REVIEW OF THE MAIN FEATURES OF THE BENGUELA CURRENT 279 



of Walvis Bay. Here the surface isotherms show a tongue of warm oceanic water pressing towards 

 the coast, and influencing the region as far south as the Sylvia Hill line of stations (25 S.). The water 

 masses on the landward side of this tongue are a mixture (Fig. 33 c, d), evidently derived from oceanic 

 and coastal water types. The penetration of this influence to 25 S. creates in that latitude a reversal 

 of the normal conditions, and there we find the apparent anomaly of warmer and more saline water 

 lying along the coast, with the cooler and less saline water at the offshore stations ( WS 986 and 987). 



To the north of this wedge an extensive layer of oceanic water extended over the current, typically 

 populated with a flora in which Planktoniella sol was a conspicuous member. In the southern part of 

 the wedge, however, the flora was typically that of upwelled water, and inshore on the Sylvia Hill line 

 (25 S.) and Walvis Bay line (23 S.) the largest quantities of chaetocerids encountered on the surveys 

 were found. 



It can be seen from Figs. 57 and 65 (pp. 225 and 231) that the highest concentrations of phyto- 

 plankton extended over a considerably larger area in survey I than in survey II. This was probably 

 associated with the greater age of the upwelled water, but it may be that the greater stability of the 

 upper water layers also enhanced its growth. In the offshore, unmixed oceanic water, the converse 

 was true, and the phytoplankton there on the average was in fact four or five times less plentiful than 

 on survey II. This may be associated with the more pronounced divergence of subsurface water which 

 Occurred seawards of the boundary on the second survey. 



This great standing crop of phytoplankton, supporting a correspondingly large crop of zooplankton, 

 must eventually sink down and die, and the nature of the bottom deposits is sufficient evidence that 

 a very large part of the sinking and death takes place in the coastal region. The conditions prevailing 

 at the time of the first survey would certainly have been conducive to this. The meteorological records 

 show that there was little wind, particularly in the vicinity of Walvis Bay, and the currents appear to 

 have been equally sluggish. Indeed, while the ship steamed across these waters, masses of moribund 

 organisms, both phytoplankton and zooplankton, were encountered. 



All this organic material sinking to the sea-floor decomposes fairly rapidly, and in so doing will both 

 liberate nutrients and consume oxygen. That nutrients are released in some quantity is strongly 

 suggested by the phosphate sections (Figs. 50-56) which show concentrations of phosphate in the 

 waters of the continental shelf which exceed those in the water upwelling from greater depths. This 

 local nutrient regeneration must play an important part in supplying the dense growth of plants with 

 sufficient nourishment. 



The removal of oxygen by decomposition has, however, less beneficial effects. So complete is this 

 process that the water overlying the sea-bed of the continental shelf becomes completely anaerobic, 

 and permits the growth of sulphate-reducing bacteria in the sediment. These bacteria, liberating 

 hydrogen sulphide, enhance the effects of decomposition, and the hydrogen sulphide further reacts 

 with the dissolved oxygen in the overlying waters. 



We have already noted that the upwelling water originates from a layer of low dissolved oxygen 

 concentration, and with the further removal of oxygen on the continental shelf it becomes still further 

 depleted. In the normal conditions of flow of the current it does not become seriously reduced, as we 

 can see from the oxygen sections of survey II (Figs. 41-44). 



Presumably there is at these times a sufficient flow of water over the region to prevent stagnation. 

 In abnormal conditions, however, the cessation of upwelling, and influx of warmer water on the 

 surface, stopping or even reversing the current, has disastrous effects. These are demonstrated at 

 Walvis Bay, on survey I, where the computed currents (Fig. 34) show a southerly flow in the wedge 

 of oceanic water penetrating into the region, and where the section of dissolved oxygen (Fig. 40) shows 

 that the oxygen depletion actually extended to the sea-surface inshore. 



