To quantify the contribution of submarine groundwater discharge (SGD) to the nutrient budget in tropical embayments, naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) were investigated as SGD tracers in Sanya Bay, China. Higher activities of radium were present along the north coast and near the Sanya River estuary. Using the activity ratio of 224Ra/228Ra, the apparent water age in Sanya Bay was estimated to be 0–13.2 days, with an average of 7.2 3.2 days. Based on the mass balance of 226Ra and 228Ra, SGD was calculated to be 2.79 1.39–5.07 2.67 106 m3 d-1 (or 4.3 2.1–7.8 4.1 cm d-1). SGD associated dissolved inorganic nutrient fluxes into Sanya Bay were estimated to be 3.94 2.00–7.15 3.85 105 mol d-1 for oxidized inorganic nitrogen, 4.64 2.74–8.42 5.19 103 mol d-1 for phosphate, and 6.63 3.29–12.0 6.34 105 mol d-1 for silicate. The estuarine nutrient flux from the Sanya River was a few times smaller than the phosphate flux via SGD and at least an order of magnitude smaller than the oxidized inorganic nitrogen and silicate fluxes carried by SGD. SGD was also more important than atmospheric deposition and nitrogen fixation in the nutrient budget. Our results demonstrate that SGD compensated for at least 15% phosphate, 90% oxidized inorganic nitrogen, and 60% silicate of the nutrients sink in Sanya Bay.
