Submarine Groundwater Discharge and Nutrient Fluxes Along the Bight of Benin Shoreline

Submarine groundwater discharge delivers dissolved nutrients, carbon, and trace metals to coastal waters through a diffuse seepage process that has been chronically underestimated in coastal nutrient budgets. This study quantifies submarine groundwater discharge and associated nutrient fluxes along a 60-kilometre segment of the Bight of Benin shoreline between Lagos and Badagry, South West Nigeria, using a multi-tracer approach. Radium quartet activities (224Ra, 223Ra, 228Ra, 226Ra) and radon-222 activities in nearshore and offshore coastal waters are measured using RaDeCC radium delayed coincidence counting and RAD-7 radon-in-water systems, respectively, to estimate submarine groundwater discharge rates through mass balance calculations. Porewater samples extracted from intertidal beach sediments at eight transects spanning various land use settings (residential, commercial, agricultural) are analysed for dissolved inorganic nitrogen, dissolved inorganic phosphorus, dissolved silica, and dissolved organic carbon. Porewater-to-coastal water concentration gradients are used to compute net nutrient fluxes associated with the estimated discharge rates. Terrestrial and recirculated marine submarine groundwater discharge components are distinguished using salinity-endmember mixing analysis. Findings are expected to reveal that submarine groundwater discharge contributes up to 35 percent of total dissolved inorganic nitrogen inputs to the inner shelf during the dry season, with hotspots of discharge near densely developed urban and peri-urban waterfront areas. The implications for eutrophication risk and harmful algal bloom potential are discussed. Keywords: submarine groundwater discharge, radium tracers, nutrient fluxes, Bight of Benin, coastal eutrophication