Spatial variability of hypoxia and coupled physical-biogeochemical controls off the Changjiang (Yangtze River) Estuary in summer

Abstract

Global estuarine hypoxia is among the most acute environmental issues, yet the recognition of hypoxia variability remains patchy. The Changjiang (Yangtze River) Estuary (CE) is well known as one of the largest estuarine hypoxic systems. However, the mechanisms controlling the spatial variability and heterogeneity of summer hypoxia have been rarely examined in this region. Based on observational data in the summer of 2013, the essential linkages between physical-biogeochemical processes and spatial variability of hypoxia were revealed off the CE. The hypoxia generally occurred in a transitional region off the CE, presenting to be spatially variable and heterogeneous in its distribution and intensity. The river plume, stratification, front, upwelling and localized high phytoplankton biomass combine to shape the spatial morphology and scope of hypoxia. The spatial heterogeneity of hypoxic magnitude in horizontal direction is mainly regulated by the intensity variability of stratification and phytoplankton blooms. The position of bottom front formed by the offshore Taiwan Warm Current (TWC) and coastal water under tidal mixing controls the shoreward boundary of hypoxic zone, while the river plume front-dominated outer edge of phytoplankton blooms in upper layers plays a role in determining the seaward limit of the reach of bottom hypoxia. The stratification depth controlled by the upwelling and river plume governs the upper limit of the top reach of hypoxia, and shapes the vertical morphology of hypoxic zone. We contextualize the role of physical-biogeochemical drivers to the spatially variable hypoxia in a conceptual diagram. The findings would considerably contribute to our understanding of the spatially variable hypoxia in river plume-upwelling-front coupled estuarine systems, providing a sound basis for prediction and simulation under anticipated future conditions.

Geochemical factors associated with deposition of lower Aptian organic-rich sediments during OAE1a in the Basque-Cantabrian Basin, northern Spain

“…Based on the non-carbonate fraction, accumulation rates in the Cuchía succession can be attributed to mostly terrigenous input as indicated by Si which is of terrestrial origin because it correlates strongly with Al (r = 0.95) as observed in modern analog environments (Wei et al, 2022). In-situ production of carbonate by biomineralizers varies intermittently, such effects can be observed in many instances such as the TIC curve (Figure 3B), the variation of the major element Al (Figure 4C), and the TAR (Figure 7C).…”

Subsurface oxygen minima regulated by remineralization and bottom flushing along 123°E in the inner East China Sea

“…Furthermore, a temporal and spatial mismatch or decoupling of surface high-biomass regions and hypoxic zones was also observed. Nonuniform offshore advection and detachment of subsurface segments of water underlying the Changjiang River plume are the key drivers (Zhou et al, 2020;Wei et al, 2022). Zhang et al (2022) characterized the use of timescales of deficit and residence to evaluate near-bottom DO variations.…”

“…More recently, Wei et al (2021b) reported the mid-layer hypoxic phenomenon in the Zhejiang coastal area in August 2018 and attributed it to the seaward expansion of low-oxygen water protruding from the bottom-sloping region. As hypoxia expands off the CJE (Chen et al, 2020;Wang et al, 2021), the vertical structure of oxygen and nutrients becomes increasingly important (Wei et al, 2022). The pelagic organisms are more sensitive to oxygen than the benthos, thus the mid-layer hypoxia would be disastrous for their survival, especially for small zooplankton and juvenile fishes (Vaquer-Sunyer and Duarte, 2008;Hofmann et al, 2011).…”