ASSESSING THE IMPACT OF SUMMER FLOOD-INDUCED STRATIFICATION ON HYPOXIA IN THE ARIAKE SEA USING BUOYANCY FREQUENCY AS AN EVALUATIVE INDICATOR

Climate change is driving significant shifts in marine environments, altering coastal hydrodynamics and exacerbating hypoxia (dissolved oxygen < 3 mg/L) in many coastal waters. In the Ariake Sea, summer floods play a critical role in intensifying stratification, suppressing vertical mixing and oxygen replenishment, thereby increasing the frequency and severity of hypoxia. This presents substantial risks to benthic ecosystems and fisheries. This study used squared buoyancy frequency (N2) as a stratification indicator and investigated its relationship with hypoxia through a 30-year hydrodynamic and water quality simulation (1992–2021) using the Delft3D-Flow and WAQ modules. Specifically, the correlation between hypoxia and the rolling average of N2 over varying time windows (1–720 hours) following peak river discharge was analyzed. In addition, receiver operating characteristic (ROC) analysis was used to quantify the explanatory performance of N2, by comparing the area under the curve (AUC) and identifying the most effective threshold for predicting hypoxic events. Hypoxia classification was based on the median values of hypoxia duration (7.71 days) and hypoxia area (473.13 km2), which were used to define threshold conditions. Among the tested time-averaged N2 indicators, the 14-day (336 hours) rolling average of mean N2 (Nmean²) exhibited the strongest correlation with both hypoxia duration and hypoxia area. The ROC analysis identified 0.0090 s-2 as the optimal threshold for the 14-day rolling average of Nmean² in predicting hypoxia duration. When this threshold was exceeded, the probability of hypoxia duration surpassing the median value increased significantly. These findings demonstrate that 14-day Nmean² is an effective metric for stratifying and assessing the risk of hypoxia induced by stratification. The identified threshold could provide a valuable basis for forecasting hypoxic events and informing water quality management and ecosystem conservation efforts in the Ariake Sea.