Contribution of sediment oxygen demand to hypoxia development off the Changjiang Estuary

This study evaluates the contribution of sediment oxygen demand (SOD) to hypoxia development off the Changjiang Estuary based on SOD measurements from onboard chamber incubations and numerical experiments. Onboard core incubations were conducted for samples collected from four stations in the Yellow Sea (YS) and the East China Sea (ECS) during cruises in June, August and October 2006. The measured SOD ranges widely from 9.1 to 62.5 mmol O2 m−2 d−1 with a mean rate of 22.6 ± 16.4 mmol O2 m−2 d−1, and the maximum occurring in August. The rates in regions experiencing hypoxia vary from 13.5 to 24.0 mmol O2 m−2 d−1 with a mean of 18.9 ± 4.6 mmol O2 m−2 d−1. Additionally, the vertical oxygen flux was calculated from profile measurements at a hypoxic mooring station adjacent to the Changjiang Estuary on September 3rd and 4th, 2006. The oxygen flux across the pycnocline was about 3.1 mmol O2 m−2 d−1, much lower than the SOD, implying that the low oxygen supply associated with stratification promotes the formation and maintenance of hypoxia in bottom waters. The one-dimensional numerical experiments configured for stratified and well-mixed water columns further confirm that stratification is necessary for the development and persistence of hypoxia, while SOD is a major oxygen sink below the pycnocline. Integrated over June to August, the amount of oxygen loss beneath the pycnocline due to SOD is equal to 116%-148%, 80%-142% and 82%-179% of the total net oxygen loss in the southern hypoxic region, northern hypoxic region and the middle shelf, respectively. During June to August when hypoxia develops, SOD in the northern and southern regions has similar magnitude. The Changjiang Diluted Water (CDW) promotes hypoxia formation in both regions, while the oxygen advection caused by the Taiwan Warm Current (TWC) would alleviate hypoxia formation especially for the southern hypoxic region.