Vertical water renewal and dissolved oxygen depletion in a semi-enclosed Sea

Abstract

The limited water renewal in semi-enclosed seas may contribute to the oxygen deficiency and thereby may exacerbate the ecological degradation. Previous research has focused more on horizontal water renewal, while the role of vertical water renewal remains poorly documented despite its significance to the bottom oxygen supply. This study explores the vertical water renewal process in the Bohai Sea, a semi-enclosed sea prone to oxygen deficiency, by developing a water age model, incorporated into a developed Delft3D hydrodynamic model. Findings unveiled the key role of thermal stratification, tides, and winds on vertical water renewal. Results revealed significant spatial heterogeneity and seasonal variations in water age in the Bohai Sea. During summer, stratified from the upper water column, the deep basin water age can reach up to 30 days. Thermal stratification was crucial in regulating vertical water renewal, acting as a constraint and impediment. Spring tides and neap tides can positively and negatively influence vertical water renewal by strengthening and weakening thermal stratification, respectively. Wind forces intensified the vertical water exchange by changing the flow field structure. Additionally, higher bottom water age coincides with the oxygen-poor areas and deep-water regions. Based on this, we employed the product of the oxygen consumption rate and the bottom water age (oxygen consumption duration) to approximate oxygen depletion. This method was applied to successfully estimate vertical oxygen depletion in the Bohai Sea. The findings will serve environmental management and oxygen-poor attribution analysis.