Simulation On Formation Of Summer Low Oxygen Zone In The Bohai Sea

The Bohai Sea is a semi-enclosed shallow sea in China.Owning to the development of the cities around the Bohai Sea,the ecological environment has been deteriorated drastically.In the past forty years,the dissolved oxygen(DO)in bottom water in the Bohai Sea decreased discontinuously,and large areas of DO <3 mg/L presented at the offshore of Qinhuangdao and the Yellow River Mouth from 2014,which imposes great stresses on marine ecosystems and implies the needs of effective management to improve the eco-environments.In this study,the observation data combined with the ecosystem dynamic model is used to explore the possible reasons of the interannual decline of DO in the Bohai Sea,the seasonal variation and controlling factors of hypoxia,and hence to find out the environmental indicators that can be used for hypoxia prediction.The main conclusions are as follows:Based on the long-term DO,temperature,wind stress and surface chlorophyll a concentration,the inter-annual decline of DO in bottom water in the Bohai Sea might be mainly caused by the enhancement of summer biomass which exponentially(or linearly)correlates with time in the Bohai Sea,instead of stratification strength,wind stress and bottom temperature.Based on the model results,the seasonal hypoxia in the Bohai Sea could be characterized as: it presented from July to September and from mid July to the end of August at the offshore of Qinhuangdao and the offshore of the Yellow River Mouth,respectively;the hypoxia at the offshore of Qinhuangdao first appeared in the coastal area,then moved to the offshore area,and finally disappeared at the northern of the central shoal.However,the hypoxia at the offshore of the Yellow River Mouth first appeared in the Laizhou Bay mouth,then expanded to the northwest,and finally disappeared in the Laizhou Bay mouth.The modelled oxygen budget suggests that oxygen consumption in the bottom water via organic matter degradation and temperature induced stratification are the main factors leading to the hypoxia in the Bohai Sea.Additionally,the interannual variation of DO concentration is mainly influenced by diffusion.Furthermore,the interannual variation of DO distribution to the offshore of Qinhuangdao is mainly impacted by advection and diffusion,while that to the offshore of Yellow River Mouth is mainly affected by the freshwater discharge of the Yellow River.A statistical analysis of the model results reveals that surface temperature and the temperature difference between the surface and bottom water can be used to forecast the occurrence of hypoxia.The values of surface temperature and the temperature difference between the surface and bottom layer were 19.93±0.93? and 5.4±0.8?,when hypoxia occurred at the offshore of Qinhuangdao,and 23.02±0.89? and 3.6±0.7? for the Yellow River Mouth.