DISTRIBUTION OF ANTARCTIC MACROPLANKTON 69 



are due, and gradually works northwards (with a strong easterly component) until it 

 meets the more saline, but warmer and lighter water of the sub-Antarctic Zone, beneath 

 which it sinks. The line along which this sinking takes place is the northern boundary 

 of the Antarctic Zone and is known as the "Antarctic convergence". 



Beneath the cold surface water is a much thicker layer of warmer water whose origin 

 is in the North Atlantic or Pacific. There is a southerly component in the direction in 

 which this water drifts, and when it reaches high latitudes it becomes cooled. Part of it 

 rises to the surface to replenish the cold superficial layer, and the rest, which has been 

 cooled but not diluted, sinks to form the Antarctic bottom water. This layer flows in a 

 northerly direction. There is in fact a movement of cold water away from the pole at the 

 surface and bottom, and towards the pole at intermediate depths. The warm inter- 

 mediate layer is very rich in nutritive salts, and it is the upwelling of these salts into the 

 cold surface layer which enables the richest plankton in the world to flourish in the 

 Antarctic surface water. 



In the lower latitudes of the Antarctic Zone the surface waters are drifting in an 

 easterly direction, and Drake Passage may be described as a constriction in this drift. 

 Graham Land and the associated islands and archipelagos, which form the southern 

 promontory of this constriction, separate the Weddell Sea to the east from the Bellings- 

 hausen Sea to the west. North of the Weddell Sea lies the Scotia Sea, bounded to the 

 west by Drake Passage and on all other sides by the Scotia Arc. The Scotia Arc is a 

 loop of elevated land projecting eastwards into the Atlantic. It comprises — as existing 

 land masses — Tierra del Fuego, Staten Island, the Shag Rocks, South Georgia, and the 

 South Sandwich, South Orkney and South Shetland Islands. These, as Herdman 

 (1932) has conclusively demonstrated, are connected by well-defined submarine ridges. 

 The Burdwood Bank, to the east of Staten Island, forms part of the arc. Water passing 

 eastwards through Drake Passage is deflected a little to the north-east and flows 

 mainly through a gap in the Scotia Arc between the Burdwood Bank and the Shag Rocks. 

 The Antarctic convergence passes through the middle of Drake Passage, and the water 

 to the south of it is derived mainly from the Bellingshausen Sea. 



In the Weddell Sea there is a clockwise circulation. This great bay in the Antarctic 

 Continent receives water drifting westwards in the higher latitudes, and the flow is 

 directed northwards and north-eastwards by the Graham Land Archipelago, and flows 

 out as a very cold current towards the South Sandwich Islands and South Georgia. 

 It joins the drift from the Bellingshausen Sea along a line running from the South 

 Shetlands to South Georgia. The drift from the Weddell Sea carries with it much pack- 

 ice and innumerable icebergs, so that in longitude 20 or 30 W the pack frequently 

 extends far to the north of the South Sandwich Islands, while around 60 or 70 W it is 

 never found very much north of the South Shetland Islands. 



We are here dealing only with the larger plankton organisms and their horizontal 

 distribution in the cold surface layer. These are the true Antarctic species. At greater 

 depths we find species which are common to the deep waters of the Antarctic and the 

 tropics, but with these we are not concerned. 



