Freshwater Endmembers Influencing the Dissolved Inorganic Carbon System of the Mississippi Sound

Increasing anthropogenic inputs of atmospheric carbon dioxide (CO₂) have led to increased CO₂ in the ocean, and in turn to ocean acidification (OA). There has been extensive coastal OA research in the northern Gulf of Mexico along the Louisiana Shelf but few studies have looked at the carbon dynamics along the Mississippi (MS) shelf. Our research focuses on dissolved inorganic carbon (DIC) and total alkalinity (TA) near the coast of the MS Sound, to understand the impacts of local rivers that supply alkalinity to the area to buffer against OA. This area also has anthropogenically altered hydrology, which has led to increased freshwater inputs from the MS River through the Bonnet Carré spillway (BCS). Data from monthly sampling trips along the MS coast from August 2018 to 2019 have been collected and analyzed, in addition to weekly sampling trips from June to August 2019 in response to the BCS opening that caused marine mammal and oyster reef mortality. Prior to the 2019 BCS opening, MS Sound TA values ranged from 1000-1950 µmol/kg, with corresponding salinities of 10 to 25 psu. While the BCS was open TA values within the 10 to 25 psu salinity ranged from 1550-2200 µmol/kg, showing an influence of MS River water in the Sound. The DIC in the Sound also increased during the opening of the BCS. Previous studies have shown that waters contributed from the MS River have higher TA and DIC than those supplied from the MS and AL rivers that also influence the Sound. The contribution of these less alkaline rivers to the Sound may leave the system more susceptible to OA with future increases in CO₂. However, the BCS has been opened more in the past5 decade than in the previous 2 decades. This increased MS River flux could represent a new norm in the Sound’s hydrography and it is important to determine the impact it may have on the inorganic carbon system.