Spatial Variability of Air–Sea CO2 Flux and Their Carbon Sources During Early Spring in the Yangtze River Estuary and Adjacent Coastal Areas

Air–sea CO2 flux (FCO2) in the estuary–coastal continuum plays a vital role in global carbon sequestration; however, the mechanisms governing FCO2 spatial heterogeneity during early spring remain poorly understood, particularly the roles of distinct dissolved inorganic carbon (DIC) sources. In March 2025, we investigated the FCO2 spatial variability and DIC sources across the Yangtze River estuary and adjacent coastal areas using DIC concentration, pH, and δ13CDIC analyses. The study area was a net CO2 source (7.3 ± 8.7 mmol m−2 d−1), with the intensity declining progressively from the inner estuary to offshore areas. Physical mixing of three principal water masses established the following pattern: high-pCO2 Changjiang Diluted Water and Yellow Sea Coastal Current drove CO2 outgassing, while low-pCO2 East China Sea Shelf Water weakened it. Quantitative apportionment revealed atmospheric CO2 invasion as the dominant DIC source, followed by carbonate dissolution and organic matter degradation, with the latter declining from the inner estuary to offshore areas. The spatial variation in DIC source contributions further confirms that, superimposed on the physical mixing, biogeochemical processes—particularly biological activity—modulated reginal source intensities. This early-spring case captures a critical transitional window and highlights the necessity of integrating multi-factor regulation with DIC source partitioning to resolve carbon dynamics in the estuarine–coastal continuum.