Dominant contribution of land-based aquaculture to coastal organic matter and nutrient loads: Quantifying particulate organic carbon and ammonium fluxes from Jeju Island, South Korea

Land-based aquaculture farms (LAFs) are rapidly expanding on Jeju Island, South Korea, raising concerns about their environmental impact on coastal ecosystems. However, the quantitative contribution of LAFs to coastal nutrient loading remains inadequately understood. This study quantified the net fluxes of key pollutants from LAFs on Jeju Island by measuring concentrations of particulate organic carbon (POC), dissolved organic carbon (DOC), and dissolved inorganic nutrients (DIN, DIP, DSi) in the influent and effluent of four representative farms during the spring, summer, and autumn of 2019. Although the effluent concentrations of DOC, DIN, DIP, and DSi were comparable to influent levels, POC and ammonium (NH₄⁺) concentrations were significantly higher in the effluent (66 ± 24 µM and 24 ± 12 µM, respectively) than in the influent (7.5 ± 1.0 µM and 0.8 ± 0.2 µM, respectively), identifying them as primary pollutants associated with aquaculture activities. Effluent POC concentrations peaked in the summer, coinciding with increased feeding rates. Time-series observations capturing feeding-induced variations were used to estimate the net annual fluxes of POC and NH₄⁺ from all LAFs on Jeju Island at 35.7 × 10⁷ mol yr⁻¹ and 16.6 × 10⁷ mol yr⁻¹, respectively. Notably, the NH₄⁺ flux from LAFs was approximately 10 times greater than the combined fluxes from submarine groundwater discharge (SGD) and wastewater treatment plants (WWTPs), establishing LAFs as the primary source of bioavailable nitrogen in the coastal waters of Jeju Island. These findings highlight the urgent necessity for revising current environmental regulations by implementing nutrient-specific standards for aquaculture effluent to mitigate eutrophication and protect the ecological integrity of coastal ecosystems on Jeju Island.