Transplanted bivalves record subseasonal shifts in isotopic and elemental proxies for salinity in subtropical estuaries

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Abstract
Context: Geochemical proxies in bivalve shells can reconstruct past environmental conditions in dynamic estuaries, yet field-based validation of how quickly these proxies respond to ambient water fluctuations remains limited. Aims: This study evaluated eastern oyster (Crassostrea virginica) shell stable isotope ratios of carbon (δ13C) and oxygen (δ18O) and barium:calcium (Ba:Ca) ratio across a salinity gradient and quantified proxy equilibration times by using a field transplant experiment. Methods: Transplanted and natural shells from three estuarine sites in the north-western Gulf of Mexico were analyzed using high-resolution microsampling and laser ablation. All three proxies were compared within and between transplanted and natural shells to verify transplant-associated geochemical shifts, and Ba:Ca ratios were used to estimate equilibration timing following transplantation. Key results: Clear directional shifts in δ13C, δ18O and Ba:Ca ratios were observed. Following transplantation, Ba:Ca ratios indicated geochemical equilibrium within 31.2 ± 20.9 days. Whereas salinity appeared to be a major driver of temporal patterns recorded by all three proxies in transplanted and natural shells, additional fluctuations driven by other exogenous and endogenous drivers were present. Conclusions: Eastern oyster shells record rapid and subseasonal changes in water properties in subtropical estuaries where freshwater inflow varies spatiotemporally. Implications: These findings support the utility of oyster shells as bioindicators that archive past estuarine conditions.

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