T 



THE CIRCUMPOLAR CONTINUITY OF 

 ANTARCTIC PLANKTON SPECIES 



By A. de C. Baker 



National Institute of Oceanography 

 (Text-figs. 1-5) 



INTRODUCTION 



he Southern Ocean is a continuous circumpolar belt, the greater part of which is drifting eastwards 

 under the influence of the prevailing westerly winds. It is clear from the widespread soundings 

 taken by many ships that there is an uninterrupted belt of deep water, and Deacon (1937), Clowes 

 (1938) and Mackintosh (1946) have shown that there is a circumpolar continuity in the relative 

 positions and movements of the main water masses, 1 in the distribution of nutrient salts and in the 

 surface isotherms. As a general rule the physical features are arranged in zones (with local modifica- 

 tions and distortions) in which uniform conditions persist in east and west directions and changes or 

 gradients occur from north to south. Since the distribution of pelagic species may be expected to 

 extend as far as the environment is uniform, it would be surprising if the distribution of planktonic 

 species were not also circumpolar. 



Those who have examined collections of plankton from different parts of the Southern Ocean have 

 observed that the familiar species can indeed be looked for in any longitude at least in the Antarctic 

 zone, and the circumpolar distribution of the plankton is generally taken for granted. This, however, 

 is a far-reaching assumption, and if it is true it may sometimes allow general conclusions based on 

 samples from one sector of the Antarctic to be applied to other sectors. Although much diffuse 

 evidence of circumpolarity may be found in the reports of various expeditions which have collected 

 material during the past sixty years or so, it appears that no evidence has yet been assembled to 

 show that circumpolarity is a general character of the distribution of the plankton of high southern 

 latitudes. 



The present paper sets out to show that at least the common species of the macroplankton animals 

 and of the phytoplankton have a circumpolar distribution in the Antarctic zone, and that this circum- 

 polar distribution is continuous in so far as, with sufficient sampling, they can be found at one time 

 or another in all longitudes. It goes little further than this, but the point is considered to be of sufficient 

 basic importance to form the subject of a separate paper. 



MATERIAL AND METHODS 



The material considered here includes the most numerous Antarctic phytoplankton species, and the 

 commoner animals of the macroplankton in the Antarctic surface layer. Material could also be found 

 in collections and publications for examining the circumpolar distribution of the smaller planktonic 

 animals, those of the deeper layers, and the mammals, oceanic birds, fishes, and benthos; but such 

 a comprehensive investigation would involve more complex problems, and the work entailed would 

 at this stage be out of proportion to the particular question at issue. 



1 The Antarctic Bottom Water might be regarded as an exception. Although it can be traced all the way round, its potential 

 temperature shows a gradient from west to east (Deacon, 1937, pi. XLiv) so that it does not have uniform circumpolar 

 properties. 



