Summertime in situ monitoring of oxygen depletion in Amursky Bay (Japan/East Sea)

• Downwelling circulation induced by winds is factor in formation of Amursky Bay hypoxia.
• Ventilation process supplied DO in the bottom waters is the semi-diurnal tidal wave M2.
• Advection of “external waters” formed near mouth river is another ventilation process.
• BOD and ventilation rates were estimated by modeling.
• Upwelling circulation inducted by winds is factor in destruction of Amursky Bay hypoxia.

For more than three months in 2011, in situ monitoring of temperature (T), salinity (S) and dissolved oxygen concentrations (DO) was carried out using a Water Quality Monitor (WQM) station deployed on the seafloor of Amursky Bay (Japan/East Sea). During this period, hypoxia in the bottom waters persisted for 93 days. In the summers of 2012 and 2013, the spatial distribution of DO was measured during ship surveys. Using these time series of DO, the biological oxygen demand (BOD) and ventilation rates in bottom waters were estimated from May 10 to August 7.The seasonal change in the dominant direction of the wind, which occurs twice a year (spring and autumn), was an important natural factor in development and termination of seasonal hypoxia in the bay. Dominant southern winds in the summer induced downwelling circulation on the northwestern part of the Japan/East Sea shelf. Under this circulation, hypoxia developed in the bottom waters of Amursky Bay.In autumn, dominant northern winds induced upwelling, causing the advection of cold, oxygenated seawater into the bay, ending the period of hypoxia.Short-term fluctuations in wind direction in the summertime influenced spatial and vertical distribution of T, S and DO. At the end of the summer, the oscillation of the downwelling/upwelling circulations revealed complicated temporal-space distributions of hydrological parameters in Amursky Bay.

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