DISCOVERY REPORTS 



water. In the winter cooling and freezing at the surface leads to increased density, and 

 consequently to vertical mixing and to the homogeneity of the whole layer. Generally 

 speaking the Antarctic surface water streams away in a north-easterly direction all the 

 way round the Antarctic Continent, although the configuration of the land and of the 

 sea-floor may cause local modifications. Within the area with which we are immediately 

 concerned there are two main currents of Antarctic surface water, derived from the 

 Weddell and Bellingshausen Seas. These are described in detail below. 



The temperature of the Antarctic surface water varies from about — i-o° C. in the 

 south to 3-5° C. at its northern boundary in summer, and from — i-8 to about 0-5° C. 

 in winter. The salinity may be as low as 33-oo°/„o in the extreme south in summer, 

 increasing to an average of 33"8o7oo between 65I and 58!° S latitude. The salinity of 

 the Weddell Sea water is slightly higher than that of the Bellingshausen Sea water. 



The Antarctic surface water is extraordinarily rich in nutrient salts, the phosphate 

 content ranging from no to 150 mg. P2O5 per m.^ in winter, and from about 80 to 

 100 mg. in summer. It is safe to say that the phosphate content never falls below 

 50 mg. per m.^, from which it will be seen that there is nothing corresponding to the 

 complete utilization of nutrient ions by the phytoplankton of north temperate latitudes 

 in summer. The importance of this fact in considering phytoplankton problems in the 

 south is obvious. The nitrate content is similarly high, ranging in summer from 350 

 to 500 mg. NO3 per m.^, while from 6 to 8 mg. of nitrite nitrogen per m.^ have been 

 detected. 



The oxygen content of the Antarctic surface water is of the order 90-95 per cent 

 saturation in winter. In spring supersaturation has frequently been observed near the 

 surface, the highest values working out at about no per cent saturation. The lowest 

 values have been obtained in late summer, when, of course, the temperatures were at 

 their highest. 



The northern boundary of the Antarctic surface water is as a rule sharply defined at 

 the surface, big differences in temperature and salinity being encountered within com- 

 paratively short distances when crossing it. The efl^ect is particularly noticeable in the 

 case of ships fitted with a distance thermograph as in the R.R.S. 'Discovery II'. The 

 sharp definition of this boundary, which is called the Antarctic convergence, is due to 

 the cold Antarctic surface water meeting and sinking below the sub-Antarctic surface 

 water, which is much warmer, and therefore lighter. It is obvious that a convergence 

 of this nature will profoundly influence the nature of the plankton population to be 

 found on either side of it, though a certain amount of mixing of the water, and con- 

 sequent overlapping of species, sometimes occurs. In other respects the Antarctic con- 

 vergence may be said to have an eff'ect upon the distribution of the plankton comparable 

 to that exerted by one of the great mountain ranges on the flora and fauna of a land mass. 

 The occasional mixing and consequent overlapping, and the rare presence of " cut-ofts " 

 (e.g. a section of sub-Antarctic water surrounded by Antarctic surface water) might be 

 likened to the partial penetration rendered possible by passes through the mountains 

 on land. 



