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the particular isotope to the edible marine biota is applicable. Any specific evaluation of a particular marine locale, such as a harbor in which nuclear ship testing and servicing are carried out, or which serves as a primary port for nuclear powered vessels, can be based on the study presented here, taking into account the actual utilization of that particular marine environment by man. BIOLOGICAL SIGNIFICANCE OF THE VARIOUS PARTS OF THE MARINE ENVIRONMENT With respect to the various physical and biological processes leading to contamination of man's food from the sea, the marine envi- ronment may be subdivided in the following manner: 1. The nearshore areas, including the intertidal zone of the open coast and the areas which are partially enclosed by land, i.e., harbors, bays, estuaries, lagoons, and passages separated from the open sea by fringing islands. 2. The continental shelf, and overlying waters. The continental shelf is the submarine rim of a land mass. It extends from the beach seaward and ends at a depth of about 200 meters with a slope descending more or less precipitously into the deep sea. It may be only a few miles wide, as it is along the coast of California, or it may reach out over 100 miles into the ocean, as it does off the North Atlantic coast of North America. 3. The deep sea beyond the continental shelf. An important further subdivision of the deep sea and the outer continental shelf must be recognized for our purpose. The upper 100 meters (more or less) from the shore outward is a layer in which the water is mixed by various processes brought about by wind and seasonal changes of temperature. A sharp density gradient of considerable thickness separates this mixed layer from the water below, which is generally more stable; the density gradient constitutes a barrier which impedes exchange between the deeper water and the mixed layer. The nearshore environment is the habitat of commercially im- portant oysters and clams; it is a principal habitat of some highly valued game fishes; it is an essential nursery ground for other commercial species. Seaweed industries gather their raw material in this area. It is the only part of the sea that can be cultivated like farmland; and it provides exceedingly important fishing grounds for thousands of men, most of them working with small craft. Upon the continental shelf live many commercially important in- vertebrates and fish, together with plants and animals of indirect eco- nomic value as food for the species utilized by man. Centers of fish population concentration shift diurnally and seasonally. There are few unpopulated areas. Certain commercial species range freely both within and below the mixed layer, over the continental shelf and the 11
