The divergence between chronic offshore acidification and volatile estuarine pH dynamics: A 60-Year East China sea...

Coastal ocean acidification in river-dominated marginal seas exhibits distinct evolutionary trajectories from the open ocean. However, quantitatively decoupling its multi-stressor drivers remains challenging due to historical data scarcity and the confounding effects of shifting hydrology and localized human activities. We established a robust calibration framework using sedimentary 3‑hydroxy fatty acids to reconstruct a 60-year pH history in the Changjiang Estuary and the adjacent East China Sea shelf. Our reconstruction reveals that the shelf water pH declined at a rate of -0.00093 units yr^-1 over the past six decades, closely tracking the invasion of atmospheric CO₂, whereas the estuarine zone was governed by regional biogeochemical processes. Variation partitioning analysis and generalized additive models quantify that hydrological forcing, anthropogenic forcing and hydro-biological fluctuations accounted for independent variance contributions of 41.3%, 24.8% and 16.3% of the pH variability in the estuary. These findings suggest that the sensitivity of coastal carbonate chemistry to climate-related environmental change depends not merely on atmospheric CO₂ forcing, but also on the balance between regional hydrological regimes and ecosystem state. Scenario-based perturbation analyses further suggest that variations in freshwater discharge, warming, and oxygen dynamics will either amplify or partially buffer coastal acidification signals, depending on the dynamic competition between physical dilution and biologically mediated carbonate processes. This underscores the critical necessity of conducting integrated, multi-stressor assessments to guide adaptive watershed coastal management, rather than treating river-dominated marginal seas as a single homogeneous domain.