Threshold-based analysis of eutrophication dynamics in a semi-enclosed bay: The dominant role of dissolved inorganic nitrogen in Masan Bay, South Korea (2010–2015)

Eutrophication in semi-enclosed coastal systems remains a persistent concern due to restricted water circulation and continuous nutrient inputs. To better characterize nutrient-driven water quality degradation, we applied a threshold-based analysis to seasonal monitoring data collected from Masan Bay, South Korea, between 2010 and 2015. Eutrophication status was assessed using a composite Eutrophication Index (EI) integrating chemical oxygen demand (COD), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP). Although COD remained stable, EI increased significantly over time (p < 0.05), indicating progressive degradation. Segmented regression identified two DIN thresholds (8.74 and 18.9 μM), above which EI increased markedly. These thresholds provide a quantitative basis for identifying high-risk nutrient conditions and assessing ecological vulnerability. DIN contributed most to EI variability (52.7 %), exceeding the influence of DIP and COD. High EI values (EI > 17.05) occurred most frequently during summer and autumn at inner bay stations with limited circulation. Simultaneously, total phytoplankton abundance declined significantly (p < 0.01), and the community composition shifted from Bacillariophyceae to Dinophyceae. This shift was associated with nutrient imbalance and oxygen stress, with phosphorus limitation and nitrate enrichment favoring Dinophyceae, and hypoxia further supporting their prevalence. Our findings emphasize the utility of DIN-based thresholds in capturing eutrophication risk and support the integration of nutrient and biological indicators into coastal water quality management frameworks.