Compensatory marsh dynamics increase coastal gross primary production in a mega-delta

Global river deltas are experiencing rapid but uneven wetland gain and loss, yet the implications for key ecosystem functions remain poorly understood. Here, we quantified how changes in wetland extent, together with meteorological variability, have shaped coastal gross primary production (GPP) in the Yangtze Estuary (China), a representative mega-delta system. Using a 35-year record (1985–2020) combining satellite observation, eddy covariance measurements and scenario-based simulations, we disentangled the contributions of land cover change and meteorological variability to coastal GPP dynamics. We found that seaward marsh expansion completely compensated for landward marsh loss, with over 60,000 ha of marsh gain exceeding 51,481.0 ha of marsh loss. Across all land cover trajectories, cumulative GPP gains reached 2098.9 Gg C, more than twice the losses of 847.8 Gg C, while meteorological variability regulated compensation by accounting for 6.8% of GPP gains and 53.9% of GPP losses. Together, these findings reveal a spatially compensatory mechanism linking both wetland extent and ecosystem production along the sea-to-land continuum, highlighting the functional resilience of mega-deltas under ongoing environmental change.