234 DISCOVERY REPORTS 



due to diatoms, dinoflagellates and ciliate Protozoa, but it is quite probable that Trichodesmium may 

 also form blooms within the area sometimes. 



The conditions observed on the Orange river line during the second survey did not conform to the 

 generalizations found to hold good for the rest of the area (Fig. 72). Here a diatom-rich plankton 

 with both oceanic and neritic elements was found at the two offshore stations, and inshore there was 

 an exceptionally poor phytoplankton with a high proportion of Metazoa and Seston. The hydrological 

 data and qualitative plankton observations give some basis for the ' explanation ', necessarily somewhat 

 speculative, of this reversal of the usual pattern of quantitative distribution that has been attempted 

 later (p. 245). 



.VSIOiO WSIOSI WSOS2 WSOS3 WS054 



50 IOO 



SEA MILES FROM LAND 



Fig. 72. Distribution of the main groups of microplankton, estimated totals per net haul, survey II, 



Orange river line, 21-24 September 1950. 



Distribution of the main diatom groups 



First survey 

 The distribution of the main groups of diatoms, distinguished as described on p. 214, is summarized 

 in Table 16. This shows the estimated total diatom numbers, estimated group totals, and relative 

 percentages for each individual station. The stations are arrayed in order of the seven lines of observa- 

 tions as they were worked, from north to south (cf. Table 14). 



As a further aid to description, the estimated group totals have been plotted on a logarithmic scale, 

 in relation to the distance of each station from the coast (Figs. 73-76). In these figures the distance- 

 scale reads seawards from the left of the page for each line, departing from the chronological sequence 

 in order to secure uniformity of treatment in this respect. 



In commenting on the group distribution, reference has been made to qualitative data not given 

 here, because the group totals alone can be misleading where they unavoidably include species of 

 widely differing ecological attributes. This involves some repetition later, the whole difficulty resulting 

 from the impracticability of publishing the raw data in full. 



On the Mowe Point line, the estimated diatom totals were small in comparison with those for the 

 survey as a whole. They were highest inshore and showed a secondary peak at the shelf-edge, 



