Significant dissolved nutrient and carbon via fresh and saline groundwater discharge into a mariculture bay

Submarine groundwater discharge (SGD) is a critical pathway for transporting nutrient and carbon from land to coastal ecosystems, yet its partitioning into fresh SGD (SFGD) and recirculated seawater (RSGD) and associated solute fluxes remain poorly constrained in mariculture bays. In this study, we quantified different types of SGD and their derived dissolved nutrient and carbon fluxes in Xihu Bay, a typical mariculture-dominated semi-enclosed bay in Zhejiang, China, using a²²⁴Ra mass balance model combined with water-salt mass balance model. Results showed total SGD flux reached (2.5 ± 0.7) × 10⁷ m³ d⁻¹ (46 ± 12 cm d⁻¹), with SFGD contributing 3.5 % of the total SGD flux but only 1.6–7.5 % of SGD-derived nutrient fluxes (dissolved inorganic nitrogen: 136 ± 88 μmol m⁻² d⁻¹; dissolved inorganic phosphorous: 1.8 ± 0.60 μmol m⁻² d⁻¹; dissolved inorganic silicon: 89 ± 25 μmol m⁻² d⁻¹) and <8 % of carbon fluxes (dissolved inorganic carbon: 1352 ± 419 μmol m⁻² d⁻¹; dissolved organic carbon: 6.3 ± 2.6 μmol m⁻² d⁻¹). In contrast, RSGD dominated both dissolved nutrient (92.5–98.4 %) and carbon (>92 %) transporting, driven by tidal pumping and wave. With high nitrogen to phosphorous ratios in SFGD (54.5) and RSGD (77.9), it may exacerbate nutrient stoichiometric imbalance in Xihu Bay. Elevated nutrient and carbon concentrations in saline groundwater, highlighted RSGD as a key carrier of anthropogenically derived solutes. Given the absence of riverine inputs, SGD emerged as the predominant external source of nutrient and carbon, emphasizing its critical role in regulating biogeochemical cycles and ecological dynamics in aquaculture-impacted bays.