Mechanisms Controlling Interannual Variability of Seasonal Hypoxia Off the Changjiang River Estuary

Hypoxia has long been a symptom of deteriorating ecosystem that threatens the health of estuarine and coastal waters. Episodic hypoxia events and intraseasonal variation of coastal hypoxia have been amply investigated. However, interannual variability of coastal hypoxia has only been assessed in few regions. Bottom hypoxia forms seasonally off the Changjiang River Estuary in the East China Sea mainly due to the large riverine inputs. Large river discharge and its interactions with ambient water combine to contribute to the development of episodic hypoxia events and the intraseasonal migration of bottom hypoxia. However, little is known about the interannual variation of bottom hypoxia in this region. This study used a well-evaluated, coupled physical–biogeochemical model to explore the long-term feature of hypoxia in the East China Sea. The bottom water in the hypoxic zone lost oxygen with a rate of −1.2 mmol/m³/year. Bottom hypoxia showed large interannual variations of geographical location, severity, volume expansion, and sustainment. Large Changjiang River discharge was a prerequisite for hypoxia formation and the associated interannual variation. The interannual variations in the direction and strength of shelf wind controlled long-term distribution of Changjiang diluted water. The delivery of freshwater fundamentally determined the strength of vertical stratification and the rates of biogeochemical cycles, contributing 73% of the interannual variation of bottom hypoxia.