Abstract:To explore the paradox that the heterotrophic waters of the East China Sea (ECS) shelf act as a significant CO2 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 CO2 sink area are supersaturated with oxygen (110 ± 7%; autotrophic) but undersaturated with respect to atmospheric CO2 (ΔfCO2 = −130 ± 58 μatm; sink). In contrast, waters below the pycnocline are undersaturated with respect to oxygen (61 ± 16%; heterotrophic) but supersaturated with CO2 (ΔfCO2 = 116 ± 115 μatm; source). This demonstrates that summer stratification is the key factor maintaining the CO2 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 CO2 stored in the subsurface waters to return to the atmosphere.