8z DISCOVERY REPORTS 



content of the order of 130-140 mg. in February 1935 as opposed to about 150-160 mg. 

 in May 1934, an example of seasonal or annual variation in this current. 



In March-April 1935 between the ice-edge in 42^° E and Cape Town (section 4, 

 Plate VII), maximum phosphate at stations in the Antarctic zone was found at a depth 

 of 100-200 m. in the south and 300-400 m. near the Antarctic convergence except at 

 Sts. 1543, 1547 and 1549 where the maximum occurred in the bottom water. With these 

 exceptions, the maximum, ranging from 140 to 150 mg., was always found in the upper 

 layers of the warm deep water. In the sub-Antarctic zone the maximum content 

 occurred at a greater depth but was still situated in the upper part of the current and 

 was of the order of 142-148 mg. North of the subtropical convergence the maximum 

 was found in the Antarctic intermediate current, the warm deep water having a lesser 

 content of about 105 mg. at the depth of maximum salinity. 



South-south-west of Cape Town in 1930 (Sts. 414-421) the phosphate maximum was 

 situated in the mixed water between the intermediate and warm deep currents, whilst 

 south-east of Port Elizabeth (Sts. 424-435) it was wholly in the intermediate water. 

 South of the continental slope south of Cape Town the content at the depth of maximum 

 salinity of the warm deep water was 154 mg., and south-east of Port Elizabeth the 

 corresponding value was 143 mg. 



In May 1934 along section 6 from the ice-edge in 44 E to north of the Atlantic-Indian 

 cross-ridge the maximum phosphate content in the Antarctic zone occurred in the upper 

 layers of the warm deep water at a depth of about 200-400 m., the actual maximum 

 being of the order of 155-165 mg. ; the section is shown in Plate X. North of the 

 Antarctic convergence the maximum content was still found in the warm deep water 

 but at a greater depth, about 1000 m. at the southern end of the sub-Antarctic zone 

 and at 2000 m. south of the subtropical convergence, the maximum falling from about 

 160 mg. to 135 mg. from south to north. The section has been extended northwards to 

 near Guardafui by using observations made in April-May 1935 which are included in 

 the same plate (X). In the subtropical zone of the western part of the South Indian 

 Ocean the warm deep water contained less phosphate than the intermediate current, 

 its content being 120-130 mg. at the depth of maximum salinity as opposed to 135— 

 145 mg. at the actual maximum in the intermediate current. Farther north in the 

 tropical zone the phosphate content increased through the intermediate water to 

 become a maximum in the warm deep water. North of 20 S this warm deep water is 

 purely of North Indian Ocean origin, whilst south of this latitude the North Indian 

 warm deep water occupies the upper portion of the current, the bulk of which is of 

 North Atlantic origin. In the purely North Indian Ocean deep water the phosphate 

 content becomes a maximum at or just below the depth of the salinity maximum of the 

 layer. South of 20 S the phosphate becomes a maximum well above the depth of 

 maximum salinity. Thus in addition to other characteristics it may be possible by refer- 

 ence to the phosphate content to distinguish in observations in the tropical zone the 

 source of the deep water, i.e. if the maximum content is found close to the salinity 

 maximum of the layer it is probable that the deep water is North Indian deep water. 



