The importance of being coupled: Stable states, transitions and responses to changing forcings in tidal bio-morphodynamics.

Changes in relative sea level, nutrient and sediment loading, and ecological characteristics expose tidal landforms and ecosystems to responses which may or may not be reversible. Predicting such responses is important in view of the ecological, cultural and socio-economic importance of endangered tidal environments worldwide. Here we develop a point model of the joint evolution of tidal landforms and biota including the dynamics of intertidal vegetation, benthic microbial assemblages, erosional and depositional processes, local and general hydrodynamics, and relative sea-level change. Alternative stable states and punctuated equilibrium dynamics emerge, characterized by possible sudden transitions of the system, governed by vegetation type, disturbances of the benthic biofilm, sediment availability and marine transgressions or regressions. Multiple equilibria are the result of the interplay of erosion, deposition and biostabilization. They highlight the importance of the coupling between biological and sediment transport processes in determining the evolution of a tidal system as a whole. Hysteretic switches between stable states may arise because of differences in the threshold values of relative sea level rise inducing transitions from vegetated to unvegetated equilibria and viceversa.