220 



DISCOVERY REPORTS 



1-2 - 

 10- 

 08- 

 6 - 

 4- 

 0-2- 



,o, i-o 



ANTARCTIC 



[_ j _ 1 | 



'■■■; 



■ 





OS 



06 



4 H 



o 



02 



O 



z 



< O 6 

 LU 



21 O 4 



02 



O 

 06 

 04- 

 02 - 



O 



SUB-ANTARCTIC 



__L_ 



Zl 



SUB-TROPIC 



■r; v.;:." - '. ' . ' . 



TROPIC 



1 — • — 



~T- 



r^ 



SO 100 250 500 750 IOOO 1500 



INTERVALS OF DEPTH (metres) 

 Fig. 14. 



o-J- 



SUB-TROPIC ANTARCTIC 



TROPIC SUB-ANTARCTIC 



Fi g- IS- 



Fig. 14. Comparison of the mean concentration (i.e. volume per 50 m. haul) of zooplankton at seven intervals of depth in 



the Antarctic, sub- Antarctic, sub-tropic and tropic zones. This and Fig. 15 are based on results given in Table 7. 



Fig. 15. Comparison of the mean total volume (1000-0 m.) in the Antarctic, sub-Antarctic, sub-tropic and tropic zones. 



CIRCUMPOLAR VARIATIONS WITHIN THE ANTARCTIC ZONE 

 The data discussed in the previous section have shown that when the distribution of the standing 

 crop of zooplankton is considered in a north-south direction large-scale variations are observed. 

 These are undoubtedly related to the considerable north-south gradients that occur in the physical 

 and chemical character of the principal water masses. 



In the Southern Ocean hydrological differences in an east-west direction are small and all the main 

 water masses, with the exception perhaps of the Antarctic Bottom Water (Deacon, 1937), have a 

 circumpolar continuity. This is reflected, as Baker (1954) has shown, in the continuous circumpolar 

 distribution of nearly all the commoner species of Antarctic phytoplankton and zooplankton. 



Hart (1934, 1942) has shown that for the Antarctic phytoplankton there are some localized neritic 

 areas, particularly near South Georgia, where the standing crop is exceptionally great in summer. In 

 the Antarctic oceanic areas, however, which he provisionally divided into the Northern, Intermediate 

 and Southern regions, the standing crop was much less, and there was little regional variation during 

 the period of main increase. It is of some interest therefore to examine the zooplankton volumes in 

 comparable months for any obvious regional differences within the Antarctic zone, even though the 

 data do not allow such a detailed treatment as that of Hart (1942) or Baker (1954). 



The total volume of plankton at each station (i.e. from 1000 to o m.), has been plotted on a circum- 

 polar chart (Fig. 16.) together with the mean positions of the Antarctic and sub-tropical convergence 

 and the approximate mean limits of the West Wind, Weddell, and East Wind Drifts ; observations in 



