MICROPLANKTON 237 



Thalassiothrix longissima at the seaward end of the line (one of the few details in which this series 

 approached a norm for the area as a whole!). 



On the Walvis Bay line (Fig. 74) the estimated diatom totals reached high values at the three inshore 

 stations, with a sharp decline beyond the shelf-edge, levelling out to moderate amounts at the two 

 oceanic offshore stations. The high inshore values proved typical of the main upwelling region on 

 most of the lines worked throughout both surveys, as can be seen from the subsequent diagrams. 



Discineae were present in fair numbers inshore, represented mainly by Stephanopyxis turns and 

 Thalassiosira subtilis, the one definitely a neritic species, the other panthalassic. Offshore the numbers 

 declined, at first in parallel with the estimated diatom totals, levelling at the two outer stations to give 

 a slight secondary increase in relative importance. This was due mainly to the species Thalassiosira 



WS98I 



TOTAL DIATOMS 

 CHAETOCERACEAE 

 - -' 'PENNATAE" 

 .„„„„... BIDDULPHIINEAE 

 o DISCINEAE 

 -■ SOLENIINEAE 



n ' ' ' lio ' ' ' "" 



SEA MILES FROM LAND 



Fig. 74. Estimated total diatoms, and diatom group totals, survey I. A. Walvis Bay line, 6-8 March 1950. 



B. Middle Intermediate line, 9-10 March 1950. 



excentrica, with lesser numbers of Planktoniella sol. Biddulphiineae were present inshore and of con- 

 siderable importance at station WS 980, but were not observed seawards of the peak station WS 979. 

 The group was represented here entirely by Eucampia soodiacus, as on the Mowe Point line. 



Chaetoceros was by far the most important group over the Walvis Bay line as a whole. It was 

 strongly dominant at the two richest inshore stations, where a profusion of neritic species were present, 

 and at the first of the poorer stations, seawards. There, however, the single ' relatively oceanic ' species 

 Chaetoceros convolatum alone accounted for most of the group total. Still further offshore the group 

 dwindled rapidly, both in estimated numbers and in relative importance. 



Soleniineae were present at all stations on the line in consistently small numbers, becoming 

 relatively important in the poorest offshore plankton. Cosmopolitan Rhizosolenia spp. predominated 

 throughout, accompanied at one inshore station, where the group total was highest, by the mainly 

 neritic Dactyliosolen mediterraneus. 



' Pennatae ' were an important group on this line, especially at the innermost and outermost stations 



15-2 



