Shallow tidal basins are characterized by extensive tidal flats and salt marshes that lie within specific ranges of elevation, whereas intermediate elevations are less frequent in intertidal landscapes. Here we show that this bimodal distribution of elevations stems from the characteristics of wave-induced sediment resuspension and, in particular, from the reduction of maximum wave height caused by dissipative processes in shallow waters. The conceptual model presented herein is applied to the Venice Lagoon, Italy, and demonstrates that areas at intermediate elevations are inherently unstable and tend to become either tidal flats or salt marshes.intertidal landforms T he distribution of elevations in shallow tidal basins such as the Venice Lagoon in Italy (Fig 1, tidal range of 0.7 m) shows that tidal flats have differences in elevation of few tens of centimeters, with an average elevation between Ϫ0.50 and Ϫ1.00 m above mean sea level (MSL), whereas salt marshes lie at an average elevation higher than ϩ0.20 m, with some variability dictated by local sedimentological and ecological conditions (1-4). Few areas are located at intermediate elevations (i.e., between Ϫ0.50 and ϩ0.20 m), suggesting that the processes responsible for sediment deposition and erosion produce either tidal flats or marshes but no landforms located at intermediate elevations. In the relatively pristine northern part of the Venice Lagoon, the most frequent bottom elevation is around Ϫ0.50 m (Fig. 2), similar to natural conditions in 1901 in the Southern Lagoon ( Fig. 1 A). During the last century, anthropogenic causes produced consistent bottom erosion in the Southern Lagoon, leading to a median elevation of approximately Ϫ1.00 m above MSL (Figs. 1B and 2). Nevertheless, all three distributions of elevations show a relatively low frequency of elevations between 0 and Ϫ0.5 m.Typical conceptual and numerical models of salt-marsh formation envision a gradual transformation of sand flats and mudflats in response to sediment buildup and plant colonization (5-7). However, the evidence points to abrupt transitions to one of two distinct stable outcomes. Salt marshes emerge from tidal flats in locations where sedimentation is enhanced by lower tidal velocities, higher sediment concentrations, or the sheltering effects of splits and barrier islands (1,8). Alternatively, in areas with consistent sediment resuspension caused by a combination of tidal fluxes and wind waves, tidal flats are dominant. In tidal flats, sediment deposition is balanced by erosion, and the bottom elevation is constantly maintained below MSL (9). Sediment resuspension by wind waves is decisive, because tidal fluxes alone are unable to produce the bottom shear stresses necessary to mobilize tidal-flat sediments (10).On the basis of a simplified model for wave generation in shallow water (10), we developed a conceptual model to study the distribution of bottom shear stress as a function of elevation. The results are used to explain the bimodal distribution of bathymetry in the Ve...

Looking across a tidal landscape, can one foresee the signs of impending shifts among different geomorphological structures? This is a question of paramount importance considering the ecological, cultural and socio-economic relevance of tidal environments and their worldwide decline. In this Letter we argue affirmatively by introducing a model of the coupled tidal physical and biological processes. Multiple equilibria, and transitions among them, appear in the evolutionary dynamics of tidal landforms. Vegetation type, disturbances of the benthic biofilm, sediment availability and marine transgressions or regressions drive the bio-geomorphic evolution of the system. Our approach provides general quantitative routes to model the fate of tidal landforms, which we illustrate in the case of the Venice lagoon (Italy), for which a large body of empirical observations exists spanning at least five centuries. Such observations are reproduced by the model, which also predicts that saltmarshes in theVenice lagoonmay not survive climatic changes in the next century if IPCC’s scenarios of high relative sea level rise occ

[1] We describe and apply a point model of the joint evolution of tidal landforms and biota which incorporates the dynamics of intertidal vegetation; benthic microbial assemblages; erosional, depositional, and sediment exchange processes; wind-wave dynamics, and relative sea level change. Alternative stable states and punctuated equilibria emerge, characterized by possible sudden transitions of the system state, governed by vegetation type, disturbances of the benthic biofilm, sediment availability, and marine transgressions or regressions. Multiple stable states are suggested to result from the interplay of erosion, deposition, and biostabilization, providing a simple explanation for the ubiquitous presence of the typical landforms observed in tidal environments worldwide. The main properties of accessible equilibrium states prove robust with respect to specific modeling assumptions and are thus identified as characteristic dynamical features of tidal systems. Halophytic vegetation emerges as a key stabilizing factor through wave dissipation, rather than a major trapping agent, because the total inorganic deposition flux is found to be largely independent of standing biomass under common supply-limited conditions. The organic sediment production associated with halophytic vegetation represents a major contributor to the overall deposition flux, thus critically affecting the ability of salt marshes to keep up with high rates of relative sea level rise. The type and number of available equilibria and the possible shifts among them are jointly driven and controlled by the available suspended sediment, the rate of relative sea level change, and vegetation and microphytobenthos colonization. The explicit description of biotic and abiotic processes thus emerges as a key requirement for realistic and predictive models of the evolution of a tidal system as a whole. The analysis of such coupled processes finally indicates that hysteretic switches between stable states arise because of differences in the threshold values of relative sea level rise inducing transitions from vegetated to unvegetated equilibria and vice versa.

[1] During the last century, the Venice lagoon, Italy, has been experiencing a general degradation consisting of the deepening of tidal flats and the reduction of salt marsh areas. A conceptual model describing the long-term evolution of such lagoons has recently been proposed. According to the model, the long-term degradation consists of two steps: an initial salt marsh deterioration phase followed by a tidal flat erosion phase. In this work we test the long-term evolution model through the analysis of four different bathymetries of the Venice lagoon during the last century (1901, 1932, 1970, and 2003). The result of the analysis confirms that the recent past morphological evolution of the Venice lagoon has actually followed the proposed model and highlights a slower erosive trend characterizing the northern part of the lagoon compared to the moderately rapid erosion affecting the central southern part. This result enables us to infer the likely future evolution of the Venice lagoon as long as the present forcing conditions are maintained.

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.