Barium Isotope Fractionation, Radiogenic Strontium Isotope Signatures, and Trace Element Partitioning in Freshwater Mussels of the Upper Ohio River Watershed: Examination of Shells, Extrapallial Fluid, and River Chemistry

Abstract

The chemistry of biogenic carbonates, such as mussel shells, serve as valuable proxies for reconstructing past environmental conditions; however, interpretation requires a thorough understanding of how both environmental and biological factors affect element uptake. Mussel shell chemistry studies are particularly important in freshwater systems because freshwater mussels are severely imperiled, and elucidating the factors affecting shell chemistry can clarify our understanding of their unique biology. Because invasive mussels often co-occur with imperiled native freshwater mussel species in North American watersheds, there is growing interest in whether these invasive taxa differ in their elemental uptake and how they interact with and affect the health of native species, potentially influencing habitat or food availability.

We examined Ba isotopes, radiogenic Sr isotopes, and trace elements in shells and extrapallial fluid of 6 native freshwater mussel species, and 2 invasive co-occurring species in the Ohio River watershed to assess the influence of stream chemistry, taxonomy, and growth rates on isotope fractionation and elemental uptake. Shells of native species consistently have higher Mn concentrations, and lower Na concentrations, than invasive species, demonstrating strong effects of taxonomy in determining mussel shell composition. We found that shells consistently recorded ⁸⁷Sr/⁸⁶Sr ratios of river water irrespective of species or growth rate. Ba isotope compositions (δ¹³⁸Ba) of shells varied significantly with species and shell growth rates. Notably, native species shells of slower growth were more enriched in lighter Ba isotopes, whereas invasive shells of faster-growing C. fluminea were isotopically heaviest, which is opposite of what would be expected based on inorganic aragonite precipitation experiments. Analysis of extrapallial fluid demonstrates that Ba isotope fractionation occurs in 2 stages: during transport of Ba from river water to extrapallial fluid (up to 76% of total fractionation) and from fluid to shell (up to 26% of total fractionation). The largest magnitude fractionation therefore occurs prior to shell mineralization, and it is this fractionation step that is species-specific. Using Ba isotopes and trace metal chemistry of suspended particulate and sediment porewater, we found that shell chemical makeup may be derived from sources other than dissolved river water, such as sediment or ingested food particles.