I 



RHIZOSOLENIA CURVATA ZACHARIAS, 



AN INDICATOR SPECIES IN THE 



SOUTHERN OCEAN 



By T. John Hart, D.Sc. 

 (Plate XIV; Text-figs. 1-7) 



INTRODUCTION 



T has long been known that planktonic diatoms are the dominant constituents of the 



phytoplankton of the Southern Ocean. Nearly all the species concerned have a very 

 wide geographical range, and are able to tolerate considerable variation in the conditions 

 of their environment. The discovery of a species with a range so restricted that it appears 

 to lend itself to use as an indicator, was therefore considered of sufficient importance to 

 justify the special study of its distribution and biology described here. 



During our earlier work both Hardy (1935, p. 53) and I (1934, pp. 82, 161) found that 

 while Rhizosolenia curvata appeared to find its optimum in the southern part of the 

 sub-Antarctic Zone, it was occasionally found to the south of the Antarctic convergence, 

 notably in the South Georgia area. In the course of the third commission of the R.R.S. 

 ' Discovery II ' our work in the Southern Ocean ranged from 50 E westwards to 180 W. 

 Throughout the whole of the period November 1933-April 1935, I was able to make a 

 preliminary examination of the phytoplankton catches within a very short time of their 

 being brought on board, and I was very much struck by the consistency with which this 

 species occurred in the neighbourhood of the Antarctic convergence. It appeared to me 

 to lend itself very well to use as an indicator species, in the sense that F. S. Russell 

 (1935) has used the term with reference to certain plankton animals. 



The phytoplankton of the convergence region in which this diatom occurs is in 

 general fairly characteristic, but the other constituents of the community are either 

 cosmopolitan (e.g. R. alata) or have such a marked seasonal periodicity that they could 

 never be of much value as indicators. None of them, not even R. polydactyla (cf. Hart, 

 1934, p. 79), has such a limited geographical range as R. curvata, and although the latter 

 is rarely very numerous, it is present at all seasons, and so easily recognized by reason 

 of its large size and highly characteristic appearance, that it seems by far the best bio- 

 logical indicator of the southern limit of sub-Antarctic surface water. That this surface 

 water is frequently forced some small distance south of its normal limit, the Antarctic 

 convergence, has always been realized. The agencies by which this is brought about are 

 either temporary wind currents, or other more complicated elemental disturbances. 

 It seems very significant that wherever it has been possible to show such southward 

 movement hydrologically, there also R. curvata was to be found in the plankton. 



After my return from the third commission, therefore, I took the first opportunity of 

 working through all the material that could possibly be expected to include this species. 

 The material collected on the second commission, when the R.R.S. 'Discovery II 



