io DISCOVERY REPORTS 



Antarctic zone the greatest phosphate content is found in the upper layers of the North 

 Atlantic deep water. North of the Rio Grande ridge in about 35 S the maximum 

 phosphate content is found in the Antarctic intermediate water, and the warm deep 

 water contains less phosphate. Thus the warm deep water is not the source of the high 

 phosphate concentrations which are found in the Antarctic zone. Phosphate is added 

 to the warm deep water south of the Rio Grande ridge, and the source of this phosphate 

 must lie in the rich plankton of the Antarctic zone which decomposes in the Antarctic 

 surface water when it sinks into the bottom of the sub-Antarctic layer. It is probable 

 that the decomposition of plankton is greatest in the region of greatest phosphate con- 

 tent; this occurs between the north-going sub-Antarctic and Antarctic intermediate 

 waters and the south-going warm deep water, between 43 and 38 S. In this region the 

 greatest vertical mixing between these currents also occurs, so that the phosphate, 

 released by the decomposition of plankton, passes over into the mixed water in the upper 

 layers of the south-going warm deep water. In due course it is utilized by the phyto- 

 plankton and the cycle recommences. 



In the eastern part of the South Atlantic Ocean (section 3) a similar phosphate cycle 

 prevails ; this is illustrated in Plate V, which is a section between 69 20' S and 3 46' N. 

 Maximum phosphate occurs in 41 S at a depth of 1000 m. in the Antarctic intermediate 

 water where decomposition is probably greatest. The phosphate is returned to the south 

 in the mixed water which forms the upper layers of the warm deep water. 



The vertical section of phosphate content, between Cape Town and the ice-edge north 

 of Enderby Land in 66|° S, 42^° E (section 4, Plate VII), shows admirably the phosphate 

 cycle in this part of the ocean. Phosphate may be seen leaving the Antarctic zone both 

 in the sinking Antarctic surface water at the Antarctic convergence and in the bottom 

 water. The sinking Antarctic surface water forms part of the Antarctic intermediate 

 current. Between about 40 S and the coast of South Africa the nucleus of this current 

 reaches its greatest depth at 1500 m., as is shown by the salinity minimum at this depth. 1 

 The reason for this increased depth is the vast bulk of subtropical water brought into 

 the area by the Agulhas current system which depresses the level of the intermediate 

 water. Even at 3 8° S the phosphate content at the depth of the salinity minimum of the 

 intermediate water is 140 mg., and some of this amount is lost to the phosphate cycle in 

 the upwelling towards the coast of South Africa. The maximum content of phosphate 

 in the whole section is seen at 48 S, where it amounts to 148 mg. between 600 and 

 800 m. in the upper part of the warm deep water and where it is being returned to the 

 south in this current. North of the Antarctic convergence the sinking Antarctic water 

 is mixing with the upper layers of the warm deep water. Between 45 and 38 S the 

 phosphate content of the warm deep water is much lower. This may be seen in the value 

 of less than no mg. at the salinity maximum of the layer at about 2500-3000 m., in 

 contrast with the content of 141 mg. at the nucleus of the intermediate water in 40 S, 

 and of 130 mg. in the bottom water at the same latitude. Thus the phosphate which is 



1 The distributions of temperature and salinity in most of the sections mentioned are given by Deacon, 

 Discovery Reports, xv, 1-124, Plates, I-XLIV. 



