MICROPLANKTON 229 



Seston was recorded at the two outer stations, the first time it had been observed (in the very small 

 fractions used to obtain microplankton counts) during this first survey. 



The Sylvia Hill line (Fig. 62) showed extreme contrast between the two diatom-rich inshore 

 stations, and the impoverished offshore stations, where Dinophyceae were almost as numerous as the 

 diatoms. 



Again, the southern intermediate line (Fig. 63) showed very similar conditions; a heavy diatom-rich' 

 microplankton inshore; while offshore, though all the other groups showed some slight falling off in 

 numbers from the third station seaward (where diatoms still predominated) the relative importance 

 of those other groups was much greater in the smaller catches. 



Finally, on the Orange river line (Fig. 64) the rich diatom plankton of the two inshore stations 

 contrasted strongly with extremely small quantities of phytoplankton offshore, though moderate 

 numbers of Protozoa, Metazoa and considerable quantities of Seston were recorded there. The capture 

 of large numbers of herbivorous zooplankton especially the Pteropod Limacina bulimoides at these 

 offshore stations, and the large quantity of Seston present, suggested that recent intensive grazing may 

 have reduced a richer phytoplankton offshore not long before the stations were worked ; because the 

 greater width of the continental shelf at this point would lead one to expect greater seaward extension 

 of the rich inshore conditions here at most times. The zooplankton data derive from the sorting of 

 -material from nets of coarser mesh, so that this point cannot be indicated in the uniform series of 

 diagrams. 



Second survey 

 Primary data concerning groups totals, relative percentages, etc., for the microplankton samples 

 obtained during the second survey are given in Table 15. This has been constructed so that the 

 sequence of stations, and of the lines of stations, follows that of the corresponding observations made 

 during the first survey, regardless of their chronological order (cf. Table 14). 



The total microplankton estimates for the second survey, contoured logarithmically in Fig. 65 

 (corresponding to Fig. 57 for the first survey) show an underlying similarity of distribution, consistent 

 with the idea that upwelling activity proceeds (doubtless with minor fluctuations in intensity) through- 

 out most of the year, over the major part of the area. Except in the extreme south, where the observa- 

 tions on the Orange river line are anomalous, rich catches inshore with rapid decrease in quantity as 

 one proceeded seawards was the general rule. The heaviest catches of all were again obtained off 

 Sylvia Hill and Luderitz Bay, and yet again an incursion of offshore water with contrastingly poor 

 plankton was evident immediately seawards of the richest area, suggesting recurrence of the swirl 

 centred (perhaps) to the south of the one encountered here during the first survey. A tongue-like 

 intrusion of sparsely populated oceanic water in the north, on the Mowe Point line, was even more 

 clearly defined than during the first survey, owing to the working of three extra stations to the north 

 of the repeated series. 



With all these points of resemblance to the distribution observed during the first survey there are 

 still some differences, showing that even within a current-system apparently so persistent throughout 

 most of the year, some seasonal changes affect the plankton population. Thus during the second 

 survey the region of rich coastal plankton was smaller and did not extend so far seaward, especially 

 off Walvis Bay. Conversely, the offshore microplankton, though still much more scanty than that to 

 be found inshore, was generally somewhat richer than it had been during the first survey, especially 

 to the north. Indeed, on the Mowe Point line (Fig. 66) only the station farthest inshore was appreci- 

 ably richer in diatoms than those near the seaward end of the line. ' Other plants ' were recorded only 

 near the apparent transition point between the rapidly dwindling coastal plankton and the seasonally 



14-2 



