ZOOPLANKTON IN THE SOUTHERN OCEAN 219 



standing crop as represented in Fig. 10. It has already been noted, in the section on seasonal variations, 

 that the largest catches are very often those at stations just south of the Antarctic Convergence. 



Fig. 13 shows the total volume (1000-0 m.) at stations on five lines of observation in the meridan 

 of o° plotted according to their estimated position north or south of the convergence. The position 

 of the Antarctic Convergence in each case was taken from Mackintosh (1946), and the distances are 

 expressed in nautical miles. From the diagram it is evident that on each line there was a most marked 

 increase in volume in a region 0-200 miles south of the Convergence. It is of interest that in the 

 meridian of o° there is a second smaller increase (900 miles south of the convergence) which occurs 

 in the East Wind Drift. There are also instances of plankton concentrations near the Antarctic Con- 

 vergence in other sectors, notably in 8o° W at Sts. 991, 1224 and 1447; in 20 E at St. 2481 ; and in 

 160 E at St. 2206. 



These increases in concentration occur in latitudes well north of those areas which have yielded 

 the greatest catches of krill (Marr, personal communication), and in view of the fact that the Antarctic 

 surface current moves gradually northwards the presence of large concentrations of plankton in the 

 northern part of the Antarctic zone is consistent with the passive drift of plankton in this layer, in 

 which it is concentrated in the summer months. 



Table 7. The mean concentration (i.e. vol. per 50 m. haul) at all depths sampled and the mean total 

 volume (i.e. 1 000-0 m.) in each zone. Allowance has been made for there being different numbers of summer 

 and winter observations, except for the tropic zone where October observations only are available. Numbers 

 in brackets are the totals of winter and summer observations at each depth 



The present data have so far been treated in as detailed a manner as they allow. By using an even 

 broader treatment, it is possible to arrive at a tentative estimate of the relative standing crops of 

 plankton in the major geographical zones covered by the observations. 



Table 7 gives the mean concentration of plankton (i.e. volume per 50 m. haul) for all depth intervals 

 down to 1500 m., and the mean total quantity of plankton (1000-0 m.) in tropic, sub-tropic, sub- 

 Antarctic and Antarctic zones. Allowance has been made for the difference in each zone of the 

 number of summer and winter stations (except in the tropics, where only October results are available). 

 The results are plotted in Figs. 14 and 15 respectively. 



Fig. 14 shows that at all depths there is a general increase in concentration from tropic to 

 Antarctic waters, and that in each zone the greatest concentration occurs in the shallowest depth 

 horizon. 



Fig. 1 5 gives an overall picture of a greater standing crop of plankton in the Antarctic than in any 

 other region, the ratio tropic : sub-tropic : sub-Antarctic : Antarctic being roughly 1 : 1 -3 : 2-7 : 3 -3 . These 

 estimates are, of course, only tentative and refer to the limited part of the total zooplankton standing 

 crop sampled by the N70V. 



