Significance of Submarine Groundwater Discharge in Nutrient Budgets in Tropical Sanya Bay, China

To quantify the contribution of submarine groundwater discharge (SGD) to the nutrient budget in tropical embayments, naturally occurring radium isotopes ( 223 Ra, 224 Ra, 226 Ra, and 228 Ra) 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 224 Ra/ 228 Ra, 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 226 Ra and 228 Ra, SGD was calculated to be 2.79 ± 1.39–5.07 ± 2.67 × 10 6 m 3 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 × 10 5 mol d −1 for oxidized inorganic nitrogen, 4.64 ± 2.74–8.42 ± 5.19 × 10 3 mol d −1 for phosphate, and 6.63 ± 3.29–12.0 ± 6.34 × 10 5 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.