FRESH-WATER BIVALVES (UNIONIDAE), DISEQUILIBRIUM ISOTOPIC FRACTIONATION, AND TEMPERATURES

A one-year study of two ponds and one river in Rhode Island was implem ented to determine whether Recent freshwater bivalves (Unionidae: Elli ptio complanata) secrete aragonite in oxygen isotopic equilibrium with the water in which they grow. Water samples and animals were retrieve d from each locality at two-week intervals for one year. The oxygen an d carbon stable isotopic composition of the youngest growth increment of the shells (shell edge) and the oxygen isotopic composition of wate r were measured and compared with the temperatures recorded during sam pling, via published equilibrium fractionation relationships. Elliptio does not secrete shell material in equilibrium with the ambient water . The disequilibrium is manifested as an offset of approximately -2 pa rts per thousand in the summer and -2.5 parts per thousand in the wint er. Incorporation of metabolic CO2 may contribute in large part to thi s offset.