202 DISCOVERY REPORTS 



currents, whilst the water below 80 m. is not — an assumption which has been confirmed 

 in sub- Antarctic water by the presence of a component of movement below 80 m. in 

 the opposite direction to the surface transport. With the approach of winter cooHng 

 commences in the surface, and its eff'ect is carried into the rest of the surface stratum 

 by the sinking of the surface water as its density increases, and by vertical mixing caused 

 by the turbulent movement of the pure drift currents. The density of the whole of the 

 surface stratum is increased in this way, and also by the inflow of heavier water from 

 the south, until it can hardly be distinguished from the cold stratum and the whole 

 layer becomes almost uniform. The density at 150 m. is least in winter, probably 

 because the layer is deeper at this season. 



In Table III the changes in temperature of the Antarctic surface layer from south to 

 north are summarized. 



Table III. 



The salinity at the surface can vary enormously, from very low values in the south — 

 in summer when ice is melting — to as much as 34-5 °l^^ when ice is forming, or where 

 deep water is upwelling. In the layer as a whole the salinity decreases towards the north, 

 although at the surface the lowest salinities are found near the ice in summer. 



OXYGEN CONTENT OF THE ANTARCTIC SURFACE WATER 



The oxygen content of the surface 100 m. of water in the Falkland Sector is shown in 

 Fig. 17. The oxygen content is expressed as a percentage of the amount that the water 

 would hold if it were saturated. The diagram shows the conditions in spring before the 

 diatom growth was considerable. The smallest oxygen contents, 75-2 and 747 per cent, 

 were found off the west coast of Graham Land and in the Bransfield Strait ; low values 

 were also found over the ridge joining Graham Land to the South Orkney Islands and 

 to the South Sandwich Islands. It is probable that pack-ice was present in these 

 localities shortly before the observations were made, and also that the surface water 

 was largely mixed with deep water which had upwelled from below. 



The oxygen content of the water increases from south to north. There is also a small 

 increase from west to east in water of the same temperature, about 5 per cent where it 

 is greatest. This may be due to a change which had taken place whilst the survey was 

 being made, or it may be due to a richer phytoplankton grov^th in the east than in the 

 west. 



Fig. 18 shows the vertical distribution of oxygen content in Antarctic surface water 



