Coastal Hypoxia in the Jinhae Bay, South Korea: Mechanism, Spatiotemporal Variation, and Implications (based on 2011 survey)

Hypoxia (dissolved oxygen <= 2 mg L-1), which occurs frequently in coastal regions due to eutrophication, is a serious environmental problem in marine ecosystems. The areal extent of hypoxic regions has increased globally in recent decades. Jinhae Bay (JB) on the southeastern coast of South Korea has suffered from seasonal hypoxia due to increased anthropogenic activities since the 1970s. However, no intensive study has examined hypoxia in JB, although it is a scientific, social, and economic concern. We conducted monthly hydrographic surveys of JB in 2011 and present the mechanism of the hypoxia and its spatiotemporal variation there. The advent of hypoxic waters in the JB was initiated locally by the combination of developing stratification and increased benthic (bottom waters + sediments zone) remineralization in early June. From mid-July to early September, the hypoxia extended to the entire region, despite constant organic matter content in the benthic layer, due mainly to strong stratification, resulting in stagnant water circulation. During September, the hypoxia was maintained by a combination of physical and biogeochemical effects, although the areal extent of the hypoxic regions was substantially reduced. Overall, the hypoxia was present from early June until late September, with monthly spatiotemporal variation. The hypoxic waters tended to have low pH values, indicating an association with coastal acidification. JB, a small coastal region, suffers from serious environmental problems that urgently need our attention.