374 DISCOVERY REPORTS 



water varies considerably in different parts of the Antarctic. In some places the result- 

 ant direction of the transport of water is north-east, in others east, and sometimes it may 

 for a time be even a little south of east. In general it may be said that the easterly com- 

 ponent is considerably stronger than the northerly. 



If an organism were floating quite passively in the Antarctic surface water it would 

 eventually be transported northwards to the Antarctic convergence. Here it would be 

 carried down below the sub-Antarctic surface water and, unless it was caught up in a 

 returning current, would continue to move northwards in the increasingly warm 

 Antarctic intermediate layer, finally reaching a foreign environment in which it could 

 not possibly survive. Thus if the whole plankton population floated passively in the 



70° SOUTH 65° 60° SS° 



I , i i i | i i i i | i t ■ i | 



ANTARCTIC 

 CONVERGENCE 



Om- 



A. 



ANTARCTIC SURFACE WATER V 



250- ~~ "" -- •' 



50D- *— 



\t 



WARM DEEP WATER N^ 



750- 



\ 



1000- 



Fig. 2. Water movements in 8o° W. 



Antarctic surface layer it would soon disappear. There must therefore be some form of 

 circulation by which the organisms or their offspring are able to find their way back to 

 the southern limits of the zone they inhabit. The means by which this circulation is 

 effected is a problem of fundamental importance, for it is probably the principal factor 

 in the distribution of species and quantities of plankton. 



Although they must drift with the horizontal currents the plankton organisms are 

 able to transfer themselves from one layer of water to another, either by swimming 

 actively in a vertical direction or perhaps by altering their specific gravity. Hardy and 

 Gunther (1935, pp. 311 et seq.) discuss at considerable length the possibility that these 

 organisms can ' navigate ' themselves by making use of superimposed currents travelling 

 in different directions and at different speeds. Most organisms for instance undergo 

 daily vertical migrations, rising towards the surface at night and sinking into deeper 

 water during the day, and in some cases at least these migrations are sufficiently ex- 

 tensive to bring the organism from the Antarctic surface layer, which is moving in one 

 direction, into the warm deep water which is moving in another. It is suggested that by 

 varying the range of vertical movement, or the length of time spent in one layer or the 

 other, the organism can in some degree control the ultimate direction in which it drifts. 

 There can be no doubt that the horizontal movements of such migrating organisms are 

 the resultant effects of the two currents, but diurnal migrations will not be a sufficient 

 means of keeping the bulk of the plankton permanently within its normal boundaries. 



