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 fev/ 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 



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