Nitrogen from agriculture and temperature as the major drivers of deoxygenation in the central Bohai Sea.

Agricultural N losses strongly dominate the N delivery (average 72 % of total N delivery to rivers in the period 1980-2010) in the rivers discharging into the Bohai Sea, a semi-enclosed marginal sea, which has been suffering from eutrophication and deoxygenation since the 1980s. In this paper we investigate the relationship between N loading and deoxygenation in the Bohai Sea, and consequences of future N loading scenarios. Using modeling for the period 1980-2010, the contributions of different oxygen consumption processes were quantified and the main controlling mechanisms of summer bottom dissolved oxygen (DO) evolution in the central Bohai Sea were determined. Model results show that the water column stratification during summer impeded the DO exchange between oxygenated surface water and oxygen-poor bottom water. Water column oxygen consumption (60 % of total oxygen consumption) was strongly correlated with elevated nutrient loading, while nutrient imbalances (increasing N:P ratios) enhanced harmful algal bloom proliferation. Future scenarios show that deoxygenation may be reduced in all scenarios owing to increasing agricultural efficiency, manure recycling and wastewater treatment. However, even in the sustainable development scenario SSP1, nutrient discharges in 2050 will still exceed the 1980 levels, and with further enhancement of water stratification due to climate warming, the risk of summer hypoxia in bottom waters may persist in the coming decades.