The effects of submarine groundwater discharge and the Bonnet Carré Spillway on nutrient dynamics in the western Mississippi Sound.

The Mississippi Sound is an estuary in the northern Gulf of Mexico that is susceptible to eutrophication and hypoxia, both of which have led to habitat degradation, and organism stress and mortality. In this study, we explore potential forcing factors that impact the Sound's water quality such as local river flooding, submarine groundwater discharge (SGD), and the 2019 opening of the Bonnet Carré Spillway (BCS). Broad spatial surveys of radon along the coast and offshore indicated that areas prone to localized fish kills had higher levels of groundwater seepage. Nearshore water measurements of radium (Ra) were used to calculate the groundwater flux at five stations across the western Sound. These fluxes were on the order of ∼6 cm d. Measured reduced constituents (DON, NH and CH) introduced to the Sound from SGD have a high potential oxygen demand representing about half of the typical summer oxygen saturation. Limited measurements of S and estimates of DOC increase this demand further. Submarine groundwater discharge nutrient fluxes were also higher than that of the local rivers for reduced nitrogen species and phosphate, and when river fluxes are low, the groundwater is the dominant source of all nutrients to the Sound. However, when the Bonnet Carré Spillway was open, the western part of the Sound experienced rapid and severe drops in salinity, as well as high influxes of nutrients that changed the geochemical composition of the water that were unrelated to SGD or local rivers. Therefore, the influence of SGD, the BCS, and local rivers are all an important part of the Sound biogeochemistry, requiring consideration when making management decisions.