Emergence of Summertime Hypoxia and Concurrent Carbonate Mineral Suppression in the Central Bohai Sea, China

The Bohai Sea to the west of the North Yellow Sea in the Northwest Pacific is a semienclosed shallow-water marginal sea on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline and acidification, eight field surveys were conducted between August 2011 and September 2017, investigating seasonal and interannual variations in DO and carbonate system parameters in the central Bohai Sea. The historically lowest Bohai Sea DO was recorded at 66 mu mol O-2/kg in subsurface waters in early September 2015. By comparing our data set with literature data obtained in the 1980s, 1990s, and 2000s, the emergence history of summertime hypoxia in this marginal sea was shown. Associated with the seasonal DO shortage, the metabolism-enhanced CO2 accumulation in subsurface waters in summer resulted in critical aragonite saturation states of close to or even less than 1.5, threatening calcifiers with potentially severe consequences for valuable shellfish fisheries. Based on model results combining the aerobic respiration (as indicated by apparent oxygen utilization) and the given wintertime/springtime atmospheric CO2 level of the time, this study suggests that combined stresses of the seasonal DO shortage and CO2 acidification capable of threatening Bohai Sea ecosystems could develop within decades.