226 DISCOVERY REPORTS 



coastal zone also narrowed again to the north, and off Mowe Point the general tendency of diminishing 

 quantity of microplankton with increasing distance from land was somewhat obscured by the presence 

 of relatively poor isolated catches within the coastal region. The hydrological data and qualitative 

 plankton observations combine to show that these were due to actual tongue-like intrusions or isolated 

 patches of more oceanic surface-water (see pp. 157, 246), with inherently poorer plankton content, 

 and not merely to local impoverishment of the coastal water population. It would seem that the Mowe 

 Point line was near the northern limit of the main region of upwelling at the time of this survey, 

 and that some inter-digitation of the distinctive types of surface-water (and even some overlapping of 

 these in the vertical plane) was the result. Such effects, well known to occur on a large scale near the 



WS970 WS969 WS9b8 WS967 WS9M> WS9bS «VS9M 



'S970 WS969 



III I III 



a 



(*#•-' 



METAZOA 



""protozoa" 



NO SESTON RECORDED 



SEA MILES FROM LAND 



SEA MILES FROM LAND 



Fig. 58. Distribution of main groups of microplankton, 

 estimated totals per net haul. Mowe Point line, survey I, 

 4-5 March 1950. The histogram shows the settled volumes 

 of the catch in millimetres. 



Fig. 59. Distribution of main groups of microplankton, 

 estimated totals per net haul, survey I, Northern Inter- 

 mediate line, 5 March 1950. 



northern limits of the analogous Peru coastal current, must necessarily affect any simplified presenta- 

 tion of data collected with an arbitrarily chosen spatial limit, and plotted on the horizontal plane only, 

 as here. 



Inshore on the Sylvia Hill line the two innermost stations show, as did the stations inshore on the 

 Walvis Bay and mid-intermediate lines, the heaviest concentrations of phytoplankton. According to 

 the hydrological evidence (p. 162) these occur in old and rather mixed upwelled water. The two 

 outermost stations on the Sylvia Hill line present a strong contrast. The phytoplankton, in quantity 

 more characteristic of oceanic water, consisted of a rather neutral mixture of coastal and oceanic 

 species. The hydrological evidence we have seen tends to link these two stations with more purely 

 coastal water farther to the south, but the relatively high salinities and the salinity section (Fig. 15) in 

 particular, suggest that they may lie in rather mixed oceanic water, into which some very recent 

 upwelling was introducing a pattern of vertical layering. 



One other distributional feature well shown in Fig. 57 — the extreme poverty of the microplankton 

 offshore in the south of the area, even at moderate distances from land — is much more difficult to 

 understand. The point is reconsidered later when the results from the Orange river line are described 

 in detail. 



