Past and future marsh adaptation: Lessons learned from the Ria Formosa lagoon

Abstract

It is rational to assume that salt marshes in regions where sediment loads are high should remain stable or prograde when facing a range of sea-level scenarios, whereas those in sediment-poor systems may erode or drown. Despite extensive theoretical and laboratory studies, additional marsh ‘persistence’ indicators under human pressures and accelerated sea-level rise rates are still needed. This study investigates the recent lateral and volumetric changes occurring in the lagoon marshes of the Ria Formosa lagoon (south Portugal), under human pressures and sea-level rise. Our analysis assesses the past (1947–2014) geomorphological evolution of marshes based on aerial imagery analysis and estimates its potential future adjustment to sea-level rise (~100 years) based on SLAMM (landscape-based model) simulated land cover changes. We highlight the influence of both stressors on marsh ecosystems and examine how their interactions can contribute to understanding sea-level rise impacts and ecological resilience of lagoon marshes. Salt marshes in the Ria Formosa have slowly expanded over the last 70 years (~0.2 mm/yr), with local erosion in front of tidal inlets and along the main navigable channels, associated with inlet migration and dredging activities. Past evolution shows that the ecosystem was able to maintain its functions and cope with sea-level rise. However, future marsh trajectories under a high sea-level rise rate suggest unbalanced vertical marsh accretion and progressive migration of the tidal flat (and water bodies) towards the salt marsh area. The model results show evidence of non-linearity in marsh response to high sea-level rise rates, which could indicate the presence of a system tipping-point and potential positive (disturbance-reinforcing) feedbacks within the system, with significant implications to marsh resilience.