Biologically‐controlled multiple equilibria of tidal landforms and the fate of the Venice lagoon

Abstract

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

Influence of storm surges and sea level on shallow tidal basin erosive processes

“…If the erosion rate in the tidal flats is greater than the increased deposition rate, then a global positive feedback will be established and marsh retreat will accelerate until they disappear, while tidal flats will keep on deepening. A stable condition will eventually occur only when tidal flats become deep enough to eliminate the effect of wave induced bottom shear stress [Fagherazzi et al, 2006;Marani et al, 2007;Carniello et al, 2009b].…”

“…Two distinct erosional mechanisms are associated with wind waves. Wave-generated shear stresses, combined with tidal currents, are the main mechanism responsible for sediment resuspension on tidal flats [Carniello et al, 2005;Fagherazzi et al, 2006Fagherazzi et al, , 2007Marani et al, 2007], and regulate both sediment concentration in the water column (and hence light availability at the bed) [e.g., Lawson et al, 2007] and sediment export to salt marshes and to the ocean [Mariotti and Fagherazzi, 2010]. Waves impacting salt marsh boundaries produce intermittent forces that promote marsh edge erosion and salt marsh regression.…”

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

“…An outcome of such pursuit is the generation of some striking biological and morphological patterns at different scales. At the large scale, marshes, tidal flats and subtidal areas form the 'universal' texture of the tidal zone [3][4][5][6][7]. At a smaller scale, zonation patterns, a mosaic of sharply bounded and nearly homogeneous vegetation patches, are widespread in marshes worldwide [8][9][10].…”

“…[1][2][3][4]). An outcome of such pursuit is the generation of some striking biological and morphological patterns at different scales.…”

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].…”