microplankton 263 



The cosmopolitan distribution of marine plankton diatoms and the 

 'ecological' characterization' of the more important species from 



the benguela current 



A further selection of the most important or typical diatom species observed within the area permits 

 concise illustration of the extremely cosmopolitan distribution of these in other regions, so that it is 

 the relative importance of the various forms rather than their mere presence and absence that must 

 be studied before relationships between the floras and their 'conditions of existence' begin to be 

 perceptible. This further selection has been used to give examples of the brief 'ecological charac- 

 terizations ' of species, on Gran's lines, that are so helpful in the attempt to perceive some order in the 

 ever-changing phytoplankton communities. 



Table 28 shows reported occurrences of these typical Benguela current species in various other 

 regions, ranging from the North Atlantic to the antarctic zone of the southern ocean. Some of these 

 have received much less complete coverage than others, and yet other regions might be added, 

 especially around Japan (cf. Aikawa, 1936), but the table is surely adequate to show the very cosmo- 

 politan distribution of most of the species. This fact was generally recognized by most of the earlier 

 phytoplankton workers (cf. Gran and Braarud, 1935, and Aikawa 1936, among others), but it probably 

 became so obvious to them that they rarely sought to stress it. The species listed in Table 28 are 

 selected on the ground that we have observed them to be dominant, typical or otherwise important in 

 the Benguela current, and without reference to their occurrence elsewhere. But there is no reason to 

 suppose that the less important or typical species — except perhaps the neritic ones (see below) — are 

 any more restricted in their distribution. At least it can be said that of all the eighty-two species 

 identified from the Benguela area so far, none are ' new to science '. 



The supreme importance of the inshore chaetocerids (of the group Hyalochaete) in the upwelling 

 regions, can only be fully appreciated when quantitative data are considered, but is still apparent here 

 in the relatively large number of species recorded. It can also be seen that the holoplanktonic, mainly 

 oceanic solenoids are perhaps the most completely cosmopolitan of all plankton diatoms, though 

 panthalassic 'reversionary plankton forms' such as Nitzschia seriata are almost equally widely 

 distributed. 



The few important species that seem to be limited to the Benguela current area and adjacent South 

 African coasts are all inshore ones: Chaetoceros strictum and C. tetras, Fragilaria granulata and 

 F. karsteni. Certain other coastal species are recorded from comparatively few of the other regions 

 mentioned, but since these include major upwelling regions their very widespread distribution is in 

 no doubt. Further sampling will almost certainly reveal their presence in the less favourable inter- 

 vening areas eventually, though probably in such small numbers as to form but an insignificant pro- 

 portion of the sparse populations to be found there. These forms include C. costatum, recorded else- 

 where from the coast of southern Europe and North Africa, California and Japan ; C. psendocrinitum, 

 S. temperate coasts of Europe and California, and C. van heurckii, recorded only from California and 

 the Madras coast of India among the other localities considered. Nitzschia longissima also has a com- 

 paratively restricted occurrence, so far as it is yet known, being recorded only from the other African 

 localities, California and Japan. 



Proceeding to brief ' ecological characterizations ' of the several species on the basis of our observa- 

 tions in the Benguela current area, it is necessary to make the proviso that maximal occurrence during 

 spring or autumn may indicate either a normal seasonal effect or merely that these species tend to 

 flourish earlier or later in a succession that quite probably repeats itself several times in the course of 

 a year, as the more or less persistent upwelling system waxes and wanes. 



