SPECIES-DEPENDENT TRACE METAL UPTAKE IN FRESHWATER BIVALVE SHELLS AND IMPLICATIONS FOR GEOCHEMICAL PROXY INTERPRETATION

Trace metal concentrations in bivalve shells are commonly used as geochemical proxies because the shell chemistry is known to reflect the chemistry of the water in which these animals lived. Freshwater environments, however, are home to a diverse assemblage of bivalve species, with varying shell structures, biochemical processes, and life history strategies. Initial work focused on variations in strontium (Sr), barium (Ba), and manganese (Mn) uptake into aragonite shells among several common freshwater bivalve species, and the relationship of these metal concentrations in shells to those of the river water. We sampled river water (filtered to <0.45 µm and acidified to 2% HNO₃) approximately monthly from several streams within the upper Ohio River watershed (Pennsylvania, USA), and collected shells at the same sites of several individuals of the following common bivalve species: Corbicula fluminea and Dreissena polymorpha (invasive species); and Amblema plicata, Fusconaia flava, Obliquaria reflexa, and Potamilis alatus (all Family Unionidae and native to the Ohio River watershed). For the shells, the most recently mineralized material from each specimen was powdered, dissolved in 1M acetic acid, and analyzed by ICP-MS.

Ratios of Sr/Ca and Ba/Ca from river water collected at different times from the same site varied by up to ±25% from the average, but generally by less than ±10%. In contrast, riverine Mn/Ca could vary by a factor of 1.2 to 33, making it difficult to compare to shell ratios. A preliminary analysis suggests that the Sr/Ca and Ba/Ca in invasive species C. fluminea shells are moderately correlated with river water concentrations (R² ≈ 0.6), but that this relationship in shells of native species is less straightforward. Furthermore, shells of invasive species D. polymorpha consistently have higher Ba/Ca than shells from native mussel species from the same river. Additional data from both water and shells will be presented. Our results highlight species-specific differences in trace metal uptake, which has implications for our interpretations of bivalve shell chemistry in paleoproxy and bio-monitoring studies.