SUMMARY 2S7 



region of influence of the trade wind, a N.N.W. flow is induced and this forms the eastern limb of the 

 anti-cyclonic circulation of the South Atlantic. Nearer the coast and sharply demarcated from this 

 drift, the currents are variable, and as the oceanographical observations show, are characterized by 

 pronounced vertical movements. We have restricted the use of the name Benguela current to this 

 coastal circulation in which the upwelling movements occur and have referred to the oceanic circulation 

 as the 'south-east trade wind drift'. 



The distribution of temperature and salinity of both surveys has shown that these two currents are 

 characterized by two different types of water — the coastal or upwelled water, and the oceanic water 

 which is warmer and more saline. Varying mixtures of these two types occurred in some areas. 



A comparison of the two surveys showed that while in September-October (survey II) active 

 upwelling was taking place over most of the region surveyed, in March (survey I) the waters were 

 more quiescent and a large part of the coastal region was occupied by an influx of mixed oceanic water. 



The relationship of the water masses in the south-west African region to those in the South 

 Atlantic as a whole is discussed at some length, and it is evident that the cool, less saline coastal waters 

 are derived from the South Atlantic central water at subsurface depths (200-300 m.) offshore. 



A computation of the dynamic height anomalies of the sea-surface relative to the 600 db. surface 

 agrees essentially with the deduced water-movements and emphasizes the eddy-like nature of the 

 coastal circulation. The topography of the 200 db. surface (relative to 600 db.) depends on rather few 

 observations at this depth, but indicates a southerly flow along the edge of and converging with, the 

 continental shelf. It is suggested that this is a replacement flow for the water which is upwelled, and 

 it has been termed the 'compensation current'. 



The mechanism of the upwelling has been examined, and it was found to be essentially similar to 

 the process off California described previously by Sverdrup (1938). Where the upwelled waters of the 

 Benguela current meet the warm oceanic waters a convection cell develops, which forms a sharp 

 boundary, and the position of this, although variable, lies some 40-80 miles offshore, in the vicinity of 

 the edge of the continental shelf and the position of divergence in the surface wind system. 



The distribution of dissolved oxygen in the subsurface ' compensation current ' suggests that the 

 latter is continuous with the oxygen minimum layer in the equatorial parts of the ocean, and may 

 indeed arise from the latter. The impoverishment of oxygen in this water which is evidently the source 

 of the upwelling water, is further accentuated by the decomposition of organic matter on the sea-bed 

 of the continental shelf, and in certain areas anaerobic conditions are created where sulphate-reducing 

 bacteria thrive and liberate hydrogen sulphide into the overlying waters. In March (survey I) the 

 depletion of oxygen extended to the sea-surface inshore at Walvis Bay and we believe this may have 

 contributed to a small mortality of fish which occurred at Walvis Bay during the survey. The associa- 

 tion of this phenomenon with the incursion of mixed oceanic water and calm conditions at Walvis Bay 

 suggested a correlation between the fish mortalities and such conditions, and examination of earlier 

 records of winds, strengthens the correlation between mortalities and calms or northerly winds in this 

 area. 



The considerable amount of decomposition of organic matter taking place on the sea-bed of the 

 continental shelf led to a further enrichment of the already phosphate-rich upwelled water. 



On survey II an attempt was made to determine the total extent of the anaerobic or 'azoic' zone 

 on the sea-floor of the continental shelf. This was found to extend, at least in patches, for a distance 

 of 400 miles along the coast. 



The terminology used in the account of the microplankton and the taxonomy of the main groups 

 met with during the two surveys are discussed at length. Part arbitrary groupings, devised solely to 

 aid presentation of these data, are also described in detail. Drastic changes in the accepted practice in 



