FLORAS OF MAIN TYPES OF WATER 177 



from the other areas, which accounts for the absence of seasonal data. In this area four 

 distinct associations could be recognized : (i) a rather poor phytoplankton with Corethron 

 valdiviae and Chaetoceros neglechis dominant, which appears to be the first to develop 

 as the ice breaks up ; (ii) a rich neritic flora resembling that of the western Weddell Sea 

 water, with the addition of Nitzschia closteriiim and a larger proportion of Chaetoceros 

 tortissimus, found among the Biscoe Islands and at the Palmer Archipelago; (iii) the 

 Corethron — Rhizosolenia gracillima association developing in comparatively open water 

 to the north-east ; and (iv) a moderately rich association developing along the ice-edge 

 far to the south-west, in the vicinity of Peter I Island, with Nitzschia seriata, Fragilaria 

 antarctica, Thalassiothrix antarctica and Chaetoceros dichaeta among the more pro- 

 minent species. 



THE FLORAS OF THE MAIN TYPES OF WATER 



From the consideration of the material as a whole, the four main types of surface 

 water in the area investigated were distinguishable by the proportions of the principal 

 species found within them, and by the general nature of the floras they supported. These 

 features may be summarized as follows : 



Eastern Weddell Sea water. Chaetoceros criophilum, Rhizosolenia styliformis, 

 and Corethron valdiviae, strongly dominant. Nitzschia seriata and Thalassiosira ant- 

 arctica the most numerous of the smaller forms. Moderately rich in species, very rich 

 in quantity. 



Western Weddell Sea water. Chaetoceros socialis, Thalassiosira antarctica and 

 Chaetoceros neglectus the principal dominants, followed by Fragilaria antarctica and 

 Nitzschia seriata. Very rich in species with minute forms dominant. Very rich in 

 quantity early in the year, but falling off more rapidly than the eastern Weddell Sea 

 water in this respect. 



Bellingshausen Sea water. Corethron valdiviae, Thalassiothrix antarctica, Nitzschia 

 seriata, Fragilaria antarctica and Chaetoceros neglectus all important, with Rhizosolenia 

 alata f. gracillima a characteristic species. Very rich in species and more variable in 

 quality than either of the types discussed above. Moderate in quantity. 



Bransfield Strait water. This should be regarded as a special development of 

 the last named. Corethron valdiviae very strongly dominant. Rhizosolenia gracillima, 

 Thalassiosira antarctica and Biddulphia striata sometimes important. Rather poor 

 quantitatively and with few species. 



Where intermediate conditions obtained hydrological evidence frequently indicated 

 that mixing between the bodies of water concerned was probable. Anomalies due to 

 Httoral conditions, the seasonal succession, and the "age" of the water (cf. p. 10) were 

 not infrequent, but the cause was generally obvious. Broadly speaking, the above dis- 

 tinctions held good during the seasons studied, and were found to form a suitable basis 

 for the description of the more local associations met with. 



DVIII ^3 



