Coupled Morphological-biological Dynamics and the Fate of Transitional Coastal Systems

Transitional coastal areas, such as estuaries and lagoons, receive sediment and water from a long chain of systems in the continental interior and the piedmont/floodplain zones, and are thus strongly dynamically coupled with a wide variety of Earth’s surface systems. Coastal transitional systems, and coastal wetlands in particular, also crucially depend on local organic sediment production to keep up with rising sea levels. Hence, their dynamic responses and existence are the result of coupled biological-morphological process occurring locally and at a global scale: during the past century such responses to direct anthropogenic disturbances and to changes in rates of fluvial sediment transport, sea-level rise, and subsidence has led to a huge loss of transitional coastal areas globally. Here, through models and observations from the local to the whole system scale, I review the main mechanisms that determine coastal wetland dynamics. In particular, I show that the existence of stable equilibria in coupled bio-geomorphic dynamics leads to the characteristic landforms we universally observe in coastal intertidal areas, irrespective of details in tidal regimes, climate, sediment load, etc. Unstable equilibria also exist, that lead to runaway dynamics in wetland erosion and are responsible for a large part of wetland loss that has taken place in the Antrhopocene. Finally, I will discuss the implications of equilibrium-disequilibrium dynamics for the fate of transitional tidal systems under ongoing global environmental changes and will present possible concrete conservation steps for the case of the Lagoon of Venice (Italy), with implications of a somewhat general nature.