Unexpected high indirect impacts of riverine organic matter to coastal deoxygenation

Hypoxia has become a serious threat to the sustainability of marine ecosystems globally. As one of the leading components of eutrophication, river-delivered organic matter (ROM) regulates estuarine/coastal dissolved oxygen condition and contributes to hypoxia formation both directly via remineralization and indirectly via the nutrients released during remineralization. However, the impacts of ROM on the coastal environment have long been underestimated because the indirect contribution has been overlooked and left largely unknown. This study used the Changjiang River, the second-largest ROM source in the world, as a prototype. A coupled physical-biogeochemical model was used to explore the indirect contribution of Changjiang ROM to marginal sea deoxygenation and hypoxia formation. Here I show that ROM remineralization contributed 5.4% directly and 7.3% indirectly to the total oxygen consumption in the off-estuary hypoxic zone. The indirect contribution persistently sustained in a large part of the northwest Pacific marginal seas. Strict mitigation is required to manage the ecosystem impacts of deoxygenation. The findings of this study also indicate that river discharge, freshwater turbidity, and interactions between freshwater and shelf circulation are fundamental factors that regulate the contributions of ROM to hypoxia formation and shelf deoxygenation.