NATURAL VARIATION OF RADIONUCLIDE CS-137 CONCENTRATION IN MARINE ORGANISMS WITH SPECIAL REFERENCE TO THE EFFECT OF FOOD-HABITS AND TROPHIC LEVEL

Although a number of measurements have been made on radiocesium concen trations in aquatic organisms, no clear agreement has been reached on the factors affecting accumulation of these radionuclides. Natural var iations in the concentration of the long-lived artificial radionuclide Cs-137 in marine organisms and factors affecting variations in marine fishes were investigated through long-term and systematic measurement s in coastal waters of Japan from 1984 to 1995. Concentrations of Cs-1 37 were measured in more than 30 species of crustaceans, cephalopods a nd teleosts considered representative of the marine biotic community. A clear positive correlation (p < 0.05) was found between mean weight and concentration of Cs-137 in 276 fish samples. However, different re lationships between Cs-137 concentration and weight of fish were obser ved in different species. Within 16 studied species Cs-137 concentrati on increased with growth for 4 species, while no specific correlation was observed in the remaining species. These different patterns depend ed on a change of food habits with growth. Analysis of 6066 stomach co ntents of fish samples together with Cs-137 concentrations in the stom ach contents demonstrated that Cs-137 concentration increased with ris ing trophic level and that the biomagnification factor (Cs-137 in pred ator/Cs-137 in prey) was 2.0 (95 % confidence interval 1.8 to 2.2). Fr om the yearly change of Cs-137 in 24 marine fish species, a mean effec tive environmental half-life of Cs-137 Of 13 +/- 3 y, (range 10 to 17 yr) was calculated.