. Fig. 184. B W X- w x- a+b+c spectively the same positions as aA, a3, a2 and ax 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
. Fig. 184. B W X- w x- a+b+c spectively the same positions as aA, a3, a2 and ax 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.
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