INTRODUCTION 5 



given. For a more complete account the reader is advised to consult Deacon (loc. cit.). 

 Also, since we are principally dealing with the Antarctic zone, some remarks on the 

 close connexion between nutrient salts and plankton in this important zone will be made 

 in this introduction. 



All round the Antarctic Continent, Antarctic surface water spreads northwards in a 

 shallow layer until it reaches a position which has been termed the Antarctic convergence ; 

 here it sinks below the surface layer of sub-Antarctic water. In the area of mixing, 



1233 

 386 • / ,| 4S4 



385" .1234 .1325 

 .,_,. OBV KG.* 1 * 25 '** 



'M40 

 1139 



1434. ,337 



'2' 4 .635 «2i»" •"•38 



1429. 

 •6371333. 

 •530 1332*1492 



35°S 



Fig. 2. Positions of stations where observations for phosphate or silicate were made in the South Atlantic 



sector of the Southern Ocean. 



100-200 miles north of the convergence, the Antarctic surface water becomes mixed 

 with warmer water from the subsurface and surface waters of the sub-Antarctic zone 

 and eventually forms part of the Antarctic intermediate current. The layer immediately 

 below the Antarctic surface water is the warm deep water, whose movement has a 

 southerly component. This layer lies at a great depth north of the Antarctic convergence 

 and flows almost horizontally until it approaches the Antarctic zone. It then climbs 

 steeply over a third layer of water, the Antarctic bottom water which itself sinks towards 

 the north, and south of the Antarctic convergence continues its flow towards the south 

 at a much lesser depth. 



North of the Antarctic convergence and south of the subtropical convergence the 



