380 DISCOVERY REPORTS 



It will be remembered that the most southerly station, that is the left-hand end of each 

 section, is at the edge of the pack-ice. The number of organisms counted from each 

 haul is plotted midway between the beginning and end of the haul. For example at 

 St. 1220 in the haul from 500 to 250 m. 328 organisms were counted. The figure 328 

 is therefore placed at a depth of 375 m. Fig. 4 is drawn in just the same way, but here 

 the vertical scale is undistorted, and the figures are replaced by shaded contouring 

 which shows much more clearly the distribution of the main concentrations of plankton. 

 The contours run through arbitrary numbers, in this case 20, 100, 500, etc. It would 

 probably be safe to say that this macroplankton picked out and counted far exceeds in 

 bulk of organic matter the residue of smaller organisms left in the samples. 



The sections show that in December the largest numbers are concentrated near the 

 surface, especially in the neighbourhood of the Antarctic convergence. At this time also 

 there seems to be more plankton in sub-Antarctic than in Antarctic water. The undula- 

 tions in the contour between 750 and 500 m. are no doubt an effect of the diurnal migra- 

 tions of certain species. 



In March the numbers of organisms have increased. The largest numbers are still 

 near the convergence but there has been a marked development of plankton in the more 

 southerly part of the Antarctic surface layer. The most concentrated regions are now 

 even closer to the surface than in December, but a considerable quantity of plankton 

 appeared, presumably by sinking from above, in deep water on the north side of the 

 convergence. 



In September (late winter) the distribution of the plankton is entirely changed. In 

 the first 100 m. of water, which in summer contained the vast majority of organisms, 

 there is now less than at any other depth above 1500 m., and the main concentration lies 

 between 750 and 500 m. The total quantity of plankton at all depths is evidently rather 

 less than in summer, but it is spread over a greater range of depths. 



In October we find a reversion towards the summer conditions, but the plankton has 

 not quite risen as high as the December level. The most consistently large catches were 

 those from 250-100 m. and the majority of organisms in them may sometimes have 

 come from the top of the warm deep layer and sometimes from the lower part of the 

 surface layer. 



In November (about two weeks later) the plankton seems on the whole to have risen 

 a little higher than in October. 



Both the Antarctic surface layer and the upper part of the sub-Antarctic water move 

 gradually towards the north, and it can hardly be doubted that, when the plankton is 

 distributed as it was in 8o° W in December and March, the bulk of it must be carried 

 northwards. It seems certain also that the September distribution must involve a 

 southward movement of the plankton. We have evidence then of a general circulation 

 on a very large scale, the majority of organisms drifting northwards in the surface layers 

 in summer, and returning southwards in the warm deep water in winter. 



The figures for the total numbers of organisms suffice to show the general effect of 

 this movement, but there are marked differences in the behaviour of separate species. It 



