Nutrient biogeochemistry in a Gulf of Mexico subterranean estuary and groundwater‐derived fluxes to the coastal ocean

We hypothesize that biogeochemical processes in a subterranean estuary in the northeastern Gulf of Mexico significantly alter nutrient concentrations that are discharged into the ocean via submarine groundwater discharge (SGD). This subterranean estuary is a site of steep gradients over small spatial scales, so selecting the appropriate groundwater endmember for flux calculations is difficult. Two‐endmember mixing models typically applied to investigate biogeochemical processes in surface estuaries are not useful in this case because of aquifer heterogeneity and complex solid‐fluid interactions. Groundwater residence times, redox potential, source strength, and aquifer matrix appear to be the most important drivers for dissolved organic carbon (DOC) and nutrient concentrations in this pristine coastal plain aquifer. The distribution of dissolved species was consistent with a sequence of reactions in a narrow (~2 m) surface layer in which nitrate is initially exhausted (likely because of denitrification), organic nitrogen is remineralized releasing ammonium, and some DOC remains. On the basis of a 222 Rn‐derived non‐steady state box model, total (terrestrial + marine) SGD rates were estimated to oscillate around 11 cm d −1 . The SGD is composed almost entirely of recirculated seawater (accounting for ~95% of total SGD fluxes). Conservative estimates of groundwater‐derived nitrogen inputs into the coastal ocean were 8.2± 1.2 mmol m − 2 d −1 , consisting of ammonium (58%), organic nitrogen (28%), and nitrate (14%). We suggest that these mainly represent recycled rather than new nutrient additions. When extrapolated to the Florida Gulf coast, the nitrogen and DOC fluxes were comparable to the main regional rivers, supporting the proposition that SGD plays a major role in coastal biogeochemistry.