SEASONAL-VARIATION IN OXYGEN ISOTOPIC COMPOSITION OF 2 FRESH-WATER BIVALVES - SPHAERIUM-STRIATINUM AND ANODONTA-GRANDIS

Oxygen isotopic values have been obtained from microsamples of the ara gonitic freshwater bivalves Sphaerium striatinum (Pisidiidae) and Anod onta grandis (Unionidae) collected alive from Wellington Creek, OH. To test whether these organisms secrete their shell in isotopic equilibr ium, the delta(18)O values of shell aragonite are compared to ambient water temperature and delta(18)O values monitored for > 1 yr. These bi valves were chosen for study because they are abundant in surface sedi ments and cores front Lake Erie where they represent a source of infor mation on the environmental history of the lake. The observed mean val ues are -5.54 parts per thousand for A. grandis and -6.16 parts per th ousand for S. striatinum. The mean delta(18)O value expected for bival ve aragonite if equilibrium precipitation is occurring during May-Augu st in Wellington Creek is -5.69 parts per thousand. The similarity bet ween 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 th an one half of the calculated range of potential biogenic aragonite va lues for the stream and omits recording evaporative conditions associa ted 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 a nd/or ingestion of food carbon.