Comparing the magnitude and mechanisms of submarine groundwater discharge (SGD) and associated nutrient fluxes in estuaries and coastal karst systems: the examples of mobile bay (usa) and maro-cerro gordo (spain)
Submarine groundwater discharge (SGD) is an important source of natural and anthropogenic nutrients and contaminants in coastal waters. Nutrient inputs from SGD can cause or exacerbate eutrophication, hypoxia, seagrass beds degradation, and harmful algal blooms (HABs), among other ecological impacts. Coastal karst systems and estuaries are among the most complex coastal areas, where the assessment of SGD and derived nutrient fluxes is particularly challenging. Their typically heterogeneous hydrogeology combined with temporal fluctuations of marine and terrestrial forcing result in large variations of SGD in both systems. In this dissertation I evaluated the magnitude and mechanisms driving SGD and its importance as a source of water and nutrients to Maro-Cerro Gordo (a coastal karst system in southern Spain) and Mobile Bay (an estuary of the northern Gulf of Mexico). In Maro-Cerro Gordo I found that SGD accounted for an important part of the water budget of the coastal karst aquifer, the only source of freshwater for nearby population and agricultural activities. Additionally, SGD served as a vector for NO3- fertilizers contamination to the sea, putting at risk the endangered species of the coastal ecosystem. In Mobile Bay I found that 1/4 of the nutrient budget is delivered by SGD during the dry season as NH4+ (56% of the total) and DON (15% of the total), mostly on the east shore, where Jubilees occur. I demonstrated that these SGD-derived nutrient inputs, in contrast to previous hypotheses, are originated naturally from organic matter mineralization in a peat layer found only on the east shore of the bay. In comparison, groundwater discharge in Maro-Cerro Gordo was primarily controlled by the terrestrially driven hydraulic gradient of the karst aquifer, while in Mobile Bay SGD was mainly marine-driven by sea level fluctuations (tidal pumping). Furthermore, the extremely fast groundwater flow of the karst aquifer in Maro-Cerro Gordo always created oxic conditions, allowing the NO3- contamination to reach the sea. In contrast, in the shallow coastal aquifer of Mobile Bay I found that, while the main form of nitrogen in inland fresh groundwater was NO3-, the SGD-derived nitrogen inputs to the bay was almost entirely in the form of NH4+. These large fluxes of NH4+ were produced by two main processes: organic matter mineralization and dissimilatory nitrate reduction to ammonium (DNRA).