TRANSITIONAL PHYTOPLANKTON 



75 



generally unfavourable conditions described on the plankton survey slightly earlier in 

 the year. 



That diatom production was at a minimum in the older Antarctic surface water in 

 March 1930 is also shown by the south-easterly stations on the next line (Sts. 372-5), 

 which was worked from a point near the South Sandwich Islands west-north-west to the 

 Burdwood Bank. The full analyses of the catches taken on this line are shown in Table 

 XIX. It will be seen that in the small catches to the south-east, Coscinodisciis bouvet was 

 of fairly constant occurrence, and that at St. 374 Rhizosolenia styliformis was dominant. 

 Both these species had been taken in much greater numbers in the neighbourhood of the 

 South Sandwich Islands a short time before. The comparatively rich hauls on this line 

 were all taken to the north-west, on either side of the Antarctic convergence, which 

 was evidently crossed between Sts. WS 529 and 530 (Fig. 32). The dominant forms in 

 the old Antarctic surface water examined at Sts. WS 528, 529, were small Chaetocerids, 

 mainly Chaetoceros atlanticus, Nitzschia seriata and Fragilaria antarctica. The plankton 

 at St. WS 529 appears to indicate that the autumnal maximum observed a fortnight later 

 in the southern half of Drake Passage, in old Bellingshausen Sea water, had already begun. 

 At St. WS 530, over the convergence, a hint of the Rhizosolenia plankton met with on the 

 previous line was encountered, with R. polydactyla, R. styliformis and R. data prominent 

 in that order of importance. Here also Ceratiiim petitagonum reached its maximum for 

 the line. At the last station, WS 531, on the tail of the 

 Burdwood Bank itself, the phytoplankton was quite 

 different from anything else encountered during this 

 season, apparently owing to violent vertical mixing 

 transporting bottom forms into the surface layers. 

 Coscinodiscus spp. were dominant with moderate num- 

 bers of Fragilaria antarctica, small Chaetocerids, Hya- 

 lodisciis kergtielensis and Rhizosolenia polydactyla. The 

 apparent mixing of oceanic forms and bottom forms at 

 this station is to be expected in view of the fact that it 

 was worked on the only extensive area of shoal water 

 known in the southern ocean, just to the north-west 

 of the Antarctic convergence. The relation between 

 the bulk of the phytoplankton samples taken on this 

 line and the position of the convergence, as indicated by 

 the temperature of the upper layers, is shown diagram- 

 matically in Fig. 35. 



This brings us to the consideration of the last piece of 

 work accompHshed in the south during the 1929-30 

 season, the line worked across Drake Passage in April 

 from Livingston I. to a point near Cape Horn (Sts. 

 377-88, Fig. 32). In Table XX, in which the full analyses of the phytoplankton hauls 

 taken on this line are given, that from St. 376 worked inside the Bransfield Strait has 



2- 



ESTIMATED 

 PHYTOPLANKTON TOTALS 



MILLIONS 



..^a. 



Fig. 35. Diagram showing the sur- 

 face temperatures and phytoplankton 

 totals on a line of stations from the 

 South Sandwich Islands to the Burd- 

 wood Bank, March 1930. 



