Multiple Stressors Increase the Mortality of Eastern Oyster (Crassostrea virginica) During Freshwater Discharge Events in an Estuary

The eastern oyster ( Crassostrea virginica ) is a commercially and ecologically important species across eastern North America and the Gulf of Mexico. In the Mississippi Sound, an estuarine system between the Mississippi River and Mobile Bay, Alabama, USA, oyster reefs have declined substantially over the past two decades due to multiple stressors, which may include increasingly frequent operations of the Bonnet Carré Spillway (BCS). This flood-control structure diverts Mississippi River water away from New Orleans into the western Mississippi Sound, which can create prolonged low-salinity and unsuitable conditions for oyster reefs. Although the Mississippi Department of Marine Resources (MDMR) has monitored reefs locally for decades, researchers have not used these data to evaluate how freshwater inflow anomalies affect oyster mortality. This study used a multi-decadal MDMR dataset to assess how freshwater discharge from BCS operations and co-occurring environmental stressors influenced oyster mortality. Environmental covariates included freshwater discharge from the National Water Model, salinity and temperature from USGS monitoring buoys, precipitation, dissolved oxygen, and BCS operational data. Oyster mortality varied interannually, but mortality rates and complete mortality events were strongly influenced by BCS operations and freshwater inflow volumes. Mortality was also influenced by rainfall, prolonged suboptimal salinity, and elevated water temperatures. These patterns are consistent with freshwater-induced mortality observed elsewhere across the US Gulf Coast. Projected increases in river discharge, rainfall intensity, and BCS operation frequency under climate change highlight the need to incorporate freshwater tolerance into oyster restoration and management strategies in the Mississippi Sound.