74 DISCOVERY REPORTS 



easily recognized Rhizosolenia curva, which in many ways closely resembles the R. robusta 

 of the northern hemisphere. This type of oceanic phytoplankton with Rhizosolenia spp. 

 dominant was taken on several other occasions during this season just to the north and 

 west of the convergence. It might perhaps be compared to the styli-plankton of the 

 north temperate zone (see Gran, in Murray and Hjort, 19 12, p. 347), except that 

 Dinoflagellata were much rarer; but as several observers have noted, this class of 

 organism is noticeably unimportant in the Antarctic, and it is possible that this scarcity 

 extends to the southern part of the sub-Antarctic Zone as well. It may be mentioned 

 that two of the above-named Rhizosolenia spp., R. alata and R. styliformis, are apparently 

 ubiquitous. They are subject to a large range of individual variation, and apparently 

 occur at some time of the year in almost all seas, though very rare in the tropics. The 

 R. styliformis found in the warmer water on this line consisted of small thin individual 

 frustules very different from the robust chains met with in water of the eastern Weddell 

 Sea current farther south, although a small proportion of thin forms, presumably derived 

 from microspores, was observed there also. Speaking of the distribution of this species 

 in the northern hemisphere. Gran (1929 a, p. 18) describes it as an oceanic form reaching 

 its maximum development where Atlantic water abuts on colder water masses, and also 

 shows how it occasionally invades areas as far north as Davis Strait. In the south, as we 

 see here, it seems moderately abundant where the sub-Antarctic water approaches the 

 colder Antarctic surface water, but it was much more abundant away to the south in 

 the eastern Weddell Sea water, where even in late summer the surface temperatures 

 rarely rise above 1-5° C. The summer temperatures in the southern hemisphere are in 

 fact so much lower that little can be gained from the comparison. One fact, however, 

 is noteworthy: most workers in the northern hemisphere agree that R. styliformis is a 

 form requiring high salinity to reach its optimum, and the eastern Weddell Sea water 

 in which we have found it so abundant has a higher salinity than any other Antarctic 

 surface water so far as is known at present. 



To revert to conditions between the Falklands and South Georgia, at St. WS 521 the 

 Rhizosolenia plankton was still present, but with a marked falling off in numbers, 

 correlated with a considerable drop in temperature, and this was continued at the next 

 station to the eastward where the catch was extremely small. Here R. styliformis was 

 the only member of the genus observed, dinoflagellates were absent, and the typically 

 Antarctic spiny form of Corethron valdiviae made its appearance in small numbers. At 

 the very next station, however, despite a further drop in temperature the Rhizosolenia 

 plankton cropped up again and traces of it persisted until the last station but one on the 

 line. On the last four stations small numbers of the Antarctic forms Corethron valdiviae 

 and Thalassiothrix atitarctica made their appearance. The only obvious explanation of 

 this gradual transition, and the paucity of the phytoplankton near South Georgia, is 

 that the S-shaped curve in the convergence lying between 45 and 50° W long, must have 

 led to mixing of the surface waters, encouraged by the abnormal warmth of the old 

 Antarctic surface water during the latter half of this season. Diatom production round 

 South Georgia at the time must have been at a standstill, from the prolongation of the 



