204 DISCOVERY REPORTS 



During the Discovery Investigations some thousands of plankton samples have been taken from 

 stations spread over the whole of the Southern Ocean at all seasons of the year, the majority south of 

 the Antarctic Convergence. For the present investigation an arbitrary selection of samples has been 

 made from hauls between the surface and a depth of 250 m. in the Antarctic water, which means that 

 they have been taken from within the limits of the Antarctic surface water (Deacon, 1933). Further 

 particulars of the selection of samples are given below. 



As explained on p. 209 the material used in this paper is largely derived from the work of other 

 members of the staff of the National Institute of Oceanography and the former Discovery Committee, 

 who for other purposes have identified the species in a large number of samples. I have had the 

 advantage of advice from various specialists also in checking the identity of zooplankton species. 

 Their names are mentioned below on p. 209. My own work has been to examine such additional 

 samples as were necessary, to assemble the material, and to present the conclusions. For the phyto- 

 plankton, however, most of the work had already been done some years ago by Dr T. J. Hart, who has 

 very kindly allowed me to use, in the section which follows, his own account of the phytoplankton, 

 which he had prepared in anticipation of some more general study of circumpolar distribution. It is 

 reproduced here with only slight modifications to adapt it to the present paper. Dr Hart has had 

 much experience of the Antarctic diatoms and is confident of the specific identities of the forms listed 

 in Fig. 2. 



PHYTOPLANKTON 



Among organisms other than diatoms, only Phaeocystis and Distephanus can be considered important 

 in the plant population of the seas of the Antarctic zone. The general homogeneity of the vegetation 

 is not always obvious from the results of a single voyage owing to seasonal changes in its constituents. 

 Prolonged cruises by the research ships of the Discovery Committee have enabled us to distinguish 

 between seasonal variation and differences in regional distribution with some certainty. In the course 

 of this work it became so evident that all the more important species had a completely circumpolar 

 distribution, that in a paper on phytoplankton periodicity in those seas Dr Hart stated it as a fact 

 (Hart, 1942, p. 270). Nevertheless, no detailed exposition of common circumpolar distribution for 

 Antarctic phytoplankton organisms has yet been made, and these notes are designed to supply one. 

 The observations available to the south of the Antarctic Convergence have been grouped into 

 intervals of 20 longitude and found sufficiently numerous for the selection of ten representative 

 summer stations in all but two of these eighteen sectors. Between 140 and 160 E. only eight observa- 

 tions are available and between 180 and 160° W. only nine. Much material obtained during the second 

 and fourth commissions of the R.R.S. ' Discovery II ' has not been fully examined, but the results 

 presented here seem ample to prove the point at issue. In Fig. 2 the frequency of occurrence of each 

 species in any one sector is shown, to the nearest 5 %, as a percentage of the number of samples 

 examined in that sector. The stations selected are listed in Table 3, p. 217. 



In tabulating the results Dr Hart has considered all the more important phytoplankton species of 

 the first four ecological groupings adopted in his paper on ' Phytoplankton Periodicity in the Antarctic ' 

 (1942). It will be realized that in most sectors all the observations were obtained under strictly oceanic 

 conditions. This alone accounts for such gaps as occur in the observed distribution of diatoms of the 

 neritic/ice-edge group. Other gaps are probably due mainly to the rather rigid time limit adopted to 

 keep the observations in several sectors fairly comparable; for example, the inclusion of autumnal 

 stations would doubtless show a more complete distribution of Chaetoceros radicidum, but would 

 greatly diminish the percentage frequency of occurrence of other more important species. 



The chief source of the observations is the series of qualitative counts from hauls with the Harvey 



