Limited observations exist for a reliable assessment of annual CO 2 uptake that takes into consideration the strong seasonal variation in the river-dominated East China Sea (ECS). Here we explore seasonally representative CO 2 uptakes by the whole East China Sea derived from observations over a 14-year period. We firstly identified the biological sequestration of CO 2 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 p CO 2 . Syntheses based on both observations and models show that the annually averaged CO 2 uptake from atmosphere during the period 1998–2011 was constrained to about 1.8 ± 0.5 mol C m −2 yr −1 . This assessment of annual CO 2 uptake is more reliable and representative, compared to previous estimates, in terms of temporal and spatial coverage. Additionally, the CO 2 time series, exhibiting distinct seasonal pattern, gives mean fluxes of −3.7 ± 0.5, −1.1 ± 1.3, −0.3 ± 0.8 and −2.5 ± 0.7 mol C m −2 yr −1 in spring, summer, fall and winter, respectively, and also reveals apparent interannual variations. The flux seasonality shows a strong sink in spring and a weak source in late summer–mid-fall. The weak sink status during warm periods in summer–fall is fairly sensitive to changes of p CO 2 and may easily shift from a sink to a source altered by environmental changes under climate change and anthropogenic forcing.