Does the Three Gorges Dam cause significant loss in mass accumulation rate and organic carbon accumulation rate?—Insights from the largest salt marsh in the Yangtze River

The Three Gorges Dam (TGD), as one of the world’s largest water conservancy projects, plays an important role in flood control and disaster resistance. This study evaluated the accumulation rate, sources and influencing factors of organic carbon in the Eastern Chongming Wetland (ECW) before and after the construction of the TGD. The organic carbon accumulation rate (OC-MAR) was estimated using ²¹⁰Pb radionuclide dating, δ¹³C and C/N ratio mixing model, and the contribution of rivers, vegetation and ocean to organic carbon in the ECW was quantified. Research shows that the mass accumulation rates (MAR) in the Phragmites australis zone (CM-1), Scirpus mariqueter zone (CM-2), and mudflat (CM-3) are 2.88 ± 1.32 g cm⁻² yr⁻¹, 1.95 ± 0.45 g cm⁻² yr⁻¹ and 1.51 ± 1.22 g cm⁻² yr⁻¹ and OC-MAR are 185 ± 97 g C m⁻² yr⁻¹, 80 ± 34 g C m⁻² yr⁻¹, and 33 ± 15 g C m⁻² yr⁻¹, respectively. Integrated analysis of MAR, OC-MAR, and the three-end-member mixing model indicates that, although the construction of the TGD and other reservoirs/dams in the Yangtze River basin has reduced sediment discharge and particulate organic carbon (POC) flux in the Yangtze River, MAR and OC-MAR in the study area did not respond promptly in the short term. This may be attributed to: (1) sampling area being situated within a sedimentary belt formed by flood-dominant tidal currents; (2) a vegetation-dominated sediment trapping mechanism; and (3) continuous sediment supply from the submerged delta (below the spring low tide line). Results from the three-end-member model demonstrate that the increase in riverine OC input driven by finer sediment grain size and the rise in plant OC due to expanded vegetated area are the primary reasons for the OC-MAR increase. The current global annual carbon loss from mudflats (4.8 Tg C yr⁻¹) has reached 330 times the annual carbon burial in the ECW (1.5 × 10⁻² Tg C yr⁻¹).