A Modeling Study on Hypoxia Formation in the Bottom Water of the Youngsan River Estuary, Korea

Hypoxia negatively affects marine environment and organisms and is an important indicator of estuarine water quality. We conducted a field survey and a three-dimensional particle transport modeling study to investigate summer hypoxia formation in the bottom water of the Youngsan River estuary (YRE). The field survey showed that dissolved oxygen (DO) decreased upstream and toward the bottom. The lowest DO, at 4.4 mg L-1, occurred in the bottom water in front of the estuary dam. We found persistent two-layer estuarine circulation driven by freshwater discharge, which is the main mechanism for delivering offshore water mass upstream. Applying the total oxygen consumption rate (0.50 mg L-1 d(-1)) observed in the estuary, the offshore water mass entering the estuary loses as much as 4.4 mg L-1 of oxygen during the average travel time (8.8 days) from the mouth of the estuary to the dam. These findings indicate that bottom hypoxia can be formed in front of the estuary dam in summer if offshore water mass with relatively low DO (<8.4 mg L-1) enters the estuary. For the sound management of water quality in the YRE, therefore, we recommend improving the quality of the offshore water entering the YRE and also decreasing oxygen demand by reducing nutrient loading from the YRE dam.