Anthropogenic eutrophication and stratification strength control hypoxia in the Yangtze Estuary

Many large estuaries are threatened by intensifying hypoxia. However, due to the limited duration of available observations, uncertainties persist regarding the level of contemporary hypoxia intensity in a longer-term context and the relative contributions of climate versus human factors. Here we present sediment records for the hypoxia intensity and associated environmental parameters in the Yangtze Estuary over the past three centuries. The results show that the hypoxia intensity has been increasing during the last half century due to anthropogenic eutrophication, but the current hypoxia condition is not as severe as some preindustrial periods due to weaker stratification in the water column. Our findings suggest that if anthropogenic and climatic forcing coincide in the foreseeable future, the hypoxia intensity of the Yangtze Estuary may reach unprecedented levels. The severity of hypoxia in the Yangtze Estuary has increased in the last 50 years due to anthropogenic eutrophication and may be further exacerbated by climate-driven strengthening of stratification in the future, suggests sediment core analysis and climate modelling.