MAIN TYPES OF PHYTOPLANKTON 21 



These distinctions, however, He rather in the different proportions of the dominant 

 forms than in the presence of distinct species, and signs are not wanting that as they 

 approach the Antarctic convergence, the several types of Antarctic surface water tend 

 to merge to a certain extent, or rather the floras of old, comparatively warm waters from 

 the Weddell and Bellingshausen Seas respectively, approach one another much more 

 closely than the floras of waters from the same two areas farther south, containing melting 

 pack-ice. The chief feature of the phytoplankton of the Antarctic surface water, viz. the 

 preponderance of diatoms and its great abundance as compared with that of all other 

 seas in the world (apart from comparatively small isolated areas like the Humboldt 

 current) is well brought out by the stations worked in long. 30° W. 



In the description which follows, reference should be made to Table I, Sts. 661-6, 

 and to Table 2 below showing the relative percentages of the major categories of 

 phytoplankton organisms. It will be seen that at the two southernmost stations the total 

 number of diatoms present was estimated at some millions, and that the percentage of 

 dinoflagellates was negligible. The dominant forms were Chaetoceros criophilum, and 

 in a lesser degree, Corethron valdiviae. Among the rarer forms Rhizosolenia styliformis 

 was the most numerous, these conditions all pointing to the eastern Weddell Sea origin 

 of the water. 



St. 666 was worked only a few miles south of the convergence, and the phytoplankton 

 found was comparatively scanty. The relative percentage of dinoflagellates was still very 

 small, the dominant forms being the diatoms Chaetoceros atlanticiis and Nitzschia seriata. 

 Among the less numerous forms the chief were Chaetoceros dichaeta and Dactyliosolen 

 antarcticiis. The same dominant forms have been found in larger hauls obtained in close 

 proximity to the convergence on lines of stations worked earlier in the season farther 

 west. 



The following list gives all the species recorded at these stations within the Antarctic 

 convergence, those printed in heavy type not being found north of it. The reason for 

 such a comparatively large number of typically Antarctic species being found (in most 

 cases, in an apparently dying condition) to the north of the convergence is probably to 

 be ascribed to the time of year, and to the fact that a certain amount of mixing of the 

 surface waters takes place mainly to the north of the convergence. At this time of the 

 year most of the Chaetoceros criophilum taken were in a dead or dying condition. 



Coscinodiscus spp. Chaetoceros dichaeta, Ehrb. 



Asteromphalus regularis, Karst. Ch. neglectus, Karst. 



Dactyliosolen antarcticus, Castr. Fragilaria antarctica Castr. 



Corethron valdiviae, Karst. Thalassiothrix antarctica, Schimp. 



Rhizosolenia alata, Btw. Nitzschia seriata, Cleve 



R. styliformis, Btw. Dinophysis sp. 



Chaetoceros atlanticus, Cleve Peridinium antarcticum, Schimp. 



Ch. criophilum, Castr. Peridiniimi sp. 

 Ch. curvatus, Castr. 



This list gives but a poor idea of the rich variety, apart from the bulk, of the phyto- 

 plankton of Antarctic surface waters generally. This fact is to be attributed partly to the 



