HYDROLOGY OF THE SOUTH ATLANTIC 205 



in winter along section I. It shows that apart from the station farthest south, which 

 is influenced by upwelhng deep water, the oxygen content of the surface 100 m. is 

 about 88-5 per cent in the south and 89-2 per cent near the convergence: there is also 

 very little difi'erence between the content of the surface water and that at 100 m. Fig. 9 

 shows the vertical distribution along section II in summer. The oxygen content at the 

 surface is greater than it is in winter ; it varies from 96-5 to 99-9 per cent, and it decreases 

 with depth. The content of the cold stratum in section II is less than that in section I, 

 but the difference is due to the greater upwelling of deep water in section II. 



Near South Georgia, in January, surface water has been found to be supersaturated 

 with a content as great as 1 10 per cent. At the same time large catches of diatoms were 

 obtained, and the water had a high/)H value. 



PHOSPHATE AND NITRATE CONTENT OF THE ANTARCTIC 



SURFACE WATER 



The amount of phosphate in Antarctic surface water is always large compared with 

 that in the surface of more temperate seas. In winter it has been found that the average 

 phosphate content of the surface stratum of 100 m., calculated from twenty-one stations 

 near South Georgia, was 138 mg. of PoOs/m.^ In the following January at a time when 

 large catches of diatoms were being made, the phosphate content was 1 10 mg. PaOg/m.* 

 These values are probably both rather high, as the samples on which the phosphate 

 determination was made had been stored for some time. 



In January and February of 1930 the average content calculated from fifty-seven 

 stations was 82 mg. PoOj/m.^; and in November 1930 from forty-nine stations, 

 89 mg. PgOs/m.^ The minimum phosphate content of the sea water close inshore at 

 Grytviken has been found to be about 77 mg. PoOs/m.^' in December or January, and 

 the maximum to be about 130 mg. PaOg/m.^ in September or October. The inshore 

 results, however, are likely to be increased by the effect of the effluent from the neigh- 

 bouring whaling station, and decreased by the large amounts of fresh water, containmg 

 very little phosphate, which flow into Cumberland Bay. The lowest phosphate content 

 found in the open sea has been about 70 mg. PaOj/m.', and so many measurements have 

 been made that it can safely be said that the phosphate content of Antarctic surface 

 water in the open sea never falls below this figure. There is no evidence of the existence 

 of a secondary maximum of phosphate content in the autumn, such as is recorded m 

 the EngUsh Channel, and the 0-80 m. stratum is so well mixed that it is unlikely to 

 occur. Moreover, in the presence of such an abundance of phosphate a small increase in 

 autumn will not cause a second outburst of diatom production. 



Antarctic surface water also contains large amounts of nitrate; the lowest nitrate 

 content found has been 290 mg. nitrate + nitrite N2/m.» in summer, and the highest 

 about 550 mg. in October-November. 



The abundance of phosphate in the Antarctic surface layer is maintained by up- 

 welling from the warm deep layer. This layer in the Antarctic Zone contains at least 

 140 mg. PaOs/m.s The source of the high phosphate content in this layer will be 



5-2 



