Reconciling the paradox that the heterotrophic waters of the East China Sea shelf act as a significant CO₂ sink during the summertime: Evidence and implications

To explore the paradox that the heterotrophic waters of the East China Sea (ECS) shelf act as a significant CO 2 sink in summer, vertical structures of carbon chemistry and hydrography were examined in July 2007. The results show that waters above the pycnocline (∼10 to 30 m) in the major CO 2 sink area are supersaturated with oxygen (110 ± 7%; autotrophic) but undersaturated with respect to atmospheric CO 2 (ΔfCO 2 = −130 ± 58 μ atm; sink). In contrast, waters below the pycnocline are undersaturated with respect to oxygen (61 ± 16%; heterotrophic) but supersaturated with CO 2 (ΔfCO 2 = 116 ± 115 μ atm; source). This demonstrates that summer stratification is the key factor maintaining the CO 2 sink status in the heterotrophic ECS shelf waters. Furthermore, the shallow pycnocline can easily be broken down when strong mixing occurs, potentially allowing the respired CO 2 stored in the subsurface waters to return to the atmosphere.