Riverine input and air–sea CO2 exchanges near the Changjiang (Yangtze River) Estuary: Status quo and implication on possible future changes in metabolic status

Due to anthropogenic activities, the nutrient loadings of the Changjiang (Yangtze River) are strickly on the rise. The high nutrient concentrations notwithstanding, river water was pCO2 supersaturated in the inner estuary during summer 2003 but decreased quickly in the mid-estuary due to mixing with low pCO2 waters from offshore. In addition, settling of particles in the estuary resulted in better light conditions so that phytoplankton bloomed, driving down pCO2 to ∼200 μatm. In the outer estuary and outside of the bloom area, pCO2 increased again to near or just below saturation. Literature data also reveal that the mainstream of the Changjiang is always supersaturated with respect to CO2 probably because the decomposition of terrestrial organic matter overwhelms the consumption of CO2 due to biological production.Because the Changjiang outflow accounts for 90% of the total river flow to the East China Sea (ECS), any variation in the Changjiang could have significant implications for the ECS. For instance, completion of the Three Gorge's Dam could change the metabolic status of the estuary by cutting off 70% of the downstream transport of organic carbon-containing particles. This would reduce the extent of organic carbon decomposition, producing better light conditions and enhancing autotrophy. As a result, the estuary could become a smaller source of CO2 to the atmosphere. On the other hand, if the Three Gorge's Dam reduced freshwater output, especially in summer, upwelling of nutrient-rich offshore waters would be reduced resulting in a reduction in autotrophy in the much wider ECS shelves. This effect could outweigh the reduced heterotrophy in the estuary and the ECS as a whole could become a smaller CO2 sink.