288 DISCOVERY REPORTS 



the literature of the subject over the last few years seem to make this essential. For the same reason 

 full notes are given on recent changes in taxonomy of certain individual species better known under 

 earlier names. 



The diversity of the microplankton is demonstrated by the full list of all microplankton categories 

 recognized during the routine analyses, and tabulation of the records of frequency of occurrence and 

 of dominance. 



The next two subsections deal with the distribution of the main groups of the microplankton, and 

 in particular the distribution of the dominant group, the diatoms; arrayed according to the grouping 

 devised for this purpose. The two surveys are treated separately with the aid of diagrams and tables. 

 This shows that the general pattern of distribution of the more important microplankton species 

 conformed fairly closely to the disposition of the water masses. 



In the rich upwelled coastal waters chaetocerids were dominant, with certain markedly neritic 

 species from the other groups (e.g. Stephanopyxis turris, Encampia zoodiacus, Fragilaria karsteni and 

 Asterionella japonica). The extent and average intensity of the rich inshore phytoplankton were 

 greatest during the first (autumn) survey, though the heaviest individual hauls were obtained in spring. 

 Forms such as Planktoniella sol and Thallassiothrix longissima were typically dominant in the much 

 sparser phytoplankton of the offshore waters, together with certain solenoids during the second survey. 

 The transition zone, which seems nearly to coincide with the hydrologically determined divergence 

 region, usually contained a scanty microplankton consisting mainly of more panthalassic types from 

 either of the main habitats. 



The more extensive coastal production in autumn (survey I) with numerous resting spores of the 

 dominant chaetocerids is thought to represent a late stage in the succession of waters earlier enriched 

 by upwelling, since that process was proceeding but weakly at the time. Probably this is not a purely 

 seasonal effect, for upwelling probably persists, intermittently, throughout most of the year. Hence 

 the succession may be repeated several times annually. During the spring survey (II) the rich coastal 

 area was narrower, and the proportions of the dominant species rather different, with few resting 

 spores. Upwelling activity was great, as shown by marked instability of the surface-layers among other 

 hydrological features. Here an early stage in the probable succession is postulated, some of the 

 enriched water having risen to the euphotic zone too recently for the phytoplankton crop to have taken 

 full advantage of it. Attention is drawn to the analogy with conditions observed in the California 

 current by workers benefiting from more complete seasonal coverage (Sargent and Walker 1948). 



Conversely, the scanty offshore phytoplankton was much richer during the spring survey than it 

 had been during the autumn. Here it seemed that the increased turbulence occasioned by greater 

 wind stress was proving definitely beneficial to plant production. 



In the section on special distributional features, species have been selected whose distribution in the 

 routine hauls seemed best to illustrate either the general distributional trends or divergence from the 

 pattern typical of the group to which they belong. Their distribution is described and charted in detail. 



Observations on visibly discoloured sea-water within the area surveyed confirmed that the dark 

 green to opaque black appearance of large areas near Walvis Bay was due to the dense population of 

 inshore diatoms, as Gilchrist (1914) had previously recorded. Chaetoceros didymum and Asterionella 

 japonica were two of the species mainly responsible. Within this same area more localized brownish 

 discolorations were also seen, and three samples from these showed the development of a definite 

 ' bloom ' of Peridinium triquetrum, with lesser numbers of Prorocentrum micans and diatoms, some- 

 times associated with lanes of salps and ctenophores dying at the surface. A minor fish-mortality had 

 occurred near Walvis Bay just before. Detailed analyses of these samples are given, and their possible 

 significance in relation to mortality phenomena discussed. 



