PHYTOPLANKTON OF THE WEDDELL SEA AREA 93 



Corethron valdiviae and Chaetoceros criophilum were almost completely lacking however, 

 and among the small forms Nitzschia seriata and Chaetoceros dichaela were again the 

 most important, the former especially so. From the tables it will be seen that the phyto- 

 plankton at this station was considerably poorer than at St. 529, which might fairly be 

 described as rich in any season. All through this series of observations the hauls were 

 large, with some falling off at the south-western end of the line. This general abundance 

 of the phytoplankton was of course to be expected, as the main spring increase was 

 evidently in full swing round South Georgia less than a month before. 



An extremely rich phytoplankton was encountered at St. 531, which was situated 

 about 150 miles due south of South Georgia. Here Ch. criophilum was strongly 

 dominant, with Fragilaria antarctica and Rhizosolenia styliformis the only other forms 

 of much importance. Among the rarer species at this station Tholassiosira antarctica, 

 Chaetoceros neglectus, Ch. dichaeta and Nitzschia seriata were the most numerous, in that 

 order. It is evident that this phytoplankton was mainly the product of water of the 

 eastern Weddell Sea type, but the presence in some numbers of Fragilaria antarctica 

 and more particularly of Thalassiosira antarctica so far to the north indicates the possi- 

 bility of some slight admixture with water of western Weddell Sea origin. 



At St. 532 a catch of a more normal size was obtained, about one-tenth as rich as that 

 at the last station but still moderately heavy. Here Chaetoceros neglectus was the domi- 

 nant form, and the other principal species were Rhizosolenia styliformis, Nitzschia 

 seriata, Chaetoceros criophilum, and Fragilaria antarctica in that order of importance. 

 With this considerable increase in the percentage of the smaller forms it is evident that 

 mixing between the two types of Weddell Sea water had taken place to an appreciable 

 extent, the results indeed suggesting that the western type was most strongly re- 

 presented. This was still more evident at the next station to the south-west (St. 533). 

 Here Chaetoceros neglectus was again dominant, and the order of importance of the other 

 principal species was: Corethron valdiviae, Nitzschia seriata, Thalassiosira antarctica, 

 Rhizosolenia styliformis, and Chaetoceros criophilum. The phytoplankton at this station 

 was richer than that at St. 532. 



Conditions at St. 534, to the north-west of the South Orkney Islands, present some- 

 thing of an anomaly. From its position one would expect the surface water to be either 

 of definitely western Weddell Sea type, or the latter affected by some degree of admixture 

 with old surface water from the Bellingshausen Sea passing through the southern half 

 of Drake Passage. Yet analysis of the phytoplankton haul, which was considerably 

 smaller than any of those dealt with above, reveals the fact that among the four species 

 of most importance Rhizosolenia styliformis and Chaetoceros criophilum dominated to a 

 marked extent over Nitzschia seriata and Fragilaria antarctica f. boiivet, conditions which 

 one would normally associate with surface water of eastern Weddell Sea origin. It would 

 appear that the most probable explanation of this would lie in the supposition that as 

 the age of the western Weddell Sea water increases, the nature of the phytoplankton 

 supported by it changes, the proportion of large to small forms becoming greater. 

 Further observations may show this to be a more probable explanation of the nature of 



