Does Submarine Groundwater Discharge Play a Role in Seasonal Hypoxia in the Mississippi Sound?

Submarine groundwater discharge (SGD) is an understudied component of biogeochemistry in the Mississippi Sound, an estuary located in the Northern Gulf of Mexico. SGD includes both fresh groundwater discharge from aquifers and the circulation of seawater through sediments that occurs along the coastline and across the continental shelf. In July 2017, a low oxygen, or hypoxic, (< 2 mg/L) event took place in the Mississippi Sound causing a ‘jubilee’, where large masses of fish and crustaceans came ashore. We collected seawater samples at five locations along Mississippi beaches and analyzed them for chemical species typically enriched in SGD (i.e., Ra isotopes, Ba, methane). During this period of low oxygen, we observed an increase of groundwater tracers suggesting a correlation between the hypoxic conditions and groundwater flux. Dissolved methane (CH₄) and ²²⁴Ra (half life = 3.66 d) were significantly higher than what we observed during normal conditions. The high levels and short residence times of both indicators suggest that there was a recent release, and nearby source, of groundwater. Historically, low oxygen events have been observed in the Sound during the summer, often expressed as jubilees. Our initial results suggested that there might be a link between SGD and the observed seasonal oxygen depletion. Therefore, understanding the origins and forcing factors for local SGD may be an important aspect of predicting and managing hypoxia in the Sound. In order to determine if SGD may enhance or lead to hypoxia in the Mississippi Sound, we began an ongoing coastal time series examining radium, nutrients, barium, and oxygen data. Two more low oxygen events have been observed in the Sound during the ongoing time series, both associated with increased groundwater signatures. To better characterize the spatial distribution of SGD, radon surveys are also being conducted along the coast and throughout the Sound. Preliminary results indicate that SGD is highest along the coastline, and is highest in locations with historic jubilee events. However, more work is needed to determine the fluxes and impacts of SGD within the Mississippi Sound.