MICROPLANKTON 239 



ward ends of each series of observations. Chaetocerids were by far the most important group, as can 

 be seen from the way in which their group-totals follow closely in parallel with the figures for total 

 diatoms. Inshore they were represented by the usual rich variety of species, mainly neritic but some 

 with panthalassic tendencies. Offshore there was some apparent overspill of the most abundant 

 inshore forms, but the more definitely oceanic members of the genus, such as Chaetoceros convolution 

 and C. peruvianum, were relatively much more important in the very poor phy toplankton met with there. 



The estimated totals for other groups was also greatest inshore, but the relative importance of those 

 with offshore representatives was much greater near the shelf-edge and beyond. 



The Discineae were represented mainly by Thalassiosira spp. Of these T. hyalinum, T. rotula and 

 the panthalassic T. subtilis were commonest inshore ; T. excentrica and T. subtilis (again !) offshore. The 

 oceanic Planktoniella sol was of some relative importance in one extremely small sample from near the 

 outer end of the southern intermediate line. 



W5I002 WSIOOI WSIOOO WS999 WS998 WS997 



WS996 



-iO 



TOTAL DIATOMS 

 CHAETOCERACEAE 

 "PENNATAE" 

 BIDDULPHIINEAE 



-IO 



SO IOO 



SEA MILES FROM LAND 



Fig. 76. Estimated total diatoms, and diatom group totals, survey I, Orange river line, 12-14 March 1950. 



The Biddulphiineae were almost entirely confined to the inshore stations on both series, though 

 Eucampia zoodiacns was recorded in very small numbers from station WS 986, at the seaward end of 

 the Sylvia Hill line. This same species accounted for most of the inshore totals for the group, as we 

 had found farther north at this season, but here there was a small proportion of Cerataulina pelagica 

 in addition. 



Soleniineae formed only a small proportion of these samples. The higher numbers at three inshore 

 stations were due to the mainly neritic species Dactyliosolen mediterraneus and Leptocylindrus danicus. 

 The very small totals for this group at offshore stations show that it was of some slight relative 

 importance there, but only because of the prevailing poverty of the offshore phytoplankton. The 

 offshore species included Bacteriastrum hyalinum, the more cosmopolitan Rhizosolenia spp. and 

 R. simplex. 



The 'Pennatae' were more heavily outnumbered by the dominant chaetocerids at the inshore 

 stations of these two lines than they had been farther north, but still ranked second among the diatom 

 groups. The dominant inshore species — Fragilaria karsteni, Asterionella japonica and Nitzschia seriata 

 — were the same as those recorded on the two previous lines. Offshore small numbers of Nitzschia 

 dehcatissima, N. seriata and Thalassiothrix longissima accounted for most of the group totals, together 

 with Fragilaria granulata at station WS 994. This species was recorded only at this one station during 

 the first survey, but much more frequently and abundantly during the second, when it showed a con- 

 sistently offshore distribution. The very small totals of ' Pennatae ' recorded at stations WS 986 and 994 

 just suffice to indicate greater relative importance than that of the chaetocerids, so clearly dominant 

 at all the other stations of the Sylvia Hill and southern intermediate lines. 



