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

Abstract

Limited observations exist for a 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 14-year 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. Syntheses based on both observations and models show that the annually averaged CO2 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 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 −3.7 ± 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 pCO2 and may easily shift from a sink to a source altered by environmental changes under climate change and anthropogenic forcing.