MICROPLANKTON 243 



concentrations of phytoplankton were found at the stations nearest to the coast, and there was a rapid 

 decrease seawards before even the shelf-edge was reached. On the occasions where the stations 

 extended far enough from the coast, a secondary increase in the phytoplankton is evident — the 

 abundance beyond the shelf-edge was greater than on the first survey. The population in this secondary 

 increase included some oceanic forms among its dominants and it seems probable that it was associated 

 with the divergence of oceanic water beyond the shelf-edge, which was postulated in the interpretation 

 of the mechanism of upwelling (p. 188, Fig. 37). 



Turning to conditions observed on the Sylvia Hill line itself during the second survey (Fig. 79) it 

 can be seen that although the falling-off seawards was pronounced, diatom members were still very 



, WS075 WSI077 



ro 



W^I078 



WS079 



WSO8I 



WS 1080 



~W|i074 WSI073 WSI07 2 WSKD7I 



■ • TOTAL DIATOMS 



. « CHAETOCERACEAE 



, , PENNATAE" 



..„„„,.„ BIEXXJLPHIINEAE 

 o- o DISCINEAE 

 SOLENIINEAE 



SO IOO 



SEA MILES FROM LAND 



Fig. 78. Estimated total diatoms and diatom group totals, survey II. A. Walvis Bay line, 29 September-2 October 



1950. B. Middle Intermediate line, 28 September 1950. 



high at the second station seawards, with coastal chaetocerids and ' Pennatae ' still strongly dominant. 

 While the neritic Biddulphiineae were observed at the inshore station only, as was to be expected, 

 members of the other two groups persisted right to the outer end of this comparatively short line, and 

 it was only at the outermost station that the oceanic solenoids became equally important. This 

 distribution resembled that seen on the mid-intermediate line (that next to the N. in the series) during 

 the first survey. 



On the southern intermediate line, the falling-off in quantity from the exceptionally rich inshore 

 station was steep, but the succeeding stations seawards were still quite rich and showed a typical 

 coastal flora extending fully 50 sea-miles from the coast. It seems probable that here again the inshore 

 stations were characteristic of the active upwelling shown on the Sylvia Hill line. The sharp fall in 

 abundance from station WS 1060 to station WS 1059, and the persistence at the latter station and 

 those towards the seaward end of the line of a scanty oceanic type of plankton, was correlated with the 

 pronounced intrusion of oceanic water to the north of the Orange river line, illustrated by the surface- 

 temperature pattern (Fig. 76). 



The detailed figures for the southern intermediate line in spring show some minor anomalies at 

 group level that require comment. At the outer oceanic station Discineae and 'Pennatae', both 



16 



