Constantly drifting downward is a great volume of material—the dead bodies, skeletons, excrement, and other waste from sea life at all depths. As it sinks there is a constant exchange of matter between it and the surrounding water through chemical, physical, and biological processes. Eventually, the molecules of material added to the bottom sediments may be returned to the water mass by bacteriological action or the eating and living habits of sea floor animals. [Illustration: _A school of skipjack tuna photographed from an underwater observation chamber on the research vessel_ Charles H. Gilbert.] Biological transport works in other ways, too. Most pelagic (free-swimming) fish are great travelers. They account for a tremendous movement of material, namely themselves, from one place to another. Tuna, swordfish, whales, porpoises, and sea birds may travel thousands of miles in a single year. Such migrations may serve, variously, as mechanisms for either dispersal or concentration of elements or nutrients. The anadromous (river-ascending) fishes, such as salmon, herring, sturgeon, and shad, concentrate in freshwater streams in untold numbers to spawn. After hatching, the young seek the ocean and scatter widely until they, too, feel the urge to return to the rivers and lakes whence they came, to spawn and die there as did their ancestors. Ocean currents may transport concentrations of radionuclides essentially undiluted for thousands of miles. Surface currents move at speeds of up to five knots (nautical miles per hour). Normally current waters do not mix readily with the water mass through which they pass. Because of the slowness of vertical circulation in the ocean, radionuclides deposited on the surface of the ocean may take a thousand years to reach the bottom. But the vertical transport sometimes is much more rapid: When the wind piles too much water against a coastline, the resultant downwelling (sinking) may move radionuclides suddenly into the deeper ocean. Or, conversely, when the wind and the rotation of the earth combine to force the surface water _away_ from the coast, deep water may suddenly rise to replace it, a process known as upwelling. [Illustration: _Mechanisms of nutrient turnover in the sea._]