8o DISCOVERY REPORTS 



a trace of the March Rhizosolenia plankton of the sub-Antarctic surface water still 

 remained. 



It will be remarked that very few species entirely confined to the sub-Antarctic Zone 

 were recorded, and that many Antarctic stragglers were found to the north of the con- 

 vergence. The chief reason for this was that as one of the main purposes of these lines 

 was to determine the position of the convergence, they were not carried very far north 

 and west into the sub-Antarctic Zone ; other lesser reasons for this have already been 

 discussed. If we take the stations worked in what may be considered as the extreme 

 south of the sub-Antarctic Zone, we find that only three species were recorded that did 

 not occur sporadically farther south : Chaetoceros decipiens, Hyalodiscus kerguelensis and 

 Ceratium fusus ; but if we consider the forms found dominant to the north of the con- 

 vergence, but rarely or sporadically farther south, we have besides Hyalodiscus, the 

 spineless chain form of Corethron valdiviae and Rhizosolenia polydactyla. Next in 

 importance are the two very variable cosmopolitan forms R. styliforniis and R. 

 alata, and to these can be added a list of definitely Antarctic immigrants, sometimes 

 found in comparatively large numbers to the north of the convergence, where, however, 

 they have frequently been observed in an apparently dying condition: Dactyliosolen 

 antarcticus, Chaetoceros atlanticus, Ch. criophilum, Fragilaria antarctica and Nitzschia 

 seriata. The occurrence of these forms in numbers in the sub-Antarctic Zone is normally 

 confined to the region in the immediate vicinity of the convergence itself, or only a short 

 distance north of it. Among the rarer forms the following may be regarded as typically 

 sub-Antarctic, though occurring sporadically to the south of the convergence, Rhizo- 

 solenia curva, R. torpedo and Ceratiitm pentagonum (f. lineatiim and intermediates). The 

 form grandis of the last-named species normally shows a maximum right on the con- 

 vergence or, as in this unusually mild season, slightly farther south. 



THE SEASON 1930-1 



Towards the end of this season, in the month of March, three series of observations 

 crossing the Antarctic convergence were obtained. The positions of these stations are 

 shown in Fig. 32. The distribution of the phytoplankton species differed somewhat from 

 that found towards the latter end of the previous abnormally warm season, as would be 

 expected from the striking contrast between the meteorological and ice conditions ob- 

 taining during the two years. 



The first line of stations (Sts. 633-9) was worked from 54° 58^' S, 52° i6|' W on a 

 course leading down to the neighbourhood of the South Orkney Islands and then south- 

 westwards as far as 61° 57I' S, 51° 59' W. The full analyses of the phytoplankton 

 collected on this line will be found in Table XXI. At the northernmost station, definitely 

 to the north of the convergence, it will be seen that a rich phytoplankton was encountered, 

 indeed, the haul at this station, the earliest in the year of the series under consideration, 

 was appreciably heavier than any of the others to the north of the convergence taken later 

 in the month. The chief forms present in this haul, however, were for the most part 



