CIRCULATION OF THE M ACROPLANKTON 4" 



direction respectively. There are changes of direction, eddies, and counter currents, and 

 it is quite possible that a species living exclusively in the Antarctic surface water could 

 find eddies on a scale large enough to bring back to high latitudes a certain proportion of 

 the adult population. By active propagation such a proportion might be able to restore 

 the full numerical strength of the species. The possible methods by which the plankton 

 could be replenished in an ever-moving mass of water, and the influence on horizontal 

 distribution of the reaction between vertical migrations and currents, might be dis- 

 cussed at great length. The object of this paper however is simply to show that there is 

 positive evidence that one of the suggested methods is actually used by some species, 

 namely an annual vertical migration between superimposed layers of water moving in 



different directions. 



It might be objected that an organism starting in the Antarctic surface water in a high 

 latitude might reach the Antarctic convergence in considerably less than half a year, or 

 that if the warm deep water moves more slowly to the south than the surface water 

 moves to the north the plankton organisms would need to spend most of their time in 

 deep water. But even in summer such forms as Rhincalanus are not entirely confined to 

 the surface layer, and there seems no reason why they should not adjust the speed and 

 range of their drift by resorting to the faster or more slowly moving parts of either layer 

 or by occasionally making short extra journeys from one to the other and so achieving 

 an equilibrium in their circulation. 



SUMMARY 



The preceding pages contain an account of an annual vertical migration which is 

 undertaken by certain important plankton species in the Southern Ocean. It is shown 

 that the three species which in places make up the bulk of the Antarctic macroplankton 

 are mainly concentrated in the surface water in summer, but descend into very deep 

 water in winter. Since there is a northerly component in the direction of movement of 

 the Antarctic surface water and a compensating southerly component in that of the 

 'warm deep water' it is to be supposed that this vertical migration results in a large- 

 scale circulation by means of which these species keep within the limits of their normal 



distribution. 



The circulation is on a remarkably large scale, the vertical range being from four to 

 six hundred metres and the horizontal range being presumably some hundreds of miles. 



The data are derived principally from lines of stations repeated at different times of 

 year by the ' R.R.S. Discovery II ' in the meridian of 8o° W, between about 55 S and 

 the edge of the pack-ice. A similar vertical migration is shown to have taken place in 

 other years and in other parts of the Antarctic, and there is little doubt that the pheno- 

 menon is normal and general in the higher latitudes of the Southern Ocean. 



