Synthesis of observed air-sea CO2 exchange fluxes in the river-dominated East China Sea and improved estimates of annual and seasonal net mean fluxes.

Limited observations exist for reliable assessment of annual CO₂ uptake that takes into consideration the strong seasonal variation in the river-dominated East China Sea (ECS). Here we explore seasonally representative CO₂ uptakes by the whole East China Sea derived from observations over a 14yr period. We firstly identified the biological sequestration of CO₂ taking place in the highly productive, nutrient-enriched Changjiang river plume, dictated by the Changjiang river discharge in warm seasons. We have therefore established an empirical algorithm as a function of sea surface temperature (SST) and Changjiang river discharge (CRD) for predicting sea surface pCO₂. Synthesis based on both observation and model show that the annually averaged CO₂ uptake from atmosphere during 1998-2011 was constrained to about 1.9mol C m^-2yr^-1. This assessment of annual CO₂ uptake is more reliable and representative, compared to previous estimates, in terms of temporal and spatial coverage. Additionally, the CO₂ time-series, exhibiting distinct seasonal pattern, gives mean fluxes of -3.0, -1.0, -0.9 and -2.5molC m^-2yr^-1 in spring, summer, fall and winter, respectively, and also reveals apparent inter-annual variations. The flux seasonality shows a strong sink in spring and a weak source in late summer-early fall. The weak sink status during warm periods in summer-fall is fairly sensitive to changes of pCO₂ and may easily shift from a sink to a source altered by environmental changes under climate change and anthropogenic forcing. [ABSTRACT FROM AUTHOR]