Physical and Biogeochemical Characteristics of Hypoxia in the Jinhae Bay, South Korea

Hypoxia (O₂≤2 mg L^-1 or ∼63 μmol L^-1) frequently develops during the summer by combination of high biological production and strong stratification in the coastal oceans. The Jinhae Bay (JB), located in the southeast coast of Korea, is well recognized as an area of chronically seasonal hypoxia in sub-surface waters during the summer season. Rapid industrialization and intensive aquaculture systems have caused severe eutrophication more and more in the JB since 1970s. Although we have concerned ‘Hypoxia formation’ in the JB as a socio-economic and scientific issue, we have not yet sufficient information about formation, location, and extent of hypoxia in the JB. Here, we present preliminary results from an investigation into the mechanism and extent of hypoxia in the JHB, using monthly observed hydrographic data between 2011–2014. Eutrophication was initiated in spring (March–April) due to increase in nutrient loading by river input with the beginning of water-column stratification. High primary production caused to deliver abundant fresh organic matters into the bottom waters and sediments, fueling water-column and benthic respirations. These processes mechanistically formed bottom water hypoxia in the JB. Hypoxia was perennially formed in inshore regions, but the spatial and temporal variation (i.e., increasing or decreasing) in the aerial extent/thickness of hypoxia was not obvious. However, the areal extent and frequency of suboxic-anoxic condition (O₂≤0.14 mg L^-1 or ∼12 μmol L^-1) was likely to be increased in the JB, implying that denitrification process (i.e., NO₃^-→NO₂^-→N₂O/N₂) is active with high N₂O (a strong green-house gas) production, though the observation was not long period.