T 



ON THE PHYTOPLANKTON OF THE 



SOUTH-WEST ATLANTIC AND THE 



BELLINGSHAUSEN SEA, 1929-31 



By T. John Hart, b.Sc. 



(Text-figs. 1-84) 

 INTRODUCTION 



HIS paper forms the continuation of Prof. Hardy's pioneer work on the phyto- 

 plankton of South Georgia and gives, in addition, an account of extended work on 

 similar lines over a much wider area, comprising the waters round the South Sandwich 

 Islands, the South Orkneys, in the Bransfield Strait, and in the Weddell and BelUngs- 

 hausen Seas. Mention is also made of work carried out in more northerly waters, 

 particularly of collections showing the transition from Antarctic to sub-Antarctic 

 phytoplankton. 



A knowledge of the times and places where a rich phytoplankton development may 

 be expected, and the factors governing both the seasonal variations in production and 

 the larger fluctuations from year to year, is of obvious importance in considermg the 

 ecological relationships of the various organisms in any given sea area. Such knowledge 

 is in its turn based upon a consideration of the hydrological conditions in the region m 

 question. A clear understanding of the main features of the hydrology of the surface 

 waters over the area dealt with in this paper is therefore essential before the distribution 

 of the phytoplankton can be discussed. Papers now in course of preparation by Mr 

 A. J. Clowes and Mr G. E. R. Deacon deal with this subject,^ and another paper by 

 Mr H. F. P. Herdman on bottom-relief has already appeared.'- This last is of course 

 important in the present connection as showing the regions in which upwellmg and 

 vertical mixing are likely to occur. I am greatly indebted to the above-named members 

 of the hydrological staff for their advice and assistance. 



GENERAL HYDROLOGICAL FEATURES 



The surface waters of the South-west Atlantic may be divided into four main types : 

 Antarctic, sub-Antarctic, sub-tropical and tropical. 



The Antarctic surface water is a well-defined, cold, poorly saline layer from lOO to 

 250 m. in depth, lying over the warmer deep water. It is formed all round the Antarctic 

 Continent and contains comparatively fresh water formed by the melting of the pack-ice 

 in summer. At this season the upper layers become warmed and diluted, so that their 

 stability is greatly increased, and shallow very strongly marked discontinuity layers tend 

 to be set up. The cold nucleus of the layer, however, remains above the warmer deep 



1 Since the above was written, some of Mr Deacon's work has appeared in Discovery Reports, 

 vii, pp. 171-238. This should be consulted for a fuller account of the hydrology. 



2 Discovery Reports, vi, pp. 205-36 (1932). 



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