PHYTOPLANKTON OF THE BELLINGSHAUSEN SEA 141 



be seen that a rich variety of species was present in a haul of moderate quantity, with 

 two small species, Chaetoceros socialis and Ch. neglectus predominating in uncountable 

 colonies. This is the only occasion on which the colonial form of Ch. sociolis has been 

 observed in the open water of the Bellingshausen Sea during the course of these in- 

 vestigations, though it is possible, but not probable, that the single chains have occa- 

 sionally been confounded with those of Ch. neglectus. The only other species present in 

 large numbers at this station were Nitzschia seriata and N. closterhm, although, as can 

 readily be seen from the analysis, many other species were present in subordinate 

 quantities. 



At St. WS 504 a much richer haul was obtained, though the larger volume was mainly 

 accounted for by the larger numbers of Corethron valdiviae present. Chaetoceros socialis 

 was not observed here, but Ch. neglectus and the two species of Nitzschia were present 

 in large numbers, and Ch. dichaeta was taken in much greater quantity than at any 

 other station in the Bellingshausen Sea during this season. 



The phytoplankton at these two stations presented important differences from that 

 collected in the same locality during the following year, when the large species 

 Thalassiothrix antarctica was one of the leading forms, and Fragilaria antarctica and 

 Chaetoceros dichaeta formed much larger proportions of the catches. It would appear 

 that these differences are too great to be accounted for by the three weeks difference in 

 time of year, and constitute one of the fluctuations from year to year for which we are 

 only able to offer hypothetical explanations at present. 



On the line of stations worked off Adelaide Island in the north-eastern corner of 

 the Bellingshausen Sea in February (WS 509-17), we find a rich development of 

 the association with Corethron valdiviae and Rhizosolenia gracillima dominant; this 

 association was encountered in this area on other occasions, and it evidently invades 

 the Bransfield Strait during the latter half of the season. The full analyses of the phyto- 

 plankton hauls from these nine stations (Figs. 71-73), which were worked at 10 mile 

 intervals outwards from Adelaide Island on an approximately north-westerly course, 

 are given in Table XLVI. From this table it will be seen that the phytoplankton en- 

 countered was fairly uniform throughout, though with a considerable fluctuation in the 

 relative proportions of the two dominant species, and some falling off in the bulk of the 

 catches towards the northern extremity of the line. At the three inshore stations 

 Corethron predominated over Rhizosolenia gracillima to a marked degree, and other 

 forms present in considerable numbers were Nitzschia seriata and Fragilaria antarctica. 

 Farther offshore the phytoplankton presented a smaller variety of species, and the only 

 form apart from the two dominants that consistently occurred in moderate numbers was 

 Chaetoceros criophilwn. This form, which has been found as a dominant species in the 

 Weddell Sea area, was of fairly constant occurrence in the Bellingshausen Sea also, but 

 always in subordinate numbers. Its maximum for the sixty-seven stations we have 

 examined from the latter area occurred at Sts. WS 513 and WS 514 on this line. 

 Among the less important forms, Dactyliosolen laevis, Chaetoceros dichaeta, Synedra 

 spathulata and Thalassiothrix antarctica were confined to the outermost stations on this 



