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. Plain Language Summary: Coastal regions with large riverine inputs have long been under the threat of seasonal hypoxia. Ideally, to comprehensively understand the formation and sustainment of seasonal hypoxia has important implications for managing coastal hypoxia and minimizing its detrimental consequences. Long‐term variability of bottom hypoxia in the Long Island Sound, Chesapeake Bay, and northern Gulf of Mexico has been studied. Bottom hypoxia also forms seasonally off the Changjiang River Estuary in the East China Sea due to the large riverine inputs from the Changjiang River. A coupled physical–biogeochemical model is used in this study to advance understandings of the interannual variation. Physical and biogeochemical processes that impact oxygen dynamics are isolated to distinguish the drivers of interannual variation of bottom hypoxia off the Changjiang River Estuary. The findings in this study suggest that the long‐term variation of Changjiang freshwater distribution is the dominant contributor of bottom hypoxia interannual variability over the continental shelf. Key Points: Hypoxia location, severity, size, and sustainment off the Changjiang River Estuary all show large interannual variationsLong‐term variation of shelf wind direction/strength controls interannual variation of buoyant plume distribution and stratificationInterannual variation of freshwater distribution determines the delivery of riverine materials and the subsequent formation of hypoxia [ABSTRACT FROM AUTHOR]