THE SEASONAL CIRCULATION OF THE 

 ANTARCTIC MACROPLANKTON 



By N. A. Mackintosh, D.Sc. 

 (Text-figs. 1-9) 



INTRODUCTION 



The plankton, by its definition, is unable to control its distribution by actively 

 swimming against the currents. Some of the larger organisms might be said to 

 have some power of purposeful locomotion in a horizontal direction, but none can 

 compete with the ocean currents and they must all eventually be carried in the direction 

 of movement of the water mass which surrounds them. In the oceans of the world 

 however we find different species always characterizing different latitudes or regions and 

 inhabiting different depths, and these species keep within the normal limits of their 

 distribution in spite of the fact that the water in which they live is constantly changing — 

 drifting away to warmer or colder regions, sinking from the surface or welling up from 

 the depths. Damas (1905) pointed out that there must be some mechanism which 

 allows the characteristic fauna and flora of oceanic regions to persist. His investigations 

 were concerned with a collection of Copepods from the region between Norway and 

 Iceland, and he suggested that here the organisms were carried away in a circulating 

 current, moving anti-clockwise round the Norwegian Sea, which would bring a certain 

 proportion of the stock back to the environment in which it is able to breed. Helland- 

 Hansen and Nansen (1909) published a detailed account of the hydrology of the Nor- 

 wegian Sea and refer (on pp. 312-16) to Damas's hypothesis of plankton circulation. 

 They suggest that where certain species are found to be abundant in certain areas they 

 are in a way stationary there, and that the water masses are also more or less stationary, 

 or are renewed comparatively slowly. They found that Damas's areas of abundant 

 Copepods coincided with regions in which a great part of the water probably remains for 

 a long time, exposed only to small circulatory movements in various directions. The 

 problem was again taken up by Somme (1934) who found that Calanus finmarchicus and 

 C. hyperboreus migrated into deep water during the winter in the Lofoten area, and rose 

 towards the surface in spring. He suggested that this migration was a means by which 

 the species avoided the scattering effect of the swiftly moving surface currents. 



In the present paper it will be shown that certain species of the Antarctic macro- 

 plankton, which are usually regarded as inhabitants of the surface layers, make an 

 annual vertical migration into surprisingly deep water in winter, and that this migration, 

 by which they move from one current system to another, is probably the means by 

 which they keep within their normal geographical boundaries. A preliminary account of 

 this migration was published in Nature, October 19th, 1935. 



