Long-term evolution of salt marsh landscape change in a human-modified estuarine system (1956-2022): Implications for resilience and adaptive management.

Salt marshes are among the most threatened coastal ecosystems worldwide, increasingly affected by relative sea-level rise, sediment limitation, and long-term anthropogenic pressures. Although regional-scale assessments often report generalized wetland regression, they frequently overlook the spatial heterogeneity that governs local ecosystem resilience. This study reconstructs six decades (1956-2022) of salt-marsh evolution in inner Cadiz Bay (SW Spain) to determine whether long-term marsh regression follows a uniform trajectory or reflects contrasting local responses. Historical aerial imagery, standardized habitat mapping, and Digital Shoreline Analysis System (DSAS) analyses were combined to quantify long-term habitat and shoreline change. Results reveal a persistent regressive trajectory, with widespread salt-marsh loss and an average marsh-edge retreat of -2.4 m year-1 across the inner bay. However, local trajectories differed markedly, ranging from severe marsh degradation in El Trocadero (-7.0 m year-1) to historical marsh expansion followed by recent erosion in Punta de la Clica (-1.8 m year-1 since 1978). These contrasting responses indicate pronounced spatial differences in marsh resilience despite an overall convergence towards regression. The observed trajectories are consistent with increasing system-wide pressure associated with relative sea-level rise, sediment limitation, and restricted accommodation space. Together, they suggest that passive conservation alone is unlikely to maintain long-term marsh resilience and highlight the need for integrated adaptation strategies that combine geomorphological, ecological, and socio-economic perspectives.