Seawall-induced impacts on large river delta wetlands and blue carbon storage under sea level rise

Coastal wetlands have been enclosed by thousands of kilometers of seawalls in China to obtain extra land for rapid socio-economic development in the coastal region. Although understanding seawall-induced impacts on delta wetlands and their ecosystem can provide valuable decision-making information to support coastal management, quantifying and measuring long-term, cumulative ecological impacts of harden seawall under sea level rise (SLR) remains a vital research gap. In this study, by combining the land-use transformation trajectory analysis, ecosystem services assessment, and the SLAMM (Sea Level Affecting Marshes Model), we have explored the seawall-induced effects on temporal-spatial dynamics of tidal wetlands and the Coastal Blue Carbon storage (CBCs) in the Yellow River Delta (YRD) under the SLR by 2050 and 2100. Our study revealed that the delta wetland area would have increased by 2327.87 km² after seawall removal without regard for SLR while increasing by 3050 km² in 2100 in both seawall scenarios under SLR. The effects of driving processes trajectory on the changes in CBCs indicated two-sided seawall-induced impacts on the delta wetlands in the YRD, i.e., functioning as a physical coastal defense to prevent coastal erosion (before 2050) while intensifying coastal squeeze effects and quickening the loss in delta wetlands and the CBCs by hindering their inland migration under SLR. For example, the gap of CBCs between the seawall-impacting and seawall-removal scenarios would have reached at 9.94 × 10⁶ Mg by 2050 under the SLR, and the magnitude of the final decrease effect on CBCs induced by the seawall-impacting would be nearly 5 times higher than its gain after seawall-removal in the regressive succession, while the same magnitudes in the salinization process on both scenarios. Our study has provided valuable insights for shoreline management by mitigating seawall-induced impacts on the delta wetlands and their ecosystem services such as CBCs.