Coastal Upwelling Combined With the River Plume Regulates Hypoxia in the Changjiang Estuary and Adjacent Inner East China Sea Shelf

The Changjiang (Yangtze River) Estuary (CE) and adjacent inner East China Sea (ECS) shelf are among the most well-known hypoxic aquatic environments. However, the coupled effects of coastal upwelling and the river plume on hypoxic dynamics in this region are rarely examined. Based on observations off the CE and Zhejiang coast in the summer of 2018, the dynamic mechanisms were revealed by highlighting the combined role of the river plume and upwelling in regulating hypoxia. The bottom hypoxic water formed in the steep slope on the inner ECS shelf expands upward and seaward. Coastal upwelling and the river plume combine to regulate the primary production regime and hypoxia. The frontal regions in the upper layers are prone to phytoplankton blooms, while the convergent slope generally acts as a hot spot of bottom hypoxic center along cross-shelf sections. The regional selectivity of hypoxia is responsible for the decoupling between locations of bottom-water hypoxia and upper-water phytoplankton patches. The upwelling transports oxygen-deficient water to the upper layers, resulting in shoreward shoaling of the oxycline. Meanwhile, mainly forced by the seaward expansion of low-oxygen water protruding from the bottom hypoxia in the sloping region, a pronounced midlayer hypoxic plume forms. Together, upwelling and offshore spreading potentially expand the spatial volume of the hypoxic zone. Vertical secondary circulation and subsequent physical-biogeochemical processes leading to hypoxia are indicated off the CE and Zhejiang coast. Our results demonstrate that the combination of upwelling and the river plume plays an important role in regulating the spatial distribution of coastal and estuarine hypoxia.