As one of the largest estuaries worldwide, the Yangtze (Changjiang) River Estuary serves as an important area of fast economic development and social transformation, which exert great pressure to marine ecosystems in the estuary. In addition to riverine input, submarine groundwater discharge (SGD) plays a decisive role in the biogeochemical cycle of the estuary, but is poorly understood in the Yangtze River estuary yet. In the present study, based on nearly two-decade of radium isotopes data accumulation, we evaluate the variations of SGD fluxes, which response to the Yangtze River freshwater discharge and estuarine bank erosion and evolution, to some certain extent. Nutrients, those transported by SGD, also showed significant trends relative to the flow of the Yangtze River and the increasing of population along the estuary. Therefore, SGD-derived nutrient may be closely linked to the frequent outbreaks of red tide and hypoxia in the Yangtze Estuary.

Saltmarshes are wetland ecosystems found along the coast of the Yangtze Estuary, such as the Chongming Island and the Hangzhou Bay, they are recognized as one of the most productive ecosystems on the earth with the high carbon accumulation of all habitats. SGD can create a pathway for carbon exports to the coastal waters. Results show that carbon (including DOC, DIC, CO₂, CH₄, etc) delivered by SGD rival the Yangtze River inputs, which significantly affect the blue carbon accumulation along the estuarine coasts and the carbon budgets in the Yangtze River Estuary. This study would optimize the budgets of nutrient and carbon in a large river-dominated estuary and reveal the importance of SGD on blue carbon of the saltmarsh ecosystem.