MICROPLANKTON 247 



than in autumn (Fig. 85). This feature cannot be explained by the seasonal shift in temperature alone, 

 for the species is common in coastal waters round Cape Province, to the south of the area surveyed, 

 at most seasons, and indeed all round the coasts of South Africa. A stenohaline tendency, such as that 

 which seems to account for a similar distribution of the dinoflagellate Dinophysis tripos, might be 

 partly responsible, since the region from which these species seem to be absent corresponds with the 



20° 



25° . 



O 

 U 

 T 

 H 1 



30<? 



SURVEY:! 



IO° 



20 



25- 



S 

 O 



u 



T _| 



H 



30" 



'o-io 3 ^ 



MOWE POINT 3 4E»™a 



io-io S 



io 4 -io 5i 



WALVIS BAY 



EAST 



10" 



SYLVIA HILL 



ORANGE 

 R. 



SURVEY -II 

 1 



EAST 



Fig. 85. Distribution of Stephanopyxis turris, survey I, March 

 1950. (Station numbers are shown in Fig. 1.) 



Fig. 86. Distribution of Stephanopyxis turris, survey II, 

 September-October 1950. (Station numbers are shown 

 in Fig. 2.) 



area of maximum negative salinity anomaly. The reduction in salinity is but slight, however, and 

 Stephanopyxis shows so much adaptability elsewhere that an excluding effect of the superabundant 

 chaetocerids, operating physically by crowding and/or chemically by noxious external metabolites 

 (cf. Lucas, 1947) seems a more reasonable hypothesis. 



Fig. 87 shows the distribution of Eucampia zoodiacus during the first (autumn) survey, when it 

 seemed to be strictly confined to the rich coastal waters from Luderitz Bay northwards. The only 

 Biddulphioid diatom observed in large quantities in any of these samples, the species was not seen at 

 all in spring; when the group — almost exclusively a coastal one — was represented by forms such as 

 Eucampia cornuta, Biddulphia longicruris, B. regia and Cerataulina pelagica, irregularly distributed and 

 all in very small numbers. Eucampia zoodiacus seems to be a coastal form tending to flourish late in 



