DYNAMICS OF DISTRIBUTION 



3Si 



Fig. 184. 



B 



W 

 X- 



w 

 x- 



a+b+c 



spectively the same positions as a A , a 3 , a 2 and a x in Fig. 182. At the end of a period of 24 

 hours they will be at the positions a, b, c and d in Fig. 184, b having joined a at B. At 

 the end of two days they will be at 

 the positions shown in Fig. 185. 

 At the end of three days all four 

 organisms will be together at B 

 (Fig. 186). We should get a belt 

 of zooplankton produced between 

 the area of moderate and very poor 

 phytoplankton. The water in the 

 region BC of poor phytoplankton 

 would be depleted of the zoo- 

 plankton organisms held up at B. 

 In our survey we find the zoo- 

 plankton reduced in regions of 

 poor phytoplankton. In Fig. 166 

 we have seen a belt of Euphausia 

 superba along the edge of a zone of 

 poor phytoplankton. These belts 

 when once formed might become 

 altered in shape by the action of 

 currents, and become concen- 

 trated in patches. The concentra- 

 tion of Euphausians upon the 

 coast of South Georgia in March 

 1926 might have been due to such 

 a cause, for here the phytoplank- 

 ton was poor and the Euphausians 

 high in the water, being taken in g " • 



surface nets during the daytime on several occasions, and it will be remembered (p. 273) 

 that immediately outside the concentration there was a region devoid of them. 



So far we have considered currents going in opposite directions or at different speeds 

 in the same direction. Now let us consider the extreme, 

 and in nature probably rare, alternative of currents 



travelling at right angles to one another. In our dia- /a* 



gram (Fig. 187) we will view them in plan and observe .^ 



the movements of organisms from a point within the 

 surface current. Let the two water masses be X and 

 Y, of which X is the surface current travelling in the 



direction AB and Y the under current travelling in the ^ p ^ r- 



direction EF. Let us further suppose that the deeper ^ 



we penetrate into the water mass Y the faster it is Fig. 187. 



Fig. 185. 



B 



la+b+c+d. 



.•a 



•a. 



