Seasonal Variation in Oxygen Isotopic Composition of Two Freshwater Bivalves: Sphaerium striatinum and Anodonta grandis

Abstract

Oxygen isotopic values have been obtained from microsamples of the aragonitic freshwater bivalves Sphaerium striatinum ( Pisidiidae) and Anodonta grandis ( Unionidae) collected alive from Wellington Creek, OH. To test whether these organisms secrete their shell in isotopic equilibrium, the SO values of shell aragonite are compared to ambient water temperature and δ 18O values monitored for > 1 yr. These bivalves were chosen for study because they are abundant in surface sediments and cores from Lake Erie where they represent a source of information on the environmental history of the lake. The observed mean values are −5.54‰ for A. grandis and −6.16‰ for S. striatinum. The mean δ 18O value expected for bivalve aragonite if equilibrium precipitation is occurring during May–August in Wellington Creek is −5.69‰. The similarity between measured and predicted isotopic values for both species suggests that they are useful sources of paleoenvironmental data. Overall, the isotopic composition of the shells of the two species reflects less than one half of the calculated range of potential biogenic aragonite values for the stream and omits recording evaporative conditions associated with ponded water. Bivalve δ 18O and δ 13C data covary. The δ 13C data are highly negative and values could reflect 12C enrichment of dissolved organic carbon from organic matter oxidation and/or ingestion of food carbon.