Exploring oxygen dynamics and depletion in an intensive bivalve production area in the coastal sea off Rushan Bay, China

Abstract

Hypoxia is a mounting problem affecting the world’s coastal waters, with severe consequences for marine ecosystems. Coastal oxygen consumption has been increasing, mainly owing to the continued spread nutrient discharges. Using field observations, incubation experiments and numerical modeling, we studied the spatial and temporal variability of dissolved oxygen (DO) in the coastal area off Rushan Bay, China, a typical coastal area influenced by intensive mariculture oyster production. Results show that summer DO is increasingly declining in bottom waters below the thermocline. Oxygen input from the air−sea interface exchange, primary production and net water exchange accounted for 70, 26 and 4% of the DO supply, respectively. Oxygen consumption by organic matter decomposition in the water column and sediment contributed, respectively, 79 and 21% to the total DO removal. In regions such as the coastal area off Rushan Bay where the algal biomass filtered by bivalves is imported from elsewhere by sea currents, the carbon and nutrient release by mariculture may lead to local oxygen depletion, which increased from a negligible contribution in 1984 to up to 24% of the total DO consumption in the water column in the period of June−September 2014. This phenomenon of oxygen depletion is a concern for other coastal areas with intensive bivalve and other shellfish production.