On the seasonal variation of air – sea CO2 fluxes in the outer Changjiang (Yangtze River) Estuary, East China Sea

Based upon seven field surveys conducted during April 2005 – April 2008, we examined the surface partial pressure of CO2 (pCO2) and dissolved oxygen (DO) in the outer Changjiang (Yangtze River) Estuary, on the inner shelf of the East China Sea (ECS). This area represents a most dynamic zone of the ECS where high pCO2 riverine water meets with highly productive shelf waters, covering a 2° × 3° area, ~ 10% of the surface area of the entire ECS. Surface pCO2 ranged 320 – 380 µatm (average ~ 345 µatm) in winter, 180 – 450 µatm (average ~ 330 µatm) in spring, 150 – 620 µatm (average ~ 310 µatm) in summer and 120 – 540 µatm (average ~ 375 µatm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO2, ranging 95% – 105% in winter, 96% – 142% (average 110%) in spring, 73% – 192% (average 118%) in summer and 81% – 178% (average 102%) in autumn. The dynamics of pCO2 drawdown and DO enhancement in the warm seasons (from April to October) appeared to be controlled by primary productivity and air – sea exchange, while mixing dominated the aqueous pCO2 in the cold seasons (from November to March of the following year). This study showed that the outer Changjiang Estuary served as a moderate or significant sink of atmospheric CO2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea – air CO2 flux in the outer Changjiang Estuary was estimated as − 1.9 ± 1.3 mol m− 2 year− 1, which is double the recent sea–air CO2 flux estimation for the northern ECS.