Model-based analysis of seasonal hypoxia: The Varna Lake–Bay case study

This study employs a coupled 3D hydrodynamic–biogeochemical modeling framework to investigate the spatiotemporal evolution of seasonal hypoxia in the semi-enclosed Beloslav Lake–Varna Lake–Varna Bay system, located along the western Black Sea coast. Using realistic meteorological forcing and four nutrient-loading scenarios, we quantify the roles of stratification, internal organic cycling, and external nutrient inputs in shaping oxygen dynamics. The model successfully reproduces seasonal stratification and captures a distinct surface–bottom decoupling in the distributions of nutrients and organic matter. Under baseline conditions, hypoxia is largely confined to Beloslav Lake in late summer. However, under elevated nutrient loads from both river discharge and a wastewater treatment plant, hypoxic and anoxic zones expand significantly, affecting up to 75% of the bottom area in Beloslav Lake and over 50% of Varna Bay. Vertical oxygen depletion also intensifies, with suboxic conditions extending 10–15 meters into the water column. Daily variability, driven by wind forcing and internal waves, leads to rapid spatial shifts in hypoxia, highlighting the importance of temporally resolved monitoring. The model underscores the cumulative and dynamic nature of bottom-water oxygen depletion in shallow, stratified systems and provides volumetric, spatial, and temporal metrics for ecological risk assessment. The results demonstrate the system’s vulnerability to even moderate increases in nutrient loading and offer actionable insights for regional management in line with the EU Water Framework Directive. The modeling framework also shows strong potential for assessing future climate or land-use change scenarios and for guiding mitigation strategies in complex estuarine and coastal environments.