Response of bottom dissolved oxygen reduction to net ecosystem production observed by a wave-driven profiler in the Changjiang River Plume

Coastal hypoxia, exacerbated by the combined influence of eutrophication and global warming, presents a significant environmental challenge. However, the lag correlation between organic matter (OM) export from the upper layers and bottom dissolved oxygen (DO_BOT) reduction still lack clear elucidation. This study investigated the coupling between net ecosystem production (NEP, representing the maximum OM export) and DO_BOT in the Changjiang River plume (CRP), using a wave-driven profiler system. The high-resolution profiles revealed rhythmic fluctuations in water column NEP, with sediment-water exchange (-74.6%) and NEP (-4.0%) dominating DO_BOT reduction. Notably, surface NEP impacts DO_BOT with a lag time of 25.65 h, indicating an OM sinking speed of 1.32 mm s^-1. NEP at a depth of 3.4 m exerted the most significant influence on DO_BOT, explaining a 12% reduction. These findings elucidate the response mechanism of DO_BOT reduction to upper OM export and provide insights for hypoxia prediction in coastal and estuarine areas.