2i6 DISCOVERY REPORTS 



seen that the water of greatest phosphate content in the South Atlantic Ocean is found in 

 the bottom of the sub-Antarctic layer and in the top of the warm deep layer. Above this 

 greatest content the rest of the layer has also a very high phosphate content, and so great 

 is the ease with which vertical mixing can take place in the upper strata of the layer, that 

 the surface is never depleted of its phosphate. The lowest content measured has been 

 50 mg. PoOj/m.^; this was found in sub-Antarctic water south of the Agulhas current, 

 which contained sub-tropical water mixed with it. Close inshore near Cape Horn at 

 the end of summer the surface phosphate content was 70 mg. PgOg/m.^ 



Measurements of nitrate in sub-Antarctic water are rather few, but those made in the 

 southern half of the zone show that in 30° W the nitrate content of the water at the end of 

 summer was 210 mg. nitrate Na/m.'* ; on the eastern side of the South Atlantic a little 

 farther south it was 220 mg. nitrate Ng/m.^ There are no measurements in sub-Antarctic 

 water farther north, but from the vertical distribution of nitrate on the western side of 

 the ocean, which is shown in Plate IX, it appears that the surface content in the northern 

 half of the zone in summer will be about 150 mg. nitrate N,/m.^ 



SUB-TROPICAL AND TROPICAL WATERS 



SUB-TROPICAL SURFACE AND UNDER-LAYERS, AND THE 

 TROPICAL SURFACE LAYER 



In the sub-tropical Zone the surface water is much warmer and more saline than 

 sub-Antarctic water. Just north of the sub-tropical convergence the surface temperature 

 varies from about i5"5° C. in winter to 18-5° C. in summer. The water between the 

 surface and 50 to 60 m. is well mixed; its temperature only decreases 1° C. or less with 

 depth, and its salinity does not alter appreciably with depth. The complete mixing of the 

 surface stratum is brought about by the turbulent movement of pure wind drift currents, 

 and also probably by convection currents resulting from periodical changes in the tem- 

 perature and salinity of the surface water, which are themselves due to differences in 

 radiation, conduction, and evaporation. 



Below the surface stratum the salinity of the sub-tropical water increases slightly, and it 

 is greatest in a stratum between 80 and 100 m. There is probably a component of move- 

 ment southwards in this stratum, and where the whole layer of water moves southwards 

 the movement will be greatest in this stratum. In 30'' W, where the observations on the 

 layer were made, the wind south of 30' S blows principally from the west. It is much 

 weaker than the wind in the sub-Antarctic Zone, but it will give rise to a transport 

 northwards in the surface and a return current at a depth of 80-100 m. 



Below the stratum at 80 to 100 m. both temperature and salinity decrease with depth 

 until a sharp discontinuity marks the boundary between sub-tropical water and the sub- 

 Antarctic water which has sunk below the surface south of the sub-tropical convergence. 

 This water is now known as Antarctic intermediate water, because of its intermediate 

 position between surface and deep layers of sub-tropical origin. 



The temperature of sub-tropical water in 30° W increases gradually towards the 



