DYNAMICS OF DISTRIBUTION 343 



VERTICAL MIGRATION AND THE DYNAMICS OF 



DISTRIBUTION 



The horizontal paths of movement of planktonic animals and plants in relation to the 

 sea bed or the land will vary according to whether the animal or plant undergoes vertical 

 migration. Organisms which have no vertical migration will simply be carried along in 

 the one water mass they happen to be in, but the path of those undergoing vertical 

 migration will be complicated by the differing speed and direction of the water layers 

 between which they pass. The movements of the former need no special consideration ; 

 they are the same as the water movements. The present section will be devoted to a 

 discussion of the movements and consequent distribution of vertically migrating 

 animals 1 brought about by different current systems. In the area we are studying we 

 have seen (on pp. 231-250) that vertical migration is an important characteristic of all 

 but a few of the commoner plankton animals, and that the majority are moving between 

 depths of at least 100 m. and the surface, and many are rising from considerably deeper 

 levels. We shall see that not only will their movements be complicated by the current 

 systems, but that their relative abundance in this place or that may be governed by the 

 same systems. We can deal conveniently with the types of movement under three 

 headings. Firstly, movements consequent upon both the speeds of the current systems 

 and the extent of vertical migration remaining more or less constant. Secondly, move- 

 ments consequent upon the speeds of the currents remaining constant but the extent of 

 vertical migration varying, and, thirdly, those consequent upon a relative acceleration 

 or retardation of the different water layers with the migration remaining constant or 

 varying. Under the last heading will be included the acceleration or retardation of sur- 

 face masses by the action of wind, and the retardation of lower masses by the contours 

 of the sea floor. 



In Fig. 6 on p. 10 we show a chart of the surface water movements around South 

 Georgia kindly prepared for us by Mr A. J. Clowes by means of dynamic calculations ; 

 Mr G. E. R. Deacon has also kindly prepared one, Fig. 171, showing in less detail the 

 general movements at a depth of 150 m. Neither hydrologist would regard these charts 

 as exact, but they are as nearly correct as can be estimated from the data available. By 

 considering both charts we can work out the approximate path that would be taken by 

 animals migrating between 150 m. and the surface, assuming that the speed of the 

 currents and the range of vertical migration remain more or less constant. Fig. 172 

 shows an example. It represents in plan an area to the north-east of South Georgia 

 bounded approximately by latitudes 53 20' and 54 20' and longitudes 34 and 38 W. 

 In this diagram, and the four which follow, the fine continuous lines represent the direc- 

 tion of the surface currents and the fine broken lines that of the lower currents, in this 

 case those at 150 m. The heavy black lines represent the paths of vertically migrating 



1 The term "animals" will be used rather than "organisms", because it is simpler to use frequently, and 

 because so far we have only found one plant Coscinodiscus (see p. 45) to undergo vertical migration. But 

 what is said regarding animals will apply equally well to plants similarly migrating. 



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