250 



DISCOVERY REPORTS 



offshore flora proved richer at that season than it had been during autumn. Though essentially holo- 

 planktonic, the highly cosmopolitan species concerned easily attain greater numbers nearer land at 

 other times and in other areas, although rarely achieving dominance there. Fig. 94 shows the solenoids 

 as a percentage of total diatoms during the second (spring) survey only. The individual species mainly 

 concerned showed a fairly definite sequence as the survey proceeded from south to north. To the 

 south, far out off Luderitz Bay, Rhizosolenia alata was dominant. Proceeding northwards this was 



2C5- 



25° 



S 

 O 



u 



T -| 



H 



3Cf- 



F. KARSTENI I 1 



NIL I I 



IO-IO j } 



^ORANGE 

 R. 



SURVEY 1 



IO° 15° EAST 



Fig. 91. Distribution of Fragilaria karsteni and F. granulata, 

 survey II, September-October 1950. (Station numbers are 

 shown in Fig. 2.) 



NIL □ 



Fig. 92. Distribution of Thalassiothrix longissima, survey I, 

 March 1950. Expressed as a percentage of the total diatoms. 

 (Station numbers are shown in Fig. 1.) 



succeeded by R. styliformis, more or less co-dominant with varying proportions of R. alata, these last 

 being superseded in the extreme north of the area studied by Dactyliosolen mediterraneus. 



In Fig. 95 an attempt has been made to demonstrate another aspect of distributional study, the 

 development of a localized flora where the distinctive offshore and inshore populations most nearly 

 approached each other, near the northern limits of the rich upwelling zone in autumn. The distribu- 

 tions shown are estimated numbers of Planktoniella sol, regarded as the characteristic offshore form ; 

 Goniaulax spinifera, the dinoflagellate predominating in the intervening ' patch ' j 1 and the estimated 



1 The word ' patch ' is here used in its generally accepted sense to describe the area where Goniaulax predominated. It is 

 not intended to denote exceptional population density as in descriptions of visibly discoloured areas resulting from water- 



