Assessing stratification and hypoxia mitigation through sluice gate operation in a brackish lake: a case study of Saemangeum Lake, South Korea

Abstract

Salinity-driven stratification and bottom-water hypoxia are persistent water quality problems in semi-enclosed estuarine lakes. This study assessed how sluice gate operations influence stratification and dissolved oxygen (DO) dynamics in Saemangeum Lake, South Korea, by integrating long-term monitoring data (2013–2022), high-resolution field observations, and three-dimensional Delft3D modeling. Water-column stability was evaluated using the potential energy anomaly (PEA) and Richardson number (\(\:{R}_{i}\)), and the spatial extent of hypoxia was quantified under different gate-operation regimes. Persistent stratification was indicated by log(PEA) ≥ 1, while \(\:{R}_{i}\) remained ≥ 0.25 and commonly exceeded 100 under weak resolved vertical shear associated with reduced tidal forcing behind the dike, consistent with limited oxygen transfer to the sub-halocline layer. Depending on the operation schedule, hypoxic area (DO < 2 mg L⁻¹) ranged from 45 to 100 km², and hypoxia developed within 6–7 days when seawater exchange was suspended. Scenarios exchanging similar annual volumes of seawater produced opposite changes in hypoxic extent, and alternating operation matched the improvement obtained by raising the controlled water level while reducing annual inflow by 43% relative to the reference condition. These results indicate that gate-operation pattern is an important control on hypoxia and that inflow volume alone does not determine the response, providing a practical basis for water quality management in brackish lake systems.