6o DISCOVERY REPORTS 



SUBTROPICAL SURFACE WATER 



PHOSPHATE AND SILICATE CONTENTS AT THE SURFACE 

 Even less data exist for the nutrient salt content of subtropical surface water than 

 for the sub- Antarctic water ; our observations are mostly confined to data obtained during 

 passages to and from Antarctic regions, except round South Africa where more work 

 has been done. 



In the western part of the South Atlantic Ocean (section 2, Plate IV, Sts. 673-677) 

 in late April 1931 in 30 W the surface content of water in the subtropical zone was 

 of the order of 8-10 mg. ; the content dropped to zero immediately tropical water was 



encountered. 



In the third week of November 1933 Tristan da Cunha was undoubtedly in the sub- 

 tropical zone, the subtropical convergence lying well to the south by as much as 5-6 

 of latitude. St. 1188 in 41 56' S, 15 08-6' W, south-west of the island, was taken in 

 subtropical water and had a surface content of 32 mg. 



In the eastern part of the South Atlantic Ocean no differentiation of tropical and 

 subtropical surface waters can be made by inspection of the surface content of phosphate 

 as is possible in the western part of the same ocean. Thus in section 3 (Plate V), in mid- 

 April 1933 in the eastern side of the ocean the surface content of phosphate did not 

 drop to zero until St. 1182 (3 20-8' S, 8° 37-2' W) was reached. At this station the 

 surface temperature was 28-44° C. and the salinity was falling towards the north, 

 whereas at St. 1180 (io° 30-8' S, 4 41-6' W) with a surface temperature of 26-41° C. 

 the surface phosphate content was 16 mg. ; in this section subtropical surface water had 

 a surface phosphate content which varied from 19 to about 10 mg. from south to north 

 across the zone. Thus in the eastern part of the South Atlantic Ocean quite an appreciable 

 quantity of phosphate can exist in tropical water, whereas as we have seen in the western 

 part of the ocean the surface phosphate content was reduced to zero immediately tropical 

 water was encountered in 28° 30' S. The fact that the phosphate content of tropical 

 surface water in the eastern part of the South Atlantic Ocean is not reduced to zero until 

 the equator is almost reached must be due to the effect of the Benguela current which 

 transports water with a surface content of about 25 mg. (in August) from the region west 

 of Saldanha Bay into lower latitudes. In the western part of the ocean the surface water 

 in low latitudes is supplied from the South Equatorial and Brazil currents in which the 

 phosphate has been used up. 



It has been observed that in general the plankton in the subtropical parts of the eastern 

 and western halves of the South Atlantic Ocean is very different in concentration. The 

 plankton in the eastern half of the ocean is rich compared with that in the western part. 

 This difference must undoubtedly be connected with the difference in phosphate content 

 in the two halves of the ocean. 



Around the coasts of South Africa and south of this area rather more data are available. 

 The waters surrounding South Africa are subject to subtropical and tropical influences. 

 The Cape Peninsula is surrounded by subtropical surface water, but towards the east 



