MICROPLANKTON 233 



values for Metazoa and Seston throughout, 'other plants' mainly in the transition zone and rather 

 more diatoms, relatively, than had been found at the corresponding positions in autumn. 



The group data for the mid-intermediate line show more profound differences from the conditions 

 found there during the first survey. Then the diatom-rich coastal plankton had extended right out to 

 the seaward end of the line (Figs. 61 and 65). During the second survey (Fig. 69) the rich coastal 

 plankton with diatoms dominant was confined to the inshore station, the other groups were relatively 

 more important throughout than they had been in autumn, and the secondary increase in diatoms 

 towards the seaward end of the line was mainly due to offshore species. Conditions here and on the 



WSIOM 

 d ' 



d 

 -d 

 d 



WSI062 WSDfcl WSIObO WSI059WSIOS8 WS IOS7 



Li 



MICROPLANKTON 



MLS 

 IOOO 



IOO- 



J L 



^ T WwW ED A1 



... 1 f IVO„ 



PROTOZOA 

 NOT RECORDED 

 ""'AT WSIOM 2 



a 

 o' 

 d 

 d 

 d 

 hd 



\ 





OTHER PLANTS RECORDED 

 AT-WS K36I 



NO 

 SESTON RECORDED 

 AT WS IQbl 



SEA MILES FROM LAND 



SO 



SEA MILES FROM LAND 



Fig. 70. Distribution of the main groups of microplankton, 

 estimated totals per net haul, survey II, Sylvia Hill line, 

 21-24 September 1950. 



Fig. 71. Distribution of the main groups of microplankton, 

 estimated totals per net haul, survey II, Southern Inter- 

 mediate line, 24-25 September 1950. 



adjacent lines both north and south displayed the general narrowing of the zone of rich coastal plankton 

 that seemed characteristic of the second survey in its most extreme form. 



The extremely rich diatom plankton found at the inshore station on the Sylvia Hill line (Fig. 70) 

 showed clearly that, quite locally, the coastal population attained a density as great as that observed 

 during the autumn, but proceeding seawards we again encountered the conditions met with farther 

 north: a more rapid transition to relatively scanty offshore plankton, and greater relative abundance 

 of Metazoa and Seston than had been observed there during the first survey. 



A very similar distribution of the main groups was also evident on the southern intermediate line 

 (Fig. 71) where the diatom population inshore was one of the richest sampled during either survey. 

 ' Other plants ', chiefly represented by isolated filaments and a few rafts of Trichodesmium thiebautii, 

 attained their maximum numbers and greatest relative importance in the comparatively scanty 

 offshore plankton on this line, but it should be emphasized that the species was not observed in any- 

 thing approaching bloom-forming quantities. 1 Such blooms as we observed during the surveys were 



It is one of the commonest and most widely distributed bloom-forming organisms throughout the warm and temperate 

 regions of the South Atlantic. 



