Tropical Beaches Attenuate Groundwater Nitrogen Pollution Flowing to the Ocean

Tropical urbanized coastal regions are hotspots for the discharge of nutrient-enriched groundwater, which can affect sensitive coastal ecosystems. Here, we investigated how a beach modifies groundwater nutrient loads in southern India (Varkala Beach), using flux measurements and stable isotopes. Fresh groundwater was highly enriched in NO₃ from sewage or manure. Submarine groundwater discharge and nearshore groundwater discharge were equally important contributors to coastal NO₃ fluxes with 303 mmol NO₃ m^–1 day^–1 in submarine and 334 mmol NO₃ m^–1 day^–1 in nearshore groundwater discharge. However, N/P ratios in nearshore groundwater discharge were up to 3 orders of magnitude greater than that in submarine groundwater, which can promote harmful algae blooms. As groundwater flowed through the beach, N/P ratios decreased toward Redfield ratios due to the removal of 30–50% of NO₃ due to denitrification and production of PO₄ due to mineralization of organic matter. Overall, tropical beaches can be important natural biogeochemical reactors that attenuate nitrogen pollution and modify N/P ratios in submarine groundwater discharge.