Dynamics of the Physical and Biogeochemical Processes during Hypoxia in Jinhae Bay, South Korea

Bottom-water hypoxia occurs during the summer in Jinhae Bay, a semienclosed embayment in South Korea, and can have detrimental effects on marine ecosystems around the region. Measurements of the oxygen demand in the bottom water and sediments and time-series measurements of hydrographic characteristics were conducted in Jinhae Bay in 2015. Variations in dissolved oxygen (DO) can be divided into three distinct time periods. During period I (late March to late May), bottom DO decreased linearly with thermocline development. During period II (late May to late August), hypoxia persisted. During period III (after late August), bottom DO increased as the thermal stratification began to break down. The seasonal bottom-water oxygen concentration was significantly correlated with the temperature difference between surface and bottom water, indicating that hypoxia development and maintenance is strongly dependent on thermal stratification. Although high oxygen demand in the water column and sediments must affect summer bottom-water hypoxia, biogeochemical oxygen consumption may not be significantly correlated with the time of formation and dissipation of hypoxia. The sediment oxygen demand was comparable with the water-column oxygen demand (WOD) in period I, whereas WOD dominated the total oxygen demand in the bottom water in period II, indicating that water-column respiration played a major role maintaining the hypoxic conditions. The results suggest that physical processes, especially thermal stratification, are more important to the development of hypoxia than biogeochemical oxygen consumption in Jinhae Bay, where freshwater input is insignificant and water circulation and exchange with the outer bay are restricted.