2 o8 DISCOVERY REPORTS 



It is instructive to note the proportion of the species in the phytoplankton within the Antarctic zone. 

 For the present purpose all the categories previously examined may be divided according to the 

 measure of completeness of circumpolar distribution shown by the data examined here (Table i). 

 The figures tabulated with them are mean percentages by counts from 177 observations obtained in 

 the three main oceanic regions of the Antarctic zone over the periods of the main increase during 

 1937-8 and 1938-9. 



Table 1 . Probability of circumpolar distribution and mean percentages of each species 



in the phytoplankton population 



It will be seen that the rarer microscopic plants, not considered here, form only 6-2 % of the total 

 phytoplankton ; 68 % of the organisms captured by the NHP have been shown to have a completely 

 circumpolar distribution, while it is very highly probable that a further 25 %, representing some 

 eighteen species, are truly circumpolar also. It thus appears that the statement 'almost all the 

 important species have a completely circumpolar distribution ' is not exaggerated. 



The only important plant not dealt with is Phaeocystis brucei. Dr Hart has already explained (Hart, 

 1942, p. 266) that this organism defies existing methods of estimating even its relative abundance; 

 further, that it has a very limited time distribution as a first colonist when pack-ice disperses. Thus we 

 cannot show the distribution of the species with the precision possible when dealing with diatoms. 

 All our available records of Phaeocystis brucei plotted on a circumpolar chart are shown herewith 

 (Fig. 3). From this it appears (at the least) highly probable that this species has a completely circum- 

 polar distribution also. 



ZOOPLANKTON 

 The material used for the investigation of the zooplankton consisted of samples collected by the 

 R.R.S. 'Discovery II' between 1930 and 1951, and the R.R.S. 'William Scoresby' during the years 

 1928-31. These samples were taken with the i-m. net hauled obliquely (N100B) from a depth of 

 approximately 100 m. to the surface. The construction and operation of this net have been described 

 by Kemp, Hardy & Mackintosh (1929). Where necessary the N 100B material has been supplemented 

 with samples from similar oblique hauls with the 70-cm. net (N70B) and from vertical hauls with the 

 70-cm. net (N70V) from 100-50 m. and from 50-0 m. 



As in Dr Hart's treatment of the phytoplankton the circumpolar area has been divided into eighteen 

 sectors, each of 20 longitude. The method was to choose about ten of the richer samples spaced as 



