USE OF FRESHWATER BIVALVES (CORBICULA FLUMINEA) AS RECORDERS OF WATER COMPOSITION

Freshwater bivalves may serve as recorders of water composition much like their marine counterparts. We explored the use of mollusk shells as recorders of trace element concentration in surface waters. Several specimens of the fresh water bivalves, Corbicula fluminea (Müller, 1774) were collected from streams within the Department of Energy’s Savannah River Site in South Carolina after three years of monthly water sampling. The growth history and age of the bivalves were determined by stable isotope (d13C, d18O) analyses of their shells. The chemical composition of the water samples was determined by ICP-MS. The chemical composition of the bivalve shells was determined by ICP-MS, XRF-microanalysis and Laser Ablation-ICP-MS. Element concentration and stable isotope profiles were constructed from analyses of microgram-size subsamples of carbonate taken from the outer shell layer from the umbo to the outer shell margin along the radial axis of maximum growth.

Stable isotope profiles revealed that these clams were up to two years old, confirming that the period of water sampling covered the period of shell growth. Comparisons of oxygen isotope and trace element data indicated that the concentration of some trace elements (Sr, Ba) in the shell may vary seasonally. Comparison of trace element concentrations with organic content indicated that some elements (Mn, Fe, Cu, Pb) covaried with organic content.

Our results indicate that the incorporation of metals in bivalve shells is a complex process that depends on factors including the size and charge of the cation, the shell mineral structure, the amount of shell organic matter, water temperature and organism metabolism. In order to decipher the chemical record in bivalve shells, the effects of the above factors must be understood in order to accurately decipher environmental information from freshwater mollusk shells.