MICROPLANKTON 267 



oceanic' here, though they occasionally occurred in the same samples). Temperature is probably the 

 factor involved, having regard to the distributional records of the two forms elsewhere (cf. p. 217). 

 Similarly Corethron criophilum and even the well-known Rhizosolenia styliformis both tended to 

 display a more panthalassic trend than their usual designation as oceanic forms would lead one to 

 expect. In support we may cite the abundant occurrence of Corethron (in some of its phases) close in 

 to the land as well as offshore in the antarctic zone of the southern ocean, and the occurrence of almost 

 pure communities of Rhizosolenia styliformis in the shallow waters of the North Sea (Hardy, 1923; 

 Wimpenny, 1936). 



Concerning Dactyliosolen mediterraneas previous opinions were 'neritic' (Lebour) and 'neritic, 

 sporadically oceanic'. K. R. Gaarder (1951) found moderate numbers of it in samples from all over 

 the area covered by the 'Michael Sars' to the west and south of the British Isles. In the Benguela 

 material the species was locally abundant offshore in spring and inshore in autumn, suggesting that it 

 thrives only within narrow limits of temperature (stenothermal, with optimum range about 15 to 

 1 9 C). Certainly 'panthalassic' would seem to be the best brief description of its general distribu- 

 tional trend. 



Finally, two of the most abundant 'pennatae', Nitzschia delicatissima and N. seriata, previous 

 ecological descriptions of which range from 'perhaps oceanic, but often abundant inshore', to 

 definitely 'neritic', would in the opinion of the writer (TJ.H.) be better described as panthalassic— 

 expressing the duality of the best of the earlier descriptions in one word. They were numerous in both 

 inshore and offshore samples in the area studied here, and also in antarctic seas (Hart, 1934, 1942). 

 These minor differences from previous brief ecological descriptions of the species are so few that 

 it is the general concordance with the opinions of those with the greatest experience of working 

 through large collections of material that is striking, encouraging the belief that in this direction at 

 least plankton workers grope towards 'the truth' to some purpose. Although Gran's 'system of 

 plankton elements' must necessarily be modified as data from the less-known sea-areas becomes 

 available, the ideas implicit therein remain one of our greatest aids in the attempt to understand the 

 relations between phytoplankton communities and their 'conditions of existence'. 



This comparison of the diatom-flora of the Benguela current with those of other regions brings out 

 one important point, implicit in much that has been published already, but little emphasized by the 

 earlier authors: that there is a fundamental difference between the diatoms and, for example, the 

 species of oceanic zooplankton, in the nature of their distribution. 



Although there are some cosmopolitan species of zooplankton, the range of the majority is subject 

 to some limit beyond which they are unknown. Where there is a region offering an environment 

 similar to that occupied by such a species, but isolated from it, we normally find there a similar species, 

 but a distinct one. Most of the plankton diatom species, however, excepting certain neritic, polar, or 

 markedly stenothermal warm-water forms, are not limited in their dispersal by ' barriers ' to anything 

 like the same extent as the zooplankton. Many of the more cosmopolitan diatoms may turn up almost 

 anywhere in the oceans of the world, and become locally important or even dominant, wherever the 

 'conditions of existence' best suit them. 



It is for this reason that the plankton floras of adjacent water masses must be studied with more 

 regard to the differing proportions of the various species within them, than to the presence or absence 

 of particular forms. 



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