. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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
. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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 70 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 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 antarct
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