Long-term eco-morphodynamic evolution of lagoonal systems

Tidal environments are very delicate systems whose dynamics need be described in terms of a holistic eco-geomorphological approach. Issues of conservation of such environments can be addressed only through the description of the delicate balance and strong feedbacks characterizing hydrodynamic and sediment transport processes on the one hand, and ecological dynamics on the other hand. Improving our understanding of the main processes shaping the geomorphological and biological characters of the tidal landscape, leading to their origins and further evolution, is a key critical step in order to predict their future fate. Toward the goal of a comprehensive theoretical framework, suitable for large-scale, long-term applications, we have set up a model of the eco-morphodynamic evolution of tidal systems, which makes it possible to investigate the processes governing the strictly intertwined longterm evolution of tidal networks and of the adjacent marsh platforms. Based on observational evidence indicating the existence of different time scales governing the various landscapeforming processes, the model decouples the initial rapid network incision from its subsequent slower process-controlled elaboration (chiefly by meandering) and from eco-morphological evolution of intertidal areas. This allows us to investigate the response of tidal morphologies to different scenarios of sediment supply, colonization by halophytes and changing sea level. Different morphological evolutionary regimes are shown to depend on marsh ecology. Marsh accretion rates, enhanced by vegetation growth, and the related platform elevations tend to decrease with distance from the creek, measured along suitably defined flow paths. The negative feedback between surface elevation and its inorganic accretion rate is reinforced by the relation between plant productivity and soil elevation in Spartina-dominated marshes and counteracted by positive feedbacks in multispecies-vegetated marshes. When evolving under constant sea level, unvegetated and Spartina-dominated marshes asymptotically tend to mean high water level (MHWL), differently from multiple-vegetation species marshes, which can make the evolutionary transition to upland. Equilibrium configurations below MHWL can be reached under constant rates of sea level rise, depending on sediment supply and vegetation productivity.