On Mechanisms Controlling the Seasonal Hypoxia Hot Spots off the Changjiang River Estuary

Abstract

Large areas of seasonal hypoxia have been detected off the Changjiang River Estuary over the last decade. Although seasonal bottom hypoxia was transient due to complex dynamical processes, it occurred with high frequency in certain regions. Summer hypoxia hot spots off the estuary were first identified using a comprehensive, high-resolution ecosystem model and 17 years of observations collected from historical literature and research cruises. Comparisons between long-term observations, a 4-year averaged simulation, and a 1-year simulation implied that the geographical locations of summer hypoxia hot spots were generally consistent. This ecosystem model suggested that the Changjiang River plume predominantly spread either over the Yangtze Bank or the Submarine Canyon and the Mud Belt depending on the direction of the wind; the temperature, salinity, and residence time of bottom water mass off the estuary depended highly on the properties of the Kuroshio subsurface intrusion. The dual effects of stratification and prolonged bottom water residence time created favorable condition for hypoxia to develop. The analysis in this study identified wind and the Kuroshio subsurface intrusion as the factors that facilitated the migration of seasonal hypoxia and the establishment of summer hypoxia hot spots off the estuary. In spite of the persistent hypoxia hot spots, the timing of individual hypoxia can be variable because of distinct dynamical processes over a wide range of time scales. Tidal advection was quantified as a secondary factor that influences the spatial extent and migration of bottom hypoxia. Key Points:• Summer hypoxia hot spots off the estuary were identified based on both observed and modeled data • The geographical locations of summer hypoxia hot spots were persistent from year to year • Wind and Kuroshio subsurface intrusion were main factors facilitating the establishment of hot spots Supporting Information:• Supporting Information S1

Wind-driven stratification patterns and dissolved oxygen depletion off the Changjiang (Yangtze) Estuary

“…11), DO-depleted area could have occurred in the northern part both in 2016 and 2018, with the larger extent in the former year. The positive correlation between the river discharge and the hypoxic area has been found by the model simulation (Zheng et al, 2016). Hypoxia in the northern part in 2016 was reported (Zhang et al, 2019).…”

“…Also, it has been shown that vertical mixing caused considerable variations in DO concentrations in the near-bottom layer (Ni et al, 2016). Higher wind speed causes stronger vertical mixing and vanishing of hypoxia (Zheng et al, 2016). In July 2015, 1.5 months before our survey, hypoxia was terminated by a typhoon, but 2 d later, hypoxic conditions were reestablished (Guo et al, 2019).…”

“…Wang et al (2012) explained the annual cycle of the location of near-bottom DO minimum by the seasonal change in regional stratification and variations of the northward extension of the KSSW. An idealized numerical experiment has shown that northerly winds reduce hypoxic area while westerly winds expand it (Zheng et al, 2016).…”

Spatiotemporal Development and Dissipation of Hypoxia Induced by Variable Wind‐Driven Shelf Circulation off the Pearl River Estuary: Observational and Modeling Studies

“…Although HGI is useful to determine whether a system is favorable for hypoxia formation, it cannot fully explain the hypoxia variability in the WCTZ for different scenarios with HG1 < 1. Oxygen diffusion through wind‐induced mixing can erode the hypoxic layer and reduce HA, as found in Changjiang Estuary (Zhang et al., 2019). AKv in the WCTZ is much smaller than in the ECTZ, which also contributes to the larger HA in the WCTZ (Figure 9).…”

Multiple lipid biomarkers record organic matter sources and paleoenvironmental changes in the East China Sea coast over the past 160 years

“…The depositional environment and dynamics of the ECS have been relatively stable since the last 2000 years with slight sea-level change, because the Yangtze River delta and the local current circulation system had been basically formed (Li et al, 2014). However, large areas of seasonal harmful algal blooms and hypoxia have been detected in the ECS in recent decades (Qu et al, 2019; Zhang et al, 2019), being increasing threats to coastal ecosystems. Meanwhile, a mass of dams caused fluvial erosion, which has an important impact on the coastal environment and material supply of the ECS (Sun et al, 2020).…”