Fresh and saline groundwater nutrient inputs and their impacts on the nutrient budgets in a human-effected bay

Submarine groundwater discharge (SGD) can be highly enriched in nutrients, especially in bays with strong human activity, but has often been overlooked in coastal nutrient budgets. This study investigated the impact of both fresh and saline SGD on nutrient budgets in Sanmen Bay, China, a region heavily influenced by human activities. Based on the ²²⁴Ra mass balance model, the total SGD flux was estimated to be (1.1 ± 0.1) × 10⁸ m³ d⁻¹ (13.9 ± 0.5 cm d⁻¹). Additionally, a water-salt mass balance model revealed that fresh SGD flux accounted for ~9.0 % of the total SGD flux. The results highlight the significance of fresh SGD as a freshwater source, contributing to 35.9 % of the total dissolved inorganic phosphorus (DIP) flux via SGD. Considering all nutrient sources and sinks in the Sanmen Bay, SGD was identified as the primary source of nutrients in Sanmen Bay, contributing 53.9 % and 11.9 % of the total dissolved inorganic nitrogen (DIN) and DIP input, respectively. Furthermore, the discharge of industrial/domestic sewage and mariculture wastewater also posed a potential threat to nutrient levels in the bay. Thus, initiatives such as reasonable control of culture species and scale, strengthening wastewater discharge and SGD management are crucial for maintaining the ecological environment of the Sanmen Bay.