Reading the signatures of biologic–geomorphic feedbacks in salt-marsh landscapes

Abstract

How do interacting physical and biological processes control the form and evolution of salt-marsh landscapes? Salt marshes are shaped by the erosion, transport and deposition of sediment, all of which are mediated by vegetation. In addition, vegetation plays a key role in deposition of organic material within marsh sediments. The influence of biota on salt-marsh landscapes is indeed well established. However, a fascinating and relevant question is whether one can identify the signatures of the underlying and intertwined physical and biological processes in marsh landscapes, and indeed infer from them the dynamic behavior of these coupled physical and biological systems. Can one detect landscape features that would not have emerged in the absence of interactions and feedbacks between physical and biological processes?Here we use field evidence and a two-dimensional biomorphodynamic model to show that the interplay between physical and biological processes generates striking biological and morphological patterns. One such pattern, vegetation zonation, consists of a mosaic of vegetation patches, of approximately uniform composition, displaying sharp transitions in the presence of extremely small topographic gradients. The model describes the mutual interaction and adjustment between tidal flows, sediment transport, morphology, and vegetation distribution, thus allowing us to study the biomorphodynamic evolution of salt-marsh platforms. A number of different scenarios were modelled to analyze the changes induced in bio-geomorphic patterns by varying environmental forcings, such as the rate of relative sea level rise (RSLR) andsediment supply (SS), and by plant species with different characteristics. Model results show how marsh responses to changes in forcings are highly spatially dependent: while changes in SS most directly affect marsh areas closest to the channels, changes in the rate of RSLR affect the marsh platform as a whole. Organic sediment accretion is very important for allowing marshes to compete with increasing rates of RSLR near watershed divides, whereas inorganic sedimentation is more important closer to the channels. Increasing sediment supply to coastal marshes, therefore, might not compensate for rising sea levels, particularly in the inner marsh portions. Model results also emphasize that biodiversity is strongly controlled by environmental forcings and that zonation patterns are a signature of bio-geomorphic feedbacks with vegetation acting as a landscape constructor which feeds back on, directly alters, and contributes to shape tidal environments. Finally, the model generates realistic frequency distributions of vegetation occurrence, which nicely meet observed ones.