i 9 2 DISCOVERY REPORTS 



more desirable to look upon the difference between the two surveys as an example of the current in two 

 phases — at survey I a quiescent phase subsequent to upwelling, and at survey II a phase of active 

 upwelling. 



It is very difficult to determine how much the upwelling varies throughout the year. Clearly it is 

 a phenomenon which can occur at all seasons. Schott (1902), Franz (1920) and Bobzin (1922) have 

 all endeavoured to make estimates of the intensity of upwelling at different seasons. 



Schott based his estimate on the extent of the upwelling zone and a comparison of the temperature 

 of the inshore waters with that of the offshore waters at a distance of about 300 km. from the coast, 

 and he agrees with Franz in placing the maximum period of upwelling in August and the minimum 

 in summer. Schott, however, put the minimum in November and Franz in February. Bobzin, on the 

 other hand, derived his estimate from a comparison of the mean monthly and mean yearly temperatures 

 and their mean variation, and this led him to conclude that upwelling reached its maximum in summer 

 (October-December) and its minimum in June. 



Bobzin ascribed the discrepancy between his results and those of Schott and Franz to the effect of 

 solar heating of the upwelled water, which would be at its greatest in summer and at its minimum in 

 winter, thereby masking the true changes of temperature due to upwelling. The criteria used by 

 Bobzin to estimate the upwelling are based on somewhat abstract considerations. Moreover, his 

 conclusions are based on a large number of temperature observations taken at Swakopmund and 

 Luderitz Bay — only two points on a coastline some 1000 miles in length. It seems doubtful, therefore, 

 whether Bobzin's ' Relatives Mass des Auftreibbewegung ' produces any more accurate a picture than 

 derived by Schott and Franz. 



On the assumption that wind is the primary cause of upwelling, then we may consider the winds 

 to be as good a measure as anything else of the frequency of upwelling. In winter the trade winds are 

 slightly stronger than in summer, and one might expect a consequent intensification of the relative 

 current in winter. On the other hand, it is evident from Fig. 5 that the coastal winds reach their 

 greatest upwelling effect in that season. Clearly the crucial point is the relative effect of the trade wind 

 and the coastal wind, but without more detailed observations we can go no further. 



NON-CONSERVATIVE PROPERTIES 



The distribution of dissolved oxygen 

 Since dissolved oxygen is related to the biological process in the sea, its distribution is governed not 

 only by water-movements and interchange with the atmosphere, but also by the varying biological 

 activity in the water masses. 



The oxygen content (Figs. 38-44) usually reaches its maximum concentration in the surface-layers 

 of the sea as a result of atmospheric exchange and the photosynthesis of the phytoplankton in these 

 layers. Beneath the surface-layers the oxygen content decreases towards the sea-bed. Normally in 

 sea-waters the oxygen content remains fairly high in the deeper layers, but off South-west Africa there 

 is a very rapid decrease with depth in the inshore waters, so that even in normal conditions in this 

 region a content of less than 1 ml. 2 /l. is found in the vicinity of the sea-bed on the continental shelf. 

 The sections from survey II (Figs. 41-4) show that the layer of low oxygen slopes upwards towards 

 the surface as the coast is approached. Furthermore, it extends beyond the edge of the continental 

 shelf, producing a layer of minimal oxygen content adjacent to the continental slope, at depths of 

 about 400 m. on survey II. 



On survey I this layer of minimal oxygen is much more pronounced, not only offshore, but also 

 inshore where it extends to the sea-surface at the coastal station WS 981 off Walvis Bay (Fig. 40). 



