Seasonal nutrient inputs from submarine groundwater discharge in an urbanized bay: Impacts on nutrient inventory and...

Groundwater is increasingly threatened by anthropogenic nutrient contamination, which can transfer terrestrial pollution loads into coastal environments through submarine groundwater discharge (SGD). However, accurate quantification of net SGD-derived nutrient fluxes remains limited, particularly at seasonal scales. Here, we assessed the SGD-derived nutrient fluxes into a semi-enclosed bay (Daya Bay) and evaluated their impacts on coastal nutrient dynamics during spring and autumn. The nutrient concentrations in groundwater were higher in spring than in autumn, reflecting a combination of the fertilization-driven leaching pulse during spring and precipitation-induced dilution during autumn. After correcting for nutrient losses associated with recirculated SGD (RSGD), the net flux of total nutrients delivered by SGD was estimated at (6.55 ± 4.56) × 10⁶ mol d^-1 in spring and (1.11 ± 0.45) × 10⁶ mol d^-1 in autumn, respectively. SGD was identified as the dominant external nutrient source to Daya Bay in both seasons by comparison with riverine inputs, sediment diffusion, atmospheric deposition and mariculture. The DIN:DIP of SGD-derived nutrients exceeds Redfield ratio in both seasons, potentially enhancing phosphorus limitation in the bay. In addition, the nutrients supplied by SGD could theoretically support higher primary production than other sources, yielding upper-bound estimates of (8.63 ± 6.12) × 10⁴ kg C d^-1 in spring and (3.63 ± 1.26) × 10⁴ kg C d^-1 in autumn. These findings demonstrate that SGD-derived inputs play a key role in shaping nutrient budgets and structure in Daya Bay, suggesting that non-point source pollution in coastal aquifers must be integrated into future marine management frameworks.