the subtropical 

 convergence and in the subtropical zone the warm deep water had a maximum content 

 of 120-125 mg., which is slightly less than that of the intermediate current which con- 

 tained 130 mg. 



