2 4 o DISCOVERY REPORTS 



Although the very great difference in quantity between the rich diatoms inshore and the impoverished 

 phytoplankton population farther out is well shown by these two series of observations, the qualitative 

 distinctions are less clear than they seemed to be on other lines worked during the first survey. This 

 is believed to be due to the wide tolerance of the more panthalassic species of Chaetoceros, Nitzschia 

 and Thalassiosira. Prolongation of the lines seaward would almost certainly have demonstrated their 

 replacement as dominants by more exclusively oceanic species, but this desideratum could not be 

 foreseen in planning the surveys. ' Ideal ' coverage can never be wholly attainable in such work, and 

 had we extended these two series, practical considerations of steaming time, fuel consumption and 

 water supply must have enforced a corresponding curtailment of the rest of the ship's programme. 



The inshore zone of rich diatom phytoplankton was narrower on the Orange river line (Fig. 76) 

 although the shelf is wider there, and diatoms were exceptionally scanty for a considerable distance 

 seaward. Even at the two stations right out beyond the shelf-edge the catches were still very small. 

 Peculiar local conditions must have obtained here, for the rich zooplankton and abundance of Seston 

 indicate heavy grazing of what must have been a very much richer standing crop of plants not long 

 before. Further deviation from the conditions prevailing farther north were indicated by the presence 

 of essentially neritic species along with the oceanic ones at the stations farthest from land. 



Even so, apart from the narrowing of the rich coastal belt and extreme poverty of the outer stations 

 on the shelf, the group distribution of diatoms on this line still shows some of the main features 

 observed farther north. Thus most of the chaetocerids so completely dominant inshore were neritic 

 ones, although the panthalassic species Chaetoceros convohitum was present in greater proportion than 

 was usual inshore elsewhere. The decline in relative importance of this group at the impoverished 

 offshore stations, with correspondingly increased proportions of Discineae, Soleniineae and ' Pennatae ' 

 there, is well shown in Fig. 76. 



Specific data for the groups other than Chaetoceros show that the panthalassic Thalassiosira subtilis 

 and oceanic Planktoniella sol among the Discineae and Thalassiothrix longissima among the ' Pennatae ' 

 were relatively more important offshore as in the other more normal series. 



Second survey 

 Similar treatment of the data from the Mowe Point line on the second survey shows that at that season, 

 although the inshore station was still the richest by far, the diatom totals diminished rapidly as the 

 ship proceeded offshore. A minimum was reached at station WS 1099 near the edge of the shelf and 

 there was then a considerable secondary rise in estimated totals at the three offshore stations (Table 17 

 and Fig. 77). 



Totals for each of the four main diatom groups present 1 varied roughly in parallel with those for 

 total diatoms, but whereas the numbers and the proportion of Chaetoceros and 'Pennatae' were 

 greatest inshore, there were many more Soleniineae present at the outer stations. These outer stations 

 were somewhat less well defined qualitatively than those on some of the more southerly series at group 

 level, since they still contained a moderately high proportion of Discineae and Chaetoceros, both groups 

 containing a majority of inshore species. The more detailed data seem to show up their distinction 

 fairly clearly, though there was evidently more tendency towards offshore transport in the turbulent 

 conditions prevailing than had been observed during the autumn survey. 



Among the Discineae the inshore maximum was due mainly to Stephanopyxis turris and Thalassio- 

 sira spp. Though the secondary rise in numbers of the group at the offshore stations included some 

 of these two categories, Stephanopyxis there formed a much smaller proportion of the totals, Thalas- 



1 Biddulphiineae were not observed on this line. 



