Examining the Importance of Submarine Groundwater Discharge (SGD) in a River Dominated Estuary: Example of Mobile Bay, AL

Harmful algae blooms (HABs) have been well documented along the Alabama coastline (Gulf of Mexico) in the last century. Recent studies suggest that HABs occurring on the Alabama coastline may be initiated and sustained by nutrients delivered by submarine groundwater discharge (SGD).

Mobile Bay, the fourth largest estuary in the US, is a river-dominated system that receives surface water inputs primarily from the Mobile-Alabama River System. While river discharge and associated nutrients fluxes are very well constrained from multiple gagging systems and extensive nutrients sampling, groundwater-born nutrient budgets have been largely overlooked due to the much larger (volume wise) river contribution. To assess the significance of groundwater inputs to Mobile Bay, we quantify SGD and its associated nitrate fluxes during both wet (March-April) and dry periods (July-August).

We found that even though groundwater represents only 7% of the water budget, nutrient fluxes delivered by SGD during low river flow regime account for 54% of the total nutrient budget in Mobile Bay. These findings are critical to understand HAB events in the Bay during the summer season, when potentially toxic diatoms find optimal conditions and the Mobile-Alabama River System is at its lowest stage.