278 DISCOVERY REPORTS 



In the initial stages of upwelling the lighter surface-water must first be affected and removed away 

 from the coast. To replace it, cooler and denser water must be raised from subsurface depths. This 

 in turn will, however, be transported offshore, and where it meets the lighter surface-waters a relatively 

 strong convection is set up. For example, on the Orange river line of stations the offshore waters are 

 sharply separated from the cooler and denser upwelled waters by a pronounced convection cell 



(Fig- 37)- 



Evidently this sharp demarcation is present to a greater or lesser extent along the westward edge 

 of the upwelling waters. The rapid alterations in temperature as this boundary was crossed (Fig. 9) 

 throughout the region bear ample testimony to this, and so it appears that although no geographical 

 boundary is present, the upwelled waters are in fact separated sharply from the warmer oceanic 

 surface-waters lying to the west. 



The upwelled waters have a lower temperature and salinity than the offshore surface-waters, and 

 if one compares their T-S relationships with those at the offshore stations, it can be seen that they 

 correspond to those at subsurface depths offshore (Fig. 330) and while they undergo little or no 

 mixing in the process of uplift, they are subjected to some degree of heating when they reach the 

 surface. From this comparison we can determine the depth from which the upwelled water originates 

 and this is seen to be in the layer between 200 and 300 m. 



At this depth there is evidence of a subsurface current flowing southwards along the edge of the 

 continental shelf, which we have termed the 'compensation current', and which appears to be the 

 replacement source for water which is upwelled. In this respect it is interesting to note that the 

 compensation current decreases in width in its southward journey (Fig. 34) as would be expected in 

 a replacement flow. 



In the waters of the compensation current the content of dissolved oxygen is extremely low and this 

 can be attributed primarily to the source of the water in the oxygen minimum layer which extends 

 over the tropical South Atlantic. Within the compensation current there is a gradual increase in 

 oxygen content from north to south, probably on account of lateral mixing with the adjacent well- 

 oxygenated oceanic waters. The significance of this low oxygen content will be the more evident in 

 the next section of this paper (p. 279). 



The waters of the compensation current, and those which are upwelled, have a relatively high 

 content of dissolved inorganic phosphate phosphorus, and the entry of large supplies of this (and 

 probably other) nutrient into the euphotic zone is of major importance to the biology of the region. 



We have shown that in the coastal waters a large standing crop of phytoplankton was present. The 

 main concentrations were found along the coast in the vicinity of the inshore end of the Sylvia Hill line 

 (25 S.), where the numbers were made up by large quantities of Chaetoceros sp. and Fragillaria 

 karsteni (at the station closest to the coast). In the warmer more saline oceanic waters, several species 

 of solenoids, Thallasiothrix longissima, Fragillaria karsteni and Planktoniella sol were prominent in the 

 phytoplankton, while in the vicinity of the boundary region the sparse flora contained elements of 

 both the coastal and oceanic floras. 



The results of survey II present us, therefore, with a picture of an actively upwelling region, 

 extending in a belt some 80 miles wide along 1000 miles of the western coast of South Africa, an area 

 within which the abundance of flora far exceeds that of the ocean to the west. 



Abnormal conditions 

 On survey I in the autumn (March) we see similar general features, that is, cooler and less saline water 

 along the coast, the eddy-like formation of the surface isotherms (Fig. ja), etc., but in the more 

 detailed features there is a marked contrast with survey II. This is most clearly seen in the vicinity 



