i83 DISCOVERY REPORTS 



of the South Sandwich Islands, and westwards across the north of the sea towards the 

 South Orkney Islands. Here again the influence of the mixing of the currents may be 

 the determining factor, but the concentration to the east of the South Sandwich Islands 

 appears to be of purely eastern Weddell Sea type. Doubtless the hindrance to the sur- 

 face drift afforded by the chain of islands and the submarine ridge connecting them is 

 involved. Although we have as yet no direct evidence, it is possible that upwelling, 

 which brings spores within the photic zone, is the primary cause. On this point further 

 hydro logical evidence is awaited. 



In Bransfield Strait the richest phytoplankton was found mainly to the east. Stations 

 just inside the chain of islands guarding the western end of the strait yielded scanty 

 hauls at all seasons of the year. Outside the South Shetland Islands to the north, in the 

 southern half of Drake Passage, a rich phytoplankton was found in autumn, and there 

 are indications that this locality may be a comparatively rich one at other seasons, more 



particularly in spring. 



In the channels of the Palmer Archipelago, and farther to the south-west close in- 

 shore along the Biscoe Islands and Adelaide Island, a rich neritic association was found 

 after mid-season. Here several species were abundant which commonly attain their 

 maximum in spring farther north. In the Bellingshausen Sea itself the phytoplankton 

 tended to be uniformly moderate in quantity. The nearest approaches to any local con- 

 centration observed were the moderately rich Corethron—Rhizosolema gracillima as- 

 sociation developing to the north of Adelaide Island and the slightly richer mixed 

 plankton far to the south-west. The former evidently had its centre of distribution to 

 the north of Adelaide Island during the seasons studied, and increased in that region as 

 the season advanced, at the same time invading other areas farther to the north-east in 

 a lesser degree. 



COMPARISON OF THE FERTILITY OF 



THE DIFFERENT AREAS 



Table XX shows the average estimated numbers of the catches in the several areas. 



The numerous large sources of error in such estimations have already been referred to, 



but most of the differences recorded are sufficiently large to leave little doubt that they 



have a real significance, and provide a useful basis for discussion of the probable 



limiting factors. 



From this table the great quantity of phytoplankton present around South Georgia 

 in spring, and its almost equally great scarcity in the late summer of the abnormally 

 warm season 1929-30, will at once be noted. The evidence for the late spring maximum 

 in Bransfield Strait is also well shown, and the poverty of this region as compared with 

 others investigated. It will be seen that the mean value obtained from the Weddell Sea 

 area, was more than twice that from the Bellingshausen Sea. This is not altogether a fair 

 comparison, as many of the Weddell Sea stations were worked in relatively low latitudes. 

 This source of distortion is, however, very largely compensated by the very poor catches 

 obtained to the north-west of the Weddell Sea area in the second half of the season. 



