Using Radium Isotopes to Compare Nutrient Inputs via Submarine Groundwater Discharge to Offshore Export on the West Florida Shelf

Radium isotopes have been used to trace submarine groundwater discharge (SGD) and associated nutrient and metal inputs into the coastal ocean. They have also been used to estimate rates of diffusive mixing and export fluxes of dissolved constituents offshore. However, use of both approaches in tandem to determine the fate of groundwater inputs after they enter the coastal ocean is still relatively unexplored. Radium isotopes and nutrients were measured in two cross-shelf cruises on the West Florida Shelf in different seasons and in quarterly sampling of offshore wells. Through 226Ra mass balance and 223Ra-derived diffusive mixing rates, inputs of nutrients to the shelf through SGD and transport of nutrients towards the shelf break were quantified for the winter season in February and summer season in July. Radium distributions and estimated mixing rates varied considerably between each season, as did physical stratification. However, calculated fluxes of SGD and associated nutrient loading were fairly similar. Balances of nutrient inputs to exports varied by species, with effectively all silicate inputs making it offshore of approximately the 25 m isobath, whereas nitrogen was exported nearly completely but underwent transformation from the aquifer to the offshore edge of the considered shelf area; while nitrogen inputs consisted of both inorganic and organic nitrogen, all nitrogen exports were in the form of organic compounds (with potentially limited bioavailability). Phosphorus exports could not be well-constrained with the available measurements. Radium isotopes proved to have considerable utility in quantifying the role of SGD in biogeochemical cycling on the continental shelf and in assessing the balance of nutrient inputs to the shelf versus exports offshore.