Spatiotemporal dynamics of hydrodynamics and water quality in Liaodong Bay under seasonal ice cover: A coupled 3D modeling and WQI assessment

The northern waters of Liaodong Bay (LDB) frequently exhibit low water quality due to the region's semi-enclosed hydrological conditions, high terrestrial pollutant inputs, and seasonal sea ice cover. Existing studies have not systematically elucidated the comprehensive regulatory mechanisms of sea ice on the marine environment. A three-dimensional (3D) hydrodynamic module was coupled with the ECO Lab module to reveal the spatiotemporal evolution of water quality parameters in the study area under ice-covered conditions, and subsequently a comprehensive water quality index (WQI) assessment was conducted. Sea ice exerts both thermal insulation and flow-suppressing effects: it elevates sub-ice water temperature, weakens flow velocity and vertical mixing, and reduces the extent of low-salinity zones around river estuaries. Dissolved oxygen (DO), nutrients, and chlorophyll a (Chl-a) all exhibit higher concentrations near estuaries and lower levels offshore, with their seasonal peak values appearing at different times. The overall water quality of LDB is rated as good, with an annual average WQI of 86.32, although substantial spatial differences and seasonal fluctuations are observed. Offshore water quality is superior to that near estuaries; seasonally, summer exhibits the most severe pollution and poorest water quality, whereas ice-covered winter shows the best. This study clarifies the integrated mechanisms of seasonal sea ice on hydrodynamic, thermohaline, and ecological variables within semi-enclosed bays, and fills the research gap regarding coupled water environment simulation for cold-region ice-covered bays.