Drought and flood signals in subtropical estuaries recorded by stable isotope ratios in bivalve shells

Isotope ratios of carbon and oxygen recorded in biogenic carbonates can be effective proxies for ambient conditions in estuaries including salinity and temperature. Together, they have the potential to allow periods of drought and flooding to be identified in subtropical estuaries that receive stochastic and aperiodic delivery of freshwater inflow. We investigated the ability of δ13C and δ18O values in shell increments from the eastern oyster Crassostrea virginica sampled from subtropical estuaries in the western Gulf of Mexico to indicate differences in temperature and salinity dynamics at fine spatial scales. Oyster shells at locations that experienced both hypersalinity during droughts and dramatic decreases in salinity during floods showed distinct variations in shell δ13C and δ18O values that reflected local salinity conditions. In contrast, oysters at sites where no major salinity fluctuation occurred showed only seasonal fluctuations in isotopes reflecting temperature and possibly feeding patterns. Further, similar isotopic patterns were observed across multiple individuals from each site. Our results show that δ13C and δ18O values in shells measured together provide a powerful method to identify droughts and floods in subtropical estuaries and therefore extend records of dynamic inflow to these stressed ecosystems.