Hypoxia in the Chunsu Bay Bottom Water in Korea Induced by the Inland Fresh Water Discharge

This study investigated the stratification and hypoxia formation that resulted from freshwater inflow into the Chunsu Bay (CSB), Yellow Sea, Korea, during the summer season. Observations of the physical properties and water quality parameters of the CSB showed that a strong stratification formed between the upper and lower layers at a depth of 2­–5 m after freshwater inflow from the Kanwol and Bunam lakes. According to the results of 3D hydrodynamic modelling experiments, the static stability increased drastically due to the freshwater inflow, and a two-layer structure with a Richardson number > 20 was formed, limiting vertical mixing. A simplified two-layer DO budget model based on stratification strength between surface and bottom water column was designed to investigate hypoxia formation at bottom. The replenishment term through the boundary interface from surface water to bottom water took into account the proportional constant (P_R) for stratification strength, which was determined based on the Ri. According to the results of the budget modelling experiments, the DO concentration in the northern part of the CSB remained under hypoxic conditions of 2–3 mg L^-1 in the bottom water for several days during the freshwater inflow period. As the stratification strength increased, the DO supply was suppressed from the surface to the bottom layer, so that it leads to a reduction of the bottom DO concentration more rapidly. To conclude, strong stratification formation and its maintenance played a critical role in the hypoxia formation at bottom in the estuarine system.