440 



DISCOVERY REPORTS 



It is obvious that this evidence strongly supports the view that the species finds its 

 optimum in the more southerly portion of the sub-Antarctic Zone. This is also con- 

 firmed by the numbers of individuals recorded at different temperatures. Fig. 6 shows 

 the mean numbers of R. curvata per net haul for each degree of temperature. Still 

 further support is furnished by the number of positive records obtained at the various 

 temperatures. While the extreme range was - 0-40 to 8-30° C, 72 per cent of our 

 positive records come from samples obtained in waters between 2 and 7 C. ; that is, 

 within the normal seasonal range of the southern part of the sub-Antarctic Zone. 



3 

 < 



X 



\- 



UJ 



Z 



01 

 QL 



LU 

 D 



10.000- 



5.000 



5? 



LU 

 3 



T 

 2 3 4 5 6 



TEMPERATURE IN DEGREES CENTIGRADE 



Fig. 6. Variation in abundance of Rhizosolenia curvata at different temperatures. 



PHYTOPLANKTON COMMUNITIES IN WHICH RHIZOSOLENIA CURVATA 



IS USUALLY FOUND 



By far the most characteristic phytoplankton community of the convergence region 

 as a whole is a " solenoid " community. The fullest development of this was seen in what 

 I described as a " Rhizosolenia plankton" on the basis of earlier work in the Scotia Sea 

 (Hart, 1934, p. 74). It has some points of resemblance to the styliplankton of the tem- 

 perate zone in the northern hemisphere (Gran, 1912, p. 347). In this southern solenoid 

 community R. curvata, although as a rule it is present only in relatively small numbers, 

 is the only species that lends itself to use as an indicator. The others all have a wider 

 distribution, a very much higher seasonal variation, while one species at least is almost 

 completely cosmopolitan. The predominant members of the fluctuating solenoid com- 

 munity are: 



Rhizosolenia polydactyla Castracane Corethron criophilum Castracane {inerme phase) 



R. alata Brightwell (large phases) Dactyliosolen antarcticns Castracane 



Where the mixing with water from the Antarctic side of the convergence is strongest 



