TRANSITIONAL PHYTOPLANKTON 



73 



bj 5 



2- 



% 



20 



0- 



% 



20- 



5 ta f3 S IG s 



in in I/) in in in 



ui in oi ifi lO i/l 



5 s s s s s 



PHYTOPLANKTON TOTALS 



MILLIONS 



CORETHRON VALDIVIAE 



core:thron valdiviae 



spineless chains 



RHIZ050LENIA CURVA 



organisms from remaining in the optimum light zone long enough for extensive pro 



liferation. The presence oi Rhizosolenia alata f . gracillima 



at this particular station is interesting, as this form 



has been encountered in abundance only in the northern 



part of the BeUingshausen Sea at midsummer, and 



just outside the South Shetlands later in the year, and 



in Bransfield Strait. In this last locality it evidently 



comes in with the old Bellingshausen Sea water from 



the western end, and its appearance here to the east 



quite early in the season may be due to multiplication 



of the remnant of the autumn invaders of the previous 



season. 



On the line just described, the great increase in 

 phytoplankton with decreasing temperature to the south 

 of the Antarctic convergence was very well illustrated, 

 but on a series of observations taken between the 

 Falkland Islands and South Georgia at the end of 

 February 1930 (WS 518-36), the normal state of affairs 

 was reversed, there being a fairly rich phytoplankton 

 in the sub-Antarctic surface water, while that to the 

 south-east was comparatively poor. On this line also 

 the convergence was much less sharply defined than 

 usual, the probable reasons for this being mixing caused 

 by the newly discovered bend or curve in the con- 

 vergence immediately to the west of the centre of this 

 line, and as already noted, the abnormal warmth of the 

 old Antarctic surface water in the vicinity of South 

 Georgia towards the end of this season. 



It will be seen from Table XVIII, in which the full 

 analyses of the hauls on this line are given, that pro- 

 ceeding east by south from the Falkland Islands, fairly 

 heavy catches were obtained at the first three stations in 

 definitely sub-7\ntarctic water. This is perhaps rendered 

 even clearer by Fig. 34, where the proportions of some 

 of the leading forms are also shown as percentages of 

 the catch at each station. It will be seen that few 

 species were represented, and that the phytoplankton 

 was of a very distinctive type, with R. polydactylo 

 dominant, and R. styliformis and Corethron valdiviae 

 (spineless chains) as the other more numerous forms. 

 A few rarer forms were also of very constant occurrence : Rhizosolenia alata, Ceratium 

 pentagonum with intermediates between forms grandis and longisetum, and the large 



R. polyoactyla 



40- 

 20 



2D 



THALASSIOTHRIX ANTARCTICA 



I 



CERATIUM PENTAGONUM 



Fig. 34. Diagram showing the per- 

 centages of the principal species in the 

 catches on a line of stations from the 

 Falkland Islands to South Georgia, 

 February 1930. 



