223 



MICROPLANKTON 



This table provides most of the basis for selection of the relatively more important forms, without 

 which detailed consideration of the vast mass of data on distribution and relative abundance could 

 not usefully be attempted. It seemed best to insert it here, following immediately upon the full list 

 from which it has been derived, although the further consideration of individual species — distribu- 

 tions, etc. in which it has been utilized is deferred until after the description of main group distribution. 



Tables 12 and 13 suffice to bring out one important difference between the rich diatom-flora of this 

 typical subtropical upwelling region, and that of other sea areas where diatoms predominate, such as 

 the antarctic zone of the southern ocean. The colder water flora and that of the Benguela current have 

 alike been described as ' monotonous ' (in a restricted sense) since the one class of algae predominates 

 in both of them. The colder water flora, however, tends to be characterized by a more extreme 

 monotony, in that one or two species are frequently found to predominate over all the others to a very 

 marked degree, over wide areas and for long periods of time (Hart, 1934; Marumo, 1953). The 

 Benguela current samples are very much more varied, as shown by the large number of species 

 recorded as dominants on the criterion chosen here. Even if we apply the criterion used by the 

 Japanese — least number of species needed to attain 50% of the total cell count — the diversity of the 

 Benguela samples would be more than twice as great as that of many antarctic ones, at least five or 

 six species being needed to attain this proportion in most of the catches. Indeed, when we consider 

 the occasional predominance of dinoflagellates, and of Trichodesmium at some of the poorer stations, 

 and the fact that such nannoplankton forms as Coccosphaeriales and small dinoflagellates would 

 certainly bulk far more largely had it been practicable to use other sampling methods, it seems very 

 doubtful whether this plankton should be regarded as ' monotonous ', even in the restricted sense. 



Table 13. Abridged frequency data: Categories occurring as dominants at one or more of 



the thirty-nine repeated stations 



