WARM DEEP WATER 81 



Unfortunately, north of i2|° S the observations only extend just below the intermediate 

 current. This section was made in March-April 1933 as compared with that in the 

 western part of the ocean which was made in April-May 1931. The phosphate content 

 throughout was lower in the eastern section, particularly in the Antarctic zone, where 

 the maximum content occurred in the upper layers of the warm deep water and was 

 of the order of 1 15-120 mg. compared with 140-150 mg. in the western section. 

 Attention has already been drawn to the difference in the phosphate content of the warm 

 deep water at the most southerly position, St. 11 54, in the eastern section as compared 

 with a station, St. 1525, made in almost the same position 3 years later. Also in 

 February-March 1935 some observations were made along section 4 from the ice-edge 

 in 66i° S, 42|°E towards Cape Town (Sts. 1543— 1553, Plate VII), the phosphate 

 content was maximal in the warm deep water in the Antarctic zone and was of the 

 order of 140-150 mg. This amount, though not as great as that found in the western 

 part of the South Atlantic in 1931, is more comparable than the values found in the 

 eastern part of the South Atlantic in 1933. It would be expected that the average content 

 of the warm deep water would not be so great in the eastern half of the South Atlantic 

 as in the west because the strength of the Antarctic intermediate current is less in the 

 eastern half. The general lowness of the 1933 observations in the eastern part of the 

 ocean must be due to annual variation of which we have very little data at present. 



In the Southern Ocean south of the Atlantic Ocean the phosphate content of the 

 deep layers is rather confusing in that the maximum content is found sometimes in the 

 warm deep layer and sometimes in the Antarctic bottom water. Thus south of 6o° S 

 in 1934 the maximum was mainly in the bottom water, but in 1935 it appeared about 

 equally in the two currents. At the stations where the maximum appeared in the bottom 

 water I have calculated the difference between this maximum value and the content in 

 the upper layers of the warm deep water. On an average this difference amounts to 

 about 6 J mg. but frequently is lower. We know from the longitudinal sections 2 and 3 

 (Plates IV and V) in the South Atlantic Ocean that the return of phosphate to the 

 Antarctic zone occurs via the warm deep water and that phosphate leaves the Antarctic 

 in the bottom water in addition to that which is carried out in the sinking surface water. 

 What we do not know at present is the amount of mixing that occurs between the lower 

 layers of the warm deep water and the bottom water, i.e. it is possible that when the 

 phosphate maximum occurs in the bottom water it is because plankton has either sunk 

 or been carried downwards in the warm deep water to decompose in the boundary layer 

 between the two currents and that the bottom water becomes enriched by mixing. On 

 the other hand, it is possible that the Deniges method of phosphate estimation is not 

 sufficiently accurate under the conditions at sea in bad weather, to furnish such fine 

 distinctions between the contents of water masses. 



The upper layers of the warm deep water appear to have a content of about 1 3 5- 1 45 mg. 

 in the region south of 6o° S in the South Atlantic Ocean ; at the stations in the extreme 

 south where the warm deep water flows towards the west and consists of water of mixed 

 North Atlantic and North Indian Ocean origin, the upper layers of the current had a 



