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

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.