Relative Importance of Food and Water in Long-Term Zinc−65 Accumulation by Marine Biota

Abstract

Shrimp, crabs, and fish were maintained for 3 mo in a 65Zn-labeled simulated ecosystem in which individuals were allowed to accumulate the radiotracer either directly from the water or from a combined food and water pathway. Shrimp and crabs, receiving 65Zn from the food-water milieu, did not attain significantly higher 65Zn body burdens than those individuals that accumulated the isotope from water only. For fish, the food pathway accounted for only 2.5 times more 65Zn than that obtained after direct accumulation from water. Specific activity measurements of both organisms and water as well as comparisons of radioactive concentration factors with those based on stable element measurements indicated that 65Zn in organisms had not reached isotopic equilibrium with the isotope in the water, even though net 65Zn accumulation in shrimp and crabs had ceased by the end of the experiment. It was concluded that there are zinc pools within adult organisms that exchange only slowly, if at all, with zinc atoms available in the organisms’ food or surrounding water. Hence, aquatic organisms such as those used in this study can most likely achieve true isotopic equilibrium only after living significant portions of their actively growing life stages in a radioactive environment.