The importance of being coupled: Stable states and catastrophic shifts in tidal biomorphodynamics

Abstract

[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.

The secret gardener: vegetation and the emergence of biogeomorphic patterns in tidal environments

“…Marani et al [3,20] and D'Alpaos et al [21] have examined the numerical solution of equation (2.1) when this is integrated in space to describe the time evolution of the mean elevation of a tidal platform (scales indicatively larger than 1 km). Hydrodynamic flows and sediment transport in this lumped approach are parametrized to describe the exchange of water and sediment between the channel (where the tidal wave propagates and suspended sediments are transported) and the tidal platform.…”

“…C is the instantaneous suspended sediment concentration (SSC),C(z, B, t) = C 0 when dh/dt > 0 (C 0 being the prescribed SSC in the channel), whereasC(z, B, t) = 0 when dh/dt < 0. Note that w s C(t)/ρ b and αB β C(t)/ρ b are the instantaneous settling and trapping fluxes, respectively, which, by integration over all tidal cycles in 1 year yield the yearly mean total inorganic flux, Q s (t), from the water column to the surface [20]. We assume w s = 0.2 mm s −1 for the settling velocity (typical of a sediment size of 20 µm [36]), whereas α and β synthetically account for suspended sediment trapping by the canopy (see [20] for details).…”

“…Note that w s C(t)/ρ b and αB β C(t)/ρ b are the instantaneous settling and trapping fluxes, respectively, which, by integration over all tidal cycles in 1 year yield the yearly mean total inorganic flux, Q s (t), from the water column to the surface [20]. We assume w s = 0.2 mm s −1 for the settling velocity (typical of a sediment size of 20 µm [36]), whereas α and β synthetically account for suspended sediment trapping by the canopy (see [20] for details). ρ b = ρ s · (1 − p) is the bulk density of the sediment after compaction has taken place, with ρ s = 2650 kg m −3 and porosity p = 0.5.…”

“…In particular, we show that depositional intertidal patterns emerge from, and are the signature of, two-way feedbacks between accretion rates, inorganic sediment transport and biological processes controlling organic sediment productivity. To this end, we first briefly review contributions on biogeomorphic equilibria in a conceptual lumped setting [20,21]. Subsequently, building on new observational and modelling results [22], we introduce original analyses on the spatially extended equilibrium configuration of salt marshes, showing how multiple competing stable states, largely regulated by biota, provide a unifying framework to explain emerging patterns in tidal environments.…”

Understanding and predicting wave erosion of marsh edges

“…Much attention has been devoted to the understanding and description of the processes which lead to observed equilibria in the vertical direction, or lack thereof, producing a rather comprehensive understanding of the controlling biological and physical processes [Allen, 1990;Morris et al, 2002;D'Alpaos et al, 2007;Kirwan and Murray, 2007;Marani et al, 2007;Mudd et al, 2009;Marani et al, 2010]. On the contrary, "lateral" evolution mechanisms have received comparatively much less attention, even though marsh degradation associated with edge erosion is arguably the chief mechanism by which marshes in coastal areas worldwide are being lost [Schwimmer, 2001;Gedan et al, 2009;van de Koppel et al, 2005;Mariotti and Fagherazzi, 2010].…”