LOCAL CONCENTRATION i8i 



previously outlined, so that their seasonal distribution can be conveniently summarized 

 without further detailed discussion. 



(i) Spring forms: Thalassiosira antarctica, Dactyliosolen laevis and Chaetoceros 

 schimperiamis. 



(ii) Forms with a spring maximum remaining numerous throughout the season: 

 Corethron valdiviae, Rhizosolenia styliformis, Chaetoceros criophiliim, and Thalassiothrix 

 mitarctica. 



(iii) Forms with spring and autumn maxima: Chaetoceros atlanticus, Ch. dichaeta, 

 Ch. neglectus, and Fragilaria antarctica. 



(iv) Forms with a maximum in late summer and autumn: Dactyliosolen antarcticus, 

 and, in exceptionally mild years, the dinoflagellates Peridinium antarcticum and 

 Ceratium pentagomim f. grandis, with the spineless chain form of Corethron valdiviae. 

 In normal years these last are probably only of importance in the very oldest and 

 warmest of Antarctic surface waters just to the south of the convergence ; they do not 

 reach South Georgia except in warm seasons. It is only in such years, with the mid- 

 summer diatom minimum strongly marked, that the dinoflagellates become important, 

 as they almost invariably do in corresponding latitudes in the northern hemisphere. 

 Even in the warm years the dinoflagellates are outnumbered by the diatoms in the area 

 investigated, and they are not numerous more than about 200 miles south of the Ant- 

 arctic convergence. 



AREAS OF LOCAL CONCENTRATION 



From the conditions described on the November 1930 survey and the survey dealt 

 with by Hardy, it would appear that the very rich phytoplankton, with small forms 

 dominant, found off South Georgia during the first half of the season, is confined to the 

 region to the south and south-west of the island. It is here that considerable mixing 

 probably takes place between water from the western Weddell Sea, and old water of 

 Bellingshausen Sea origin. Both Ruud (1932, p. 99) and Hardy have commented on 

 the fact that regions with converging currents tend to encourage a rich production of 

 plankton, and Hardy has suggested that this is due to the stimulus to growth con- 

 sequent upon slight changes in the external conditions of the environment. 



The tongue of eastern Weddell Sea water reaching up the eastern side of the island 

 some distance ofl^shore also supported a very rich flora during the survey of November 

 1930. Here large forms predominated, so that while the average volumes of the catches 

 were as great or even greater than those obtained to the south-west, the numerical 

 estimations indicate that the region was poorer. This may be so in actual fact from the 

 point of organic production, as the proportionate bulk of the chloroplasts to the whole 

 frustule is evidently very much smaller in the large species. It is to the east and south- 

 east of the island that a comparatively rich phytoplankton persists longest throughout 

 the season. 



In the Weddell Sea area the only definite localizations were observed in the vicinity 



