The major ocean currents have been described earlier (see page 9 ff.). Currents keep the water well mixed, influence the distribution of salinity and heat, bring to the surface the nutrients necessary for plant growth and carry down supplies of oxygen to deep levels. Because the bottom layers of water are in movement, it has been possible for a diverse benthic population to evolve which includes many forms living attached to or embedded in the sea bottom, and relying upon the flow of water to carry food and oxygen to them and waste products away. The benthic population is also influenced by the speed at which the bottom water moves because of effects on the nature of the sediment and the settlement of pelagic larvae.
Currents have a direct influence on the distribution of many species by transporting them from place to place, especially the smaller holoplanktonic forms. Many benthic and nektonic species also start life with a planktonic phase, and the direction of drift of their eggs and larvae must to some extent determine their areas of colonization. Currents probably also provide a means of navigation for animals such as turtles (Carr, 1974) and fish (Harden Jones, 1968). Some fish show a tendency to swim against the stream, at least during certain phases of life, and at the approach of the spawning period this behaviour is important in determining the position of spawning areas. Afterwards, the success of the brood depends upon the drift of larval stages to suitable nursery grounds.
How and to what extent can aquatic creatures detect water currents? Close to the bottom or any other fixed object they can obviously be aware of water movements through sight or touch; but where the water is flowing in a straight line at constant velocity, a floating organism out of sight or contact with fixed reference points can have little evidence of movement. Probably all animals are sensitive to acceleration and may therefore respond to velocity gradients or rotational flow of water. Because different water layers usually move at different velocities and often in different directions, there are possibilities of detecting water movement from effects of pressure or turbulence, or by observation of the movement of other floating objects above or below. There is also a theoretical possibility, though no firm evidence supports the idea, that organisms may be sensitive to the slight electromotive forces generated in the water or within themselves by movement through the earth's magnetic field.
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