ZOOPLANKTON IN THE SOUTHERN OCEAN 223 



Pacific sector. From the results it is apparent that there are no large-scale variations in the mean 

 volumes in the different oceanic areas. 



The presence in the Antarctic of three main surface drifts, the West Wind, East Wind and Weddell 

 Drifts, suggests the possibility of comparing the east-west distribution of plankton within their 

 approximate boundaries. For this purpose the volumes for the 100-0 m. horizon are more realistic 

 since they are referable to the Antarctic surface layer (p. 200). The results for the main period of 

 increase are given in Table 9 and Fig. 18, and show that the observations are too few to allow anything 

 except tentative conclusions to be drawn from them. In the Weddell and East Wind Drifts there 

 appears to be no large-scale regional variation. The results from the West Wind Drift are very irregular, 

 and as the largest mean volumes occur between longitudes where there are few observations it is 

 unlikely that the variations are significant. The figure shows, however, that, compared to the East 

 Wind Drift, the quantity of plankton is greater in the West Wind Drift in all longitudes. 



Table 9. Comparison of the mean volume of plankton in the upper 100 m. (100-0 m.) of main surface 

 drifts in differetit areas during the main period of increase (November- April). Numbers of observations 

 are given in brackets 



[WWD = West Wind Drift; WC = Weddell Current; EWD = East Wind Drift] 



COMPARISON WITH THE FINDINGS OF OTHER WORKERS 



The conclusions reached in the previous two sections regarding the regional distribution of the 

 standing crop of zooplankton in the Southern Ocean may be summarized as follows. 



From tropical latitudes there is a north-south gradient in the quantity of plankton which reaches 

 a maximum between 50 and 55 S. This is an area which in most meridians lies just south of the 

 Antarctic Convergence in the West Wind Drift. In more southerly waters there is an apparent 

 decrease in the quantity of plankton, which may, however, be accounted for by the increased concen- 

 tration of krill (a large active animal poorly sampled by the N70V except as larvae) that occurs in 

 these regions. 



In the Antarctic zone there appears to be, from existing evidence, little circumpolar variation at 

 comparable times of the year. 



It remains now to compare the major conclusions of this work with the results of other workers. 

 Although there is a great deal of information on the distribution and variation in standing crop in all 

 oceans of the world there is such diversity in the gear and methods used both to catch the plankton and 

 to measure its volume that any comparison on anything except a very general basis is of doubtful value. 



Much of the work is concerned with variations in coastal and offshore waters, and relatively little 

 is known about conditions in the open ocean apart from the results of a few major oceanic expeditions 

 on which quantitative plankton work has formed part of the programme. The relative distribution of 

 the standing crop of plankton in the North and South Atlantic has been described in some detail by 

 Friedrich (1950), who used the Meteor data supplemented by observations in other regions not 

 covered by the 'Meteor'. These allowed areas of relative plankton density to be plotted on a chart in 

 a similar fashion to that of Henschel (1936) but with the geographical range considerably extended 

 (Friedrich, 1950, fig. 2, p. 113). It is not intended to treat the 'Discovery' data in such detail, but 

 rather will they be compared with the results of other expeditions, including those of the 'National', 



