PHYTOPLANKTON OF THE BELLINGSHAUSEN SEA 145 



essential factor. This would explain its absence from the innermost stations on this line, 

 as it was known that pack-ice had been present for a considerable distance to the north- 

 ward of Adelaide Island at the beginning of January. At the northernmost station on 

 this line, St. 592, a moderately rich and varied phytoplankton was encountered, with 

 Nitzschia seriata strongly dominant. The other leading species at this station were: 

 Fragilario antarctica, Corethronvaldiviae, Chaetoceros neglectus, Ch. dichaeta, Thalassiosira 

 antarctica and Thalassiothrix antarctica in that order of importance. When the catches 

 obtained in the far south-west of the Bellingshausen Sea a httle earlier are considered, it 

 seems highly probable that the surface water at this station had originated in that region, 

 and drifted to the north-east fairly rapidly. It will be noted that the phytoplankton 

 became moderately rich only at the five more northerly stations on this line. There is 

 thus some support for the view that the persistence of the pack-ice in the Bellingshausen 

 Sea throughout spring and early summer has a strong retarding effect upon the develop- 

 ment of the phytoplankton. 



From the position of the outermost station on this line, four stations were worked on 

 an approximately south-easterly course, in the direction of the Biscoe Islands. The 

 analyses of the phytoplankton hauls from these stations are given in Table XLVIII. 

 It will be seen that at the three outermost stations Nitzschia seriata predominated in a 

 fairly rich mixed plankton, which became less varied and showed a stronger tendency 

 to develop into the Corethron — Rhizosolenia gracillima association, as one proceeded in 

 a south-easterly direction. The varied plankton at St. 593 contained species giving good 

 evidence of the origin of the surface water far to the south-west, for example Thalas- 

 siothrix antarctica, and the high proportion of Chaetoceros dichaeta at the succeeding 

 station also indicated that the surface water was largely of similar origin. At St. 595 it 

 was evident that a transition stage between the three major types of phytoplankton de- 

 velopment observed in the Bellingshausen Sea on this occasion was being reached ; the 

 large numbers of Nitzschia seriata and Fragilaria antarctica afl^orded good evidence of 

 the origin of some of the surface water far to the south-west, the high proportion of 

 Chaetoceros neglectus suggested mixing with water drifting up from the region of 

 Alexander the First Island, and finally there was an obvious increase in the tendency 

 towards the development of the Corethron — Rhizosolenia gracillima association. It 

 would seem probable from the conditions on this line that there is a general tendency 

 towards the development of this association in the older surface water of the whole of 

 the Bellingshausen Sea as it drifts north-eastward, and that the reason for the observed 

 maximum of this type of development to the north of Adelaide Island will be found in 

 the comparatively early breaking up of the pack-ice in that region. At the most southerly 

 station (St. 596) on this line a rich development of this association was encountered, 

 and other species prominent at the three more northerly stations in more mixed water 

 were either absent or present only in much smaller relative proportions. 



Table XLIX shows the analyses of the phytoplankton material from all stations 

 worked on this survey between 64 and 70° W, excluding any from the two definite lines 

 already dealt with that fall within this area (Sts. 584 and 596). From the route followed 



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