Ecological hydrodynamic modeling and factor analysis of hypoxia dissipation in the semi-enclosed Mikawa Bay, Japan, in August 2020

A hypoxic water mass in Mikawa Bay, a semi-enclosed coastal area in Japan, dissipated significantly on August 20, 2020. Observational data suggest that the dissipation resulted from the advection of oxygen-rich water masses from the bay mouth and/or the open sea. However, the precise mechanism driving the dissipation of hypoxia remains unclear. In this study, using an ecological hydrodynamic model, we aimed to simulate the short-term dissipation of hypoxic water masses in Mikawa Bay. Sensitivity analysis was conducted to examine the interplay between density-, wind-driven, and tidal currents in the dissipation of hypoxic water masses and to identify the contributing factors. Although the modeled bottom-layer dissolved oxygen (DO) concentrations in the inner bay did not fully align with the observational data, the timing of increased DO levels was consistent between the observations and simulations, with a correlation coefficient of 0.78, root mean square error of 1.27 mg/L, and bias of -0.57 mg/L, indicating that the ecological hydrodynamic model effectively replicates the reduction of bottom-layer hypoxia in Mikawa Bay. Factor analysis revealed that density currents, driven by high salinity near the bay mouth, are the primary contributors to increased bottom-layer DO concentrations. Wind-induced and tidal currents influence DO distribution but are not primary drivers of elevated bottom-layer DO levels. Our findings highlight the importance of density currents in reducing hypoxic conditions, with significant implications for ecosystem dynamics and hypoxia management in coastal areas. These results could guide future studies and strategies to mitigate hypoxic events in similar environments.