HYDROLOGY OF THE SOUTH ATLANTIC 219 



of tropical water in the discontinuity separating it from the sub-tropical under-water 

 also varies from about 35-8 to 37-0 7oo- It is greatest between 20 and 10° S, and least 

 just north of the Equator. 



OXYGEN CONTENT OF THE SUB-TROPICAL AND TROPICAL WATERS 



The oxygen content of the sub-tropical water is approximately 95 per cent of satura- 

 tion, and the oxygen content between the surface and 50-60 m. (which must be in 

 constant circulation) is almost uniform with depth. There is, however, always a small 

 maximum at a depth of 60 m., which must be caused by the maximum growth of 

 diatoms at that level. The oxygen content in the bottom of the layer decreases from 

 about 80 per cent in 40° S, to 75 per cent in 28° S. 



The surface of tropical water is about 96 per cent saturated with oxygen between 28 

 and 20° S, but then the oxygen content decreases slightly to a minimum of about 

 93 per cent between 15 and 10'^ S in the south equatorial current. The oxygen content 

 of tropical water increases with depth and is at a maximum just above the discontinuity 

 which separates the layer from the sub-tropical under-layer. This maximum is again 

 probably due to the greater growth of diatoms at that depth, the diatoms utilizing the 

 small amounts of phosphate and nitrate liberated in tropical water as a result of the 

 slight vertical mixing which takes place across the discontinuity. 



The oxygen content of the sub-tropical under-layer can only be replenished by a 

 horizontal inflow of water, since the layer is shut ofl^ from the surface by a discontinuity 

 which does not allow vertical mixing. The oxygen content of the layer falls from about 

 80 per cent of saturation in 28° S to about 40 per cent at the Equator. 



PHOSPHATE AND NITRATE CONTENT OF THE SUB-TROPICAL 



AND TROPICAL WATERS 



In 30" W the phosphate content of the surface stratum falls at once from about 

 60 mg. PgOs/m.^ to 10 mg. on passing from sub-Antarctic to sub-tropical water. The 

 nitrate content also decreases from about 150 mg. nitrate N.,lm.^ to 50 mg., and a little 

 farther north to 10 mg. Both decrease towards the north until just south of 28° S, the 

 phosphate content of the surface stratum is 8 mg. PgOs/m.^, and the nitrate content is 

 5-6 mg. nitrate NJm.^ 



In the surface stratum of tropical water there is no phosphate and it must be com- 

 pletely used up as soon as it is regenerated. There is usually a small nitrate content of 

 0-5 mg. nitrate NJm.^, and this is replenished from rain water, which has been found 

 to contain as much as 10 mg. nitrate Nj/m.^ after a thunderstorm. The nitrate is probably 

 not used up because of the absence of phosphate. 



In sub-tropical water the phosphate and nitrate contents increase with depth until 

 there is about 35 mg. PoOs/m.^ and 150 mg. nitrate Na/m.^ in the discontinuity level. 



The sub-tropical Zone is not very favourable for diatom growth in general because 

 of the small amounts of phosphate and nitrate at the surface. It is not, however, entirely 

 depleted of them, and where they are replenished by upwelling, such as takes place off 

 the African coast, the conditions are very good. The tropical water can only support 



