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 (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.