Summer bottom oxygen depletion dynamics and the associated physical structure in the Bohai Sea

Summertime oxygen depletion has been more and more frequently observed in the bottom water of the Bohai Sea in the last decade. Based on comprehensive hydrography and microstructure measurements in summer in the Bohai Sea, the physical structure and bottom dissolved oxygen (DO) dynamics were investigated. The study area is characterized by strong tidal currents and obvious horizontal temperature and DO gradients in the bottom boundary layer. The strong tidal forcing induces large near-bottom turbulent kinetic energy dissipation rates (epsilon similar to 5 x 10(-5) W kg(-1)) which can be well parameterized by the law of the wall. Tidal horizontal advection effects dominate the short-term variations of bottom hydrography. Although the residual current is in a near-perpendicular direction with the horizontal DO gradient (similar to 94 degrees), the horizontal residual DO transport was calculated to be 67.4 mg m(-2) d(-1) which is similar to 33% of the magnitude of sediment oxygen demand. During the observation period, we observed an intense high-turbidity event leading to a severe drawdown of near-bottom DO concentration (0.16 mg L-1) in 1.5 hours. The DO consumption rate due to this event was then estimated to be similar to 33.3 g m(-2) d(-1) (1.39 g m(-2) h(-1)) which is two orders of magnitude larger than the sediment oxygen demand. Rapid DO consumption can be induced by the great increase in bioavailable surface area in bottom water and seabed when benthic organic matter is resuspended. This process should be incorporated into the coupled physical-biogeochemical model to improve accuracy in simulating DO depletion.

Automated summary

Based on comprehensive measurements in the Bohai Sea, the physical structure and dynamics of bottom dissolved oxygen were investigated. Strong tidal currents and horizontal temperature and dissolved oxygen gradients were observed. Although the residual current was perpendicular to the horizontal dissolved oxygen gradient, the horizontal residual dissolved oxygen transport was still significant. In addition, a strong high-turbidity event leading to a severe decrease in bottom dissolved oxygen concentration was observed.