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

Abstract

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.

Response Process of Coastal Hypoxia to a Passing Typhoon in the East China Sea

“…A hotspot for the accumulation of detritus before the passage of Bavi was found on the steep slope to the west of 123°E (Figure 11A), which was consistent to a recent study targeting the distribution of hypoxia (Wei et al, 2021). During Bavi's passage, the variation in the distribution of detritus (Figures 11B, C) was weaker than that of DO (Figures 10B, C).…”

“…where the x-and y-axes point horizontally eastwards and northwards, respectively, while the z-axis points vertically upwards; C stands for DO or detritus; velocity components in 11A) and low DO concentration (Figure 10A) were found before Bavi's passing. It is also a typical situation in summer related to hypoxia (Wei et al, 2021). Region II is located on the submerged river valley floor.…”

The Role of Fronts in Horizontal Transports of the Changjiang River Plume in Summer and the Implications for Phytoplankton Blooms

“…In addition to the vertical processes mentioned above, upwelling on the western flank of the submarine valley, near the tidal front, will supply nutrients from the Kuroshio water to the surface layer that is subject to the phosphorous limitation (Jiang et al., 2015; Tseng et al., 2014). Additionally, secondary circulations exist near the frontal regions (Wei et al., 2021). To comprehensively understand the mass transport and their role in phytoplankton biomass, the vertical processes need to be further considered.…”

“…Near the tidal front, upwelling resulting from the frontal secondary circulation (Lü et al, 2006;Z. Zhang et al, 2020;Wei et al, 2021) could supply nutrients from the deep-layer Kuroshio water into the surface layer (Jiang et al, 2015;S. Li et al, 2021;Tseng et al, 2014).…”

“…In the Changjiang River plume area, previous studies of frontal dynamics investigated the transport processes associated with fronts from the perspective of the plume circulation and residual currents (Su & Wang, 1989), the generation of eddies and secondary circulations (Chen et al., 2008; Lü et al., 2006; Wei et al., 2017, 2021), the tidal influence on the front formation (Wang et al., 2007), and seasonal front transition (Y. Zhang et al., 2020). For ecological importance of fronts in this area, early studies were focused on the effect of sediment fronts on sediment transport associated with biochemical processes (Chen et al., 1999; Ge et al., 2020) and the role of the plume (buoyancy) fronts on phytoplankton blooming (W. Li et al., 2021; Wang et al., 2019; Yuan et al., 2010; Z. Zhang et al., 2020).…”