Plants and animals have a capacity, which varies from species to species, for concentrating specific radionuclides. The level to which an isotope is concentrated by any organism can vary with season, geographical location and other factors not completely understood at this time. Plants and animals play an important role in the cycling of radionuclides in the sea not only while living, through metabolism, but also after death, through the process of decomposition. The availability of radionuclides to marine organisms is dependent to a large extent upon the physical state in which the isotopes occur in seawater. Both particulate and dissolved radionuclides can enter into the biogeochemical cycles of the marine environment. These materials can: (1) remain in solution or in suspension, (2) precipitate and settle on the bottom or (3) be taken up by plants and animals. Immediately upon introduction of radioactive materials into the water, certain factors interact to dilute and disperse these materials, while simultaneously other factors tend to concentrate the radioactivity. Radioactive materials upon introduction into the marine environment may be cycled through the following three components: water, sediment and biota. This cycling can be described by the three R's: routes, rates and reservoirs. Each radionuclide takes a characteristic route and has its own rate of movement from component to component prior to coming to rest in a reservoir, one of the three components of the marine environment. Plants and animals to be of any significance in the cycling of radionuclides in the marine environment must accumulate the radionuclide, retain it, be eaten by another organism and be digestible. Organisms can obtain radioactivity by absorption, adsorption and ingestion. Conversely, radionuclides may be lost by excretion and decomposition. Sediments may accumulate radionuclides through the physical processes of exchange and adsorption. Water acts as the principal medium of transport between the biota and sediments.
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