Physical Controls Of Summer Variations In Bottom Layer Oxygen Concentrations In The Coastal Hypoxic Region Off The Northeastern Shandong Peninsula In The Yellow Sea

Hypoxia has been observed worldwide and causes great damage to marine ecosystems. In the northeastern mariculture areas of the Shandong Peninsula, summer oxygen depletion and even hypoxia have been observed in the bottom layer, but the underlying dynamics are still poorly understood. In the current study, the physical controls of variations in bottom layer oxygen concentrations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations collected during 2016-2020. Over the past 5 years, the dissolved oxygen (DO) concentrations of the bottom layer have ranged from 0.8 to 13.0 mg L-1 and exhibited a robust annual cycle, with the lowest value occurring in August. Hypoxia events covering several days were observed intermittently in the summers of 2016 and 2017. Summer oxygen depletion was caused by the combined effects of ocean stratification and enhanced biogeochemical oxygen consumption. At the interannual time scale, stratification and bottom layer DO concentrations showed synchronous variations and both peaked in 2018. The underlying mechanism was related to bottom layer currents, which were created by southerly monsoon winds. Strong/weak southerly surface winds induced a strong/weak bottom shoreward advection of cold, oxygen-rich water and resulted in strong/weak stratification and high/low DO concentrations. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia.