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

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 trace metal 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 barium (Ba) isotopes, radiogenic strontium (Sr) isotopes, and trace element composition in the shells and extrapallial fluid of six native freshwater mussel species, and two invasive co-occurring species in the upper Ohio River watershed to assess the influence of stream chemistry, taxonomy, and growth rates on isotope fractionation and elemental uptake.  We found that shells consistently recorded ⁸⁷Sr/⁸⁶Sr ratios of river water irrespective of species or growth rate. Stable Ba isotope compositions (δ¹³⁸Ba) of shells varied significantly with species and shell growth rates and provide additional information about uptake processes. 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 was opposite of the fractionation expected based on previous inorganic aragonite precipitation experiments.  Our analysis of extrapallial fluid demonstrates that Ba isotope fractionation occurs in two stages: during transport of Ba from river water to extrapallial fluid (up to 76% of the total fractionation) and from the fluid to the shell (up to 26% of the 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 also show that shell chemical makeup may be derived from sources other than dissolved river water, such as sediment or ingested food particles.

Using ANOVA statistical tests, we show that elemental composition of shells and extrapallial fluid also varied significantly with species, particularly for elements like Mn, Na, Ba, and Sr, underscoring the need to consider species-specific factors in environmental proxy studies.  Shells of native species consistently have higher concentrations of Mn, and lower concentrations of Na, than invasive species, demonstrating the strong effects of taxonomy in determining freshwater mussel shell composition. This work demonstrates the utility of isotope and trace element chemistry analysis of freshwater mussel shells, allowing us to determine which factors are the most important considerations in establishing geochemical proxies while providing foundational insight into the biochemistry of biomineralization.