On the seasonal variation of air - sea CO_2 fluxes in the outer Changjiang (Yangtze River) Estuary, East China Sea

摘要

Based upon seven field surveys conducted during April 2005 - April 2008, we examined the surface partial pressure of CO_2 (pCO_2) 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 pCO_2 riverine water meets with highly productive shelf waters, covering a 2° ×3° area, ～10% of the surface area of the entire ECS. Surface pCO_2 ranged 320 - 380 uatm (average -345 μatm) in winter, 180 - 450 uatm (average ～330 μatm) in spring, 150 - 620 uatm (average -310 μatm) in summer and 120 - 540 uatm (average ～375 μatm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO_2, 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 pCO_2 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 pCO_2 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 CO_2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea - air CO_2 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 CO_2 flux estimation for the northern ECS.