Seasonal variation in oxygen isotopic composition of two freshwater bivalves: Sphaerium striatinum and Anodonta grandis

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 delta(18)O values of shell aragonite are compared to ambient water temperature and delta(18)O values monitored for > 1 yr. These bivalves were chosen for study because they are abundant in surface sediments and cores front Lake Erie where they represent a source of information on the environmental history of the lake. The observed mean values are -5.54 parts per thousand for A. grandis and -6.16 parts per thousand for S. striatinum. The mean delta(18)O value expected for bivalve aragonite if equilibrium precipitation is occurring during May-August in Wellington Creek is -5.69 parts per thousand. The similarity between measured and predicted isotopic values for both species suggests that they are useful sources of paleoenvironmental darn. 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 delta(18)O and delta(13)C data covary. The delta(13)C data are highly negative and values could reflect C-12 enrichment of dissolved organic carbon from organic matter oxidation and/or ingestion of food carbon.