Marsh Collapse Does Not Require Sea Level Rise

. Salt marshes are among the most productive ecosystems on Earth, providing nurseries for fish species and shelter and food for endangered birds. Salt marshes also mitigate the impacts of hurricanes and tsunamis, and sequester large volumes of carbon in their peat soil. Understanding the mechanisms responsible for marsh stability or deterioration is therefore a key issue for society. Sea level rise is often viewed as the main driver of salt marsh deterioration. However, while salt marshes can reach equilibrium in the vertical direction, they are inherently unstable in the horizontal direction. Marsh expansion driven by sediment supply rarely matches lateral erosion by waves, creating a dynamic landscape. Recent results show that marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is higher than the input of sediment from nearby rivers or from the continental shelf. We propose that the horizontal dynamics and related sediment fluxes are key factors determining the survival of salt marshes. Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Ancient Venetians understood this dynamic very well. They manipulated the supply of sediment to the Venice lagoon, Italy, in order to control the long-term evolution of the intertidal landscape.

It quickly developed into one of the most powerful mercantile states in human history. At its apogee, the city was the third largest in Europe and the terminus for lucrative goods that traveled from the Far and Middle East on the Silk Road.
For population density and diversity and cultural and economic relevance, Venice was qualitatively the equivalent of New York City in the twentieth century.
Venice's location-surrounded by water-was critical for its defense.
Venetians understood that the intertidal landscape is extremely dynamic, with rivers, waves, and currents constantly reshaping the coast and creating a complex succession of salt marshes, tidal flats, and channels. The ongoing silting of the lagoon was of particular concern in the fifteenth century. Large rivers, carrying sediment from the mountains to the ocean, were debouching into the lagoon, infilling large areas. Similar shallow lagoons were converted to land both north and south of the Venice lagoon, cutting off coastal cities from the ocean.
To counteract the silting of the Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Ancient Venetians understood this dynamic very well. They manipulated the supply of sediment to the Venice lagoon, Italy, in order to control the long-term evolution of the intertidal landscape. Other societies have also dealt with the delicate equilibrium between land and sea. Ancient Chinese relocated coastal cities at the mouth of the Yellow River due to complex erosion/accretion patterns (Chen and Zong, 1998), and Frisians were among the first to erect dykes to hold back the advances of the sea (Charlier et al., 2005) pers. comm., 2013). Indeed, new evidence shows that salt marshes are particularly weak when exposed to wave action.

Tidal Flat
Marsh Scarp Low Tide High Tide Wave Thrust Figure 2. Thrust exerted by waves on a marsh scarp. The thrust is maximum when the water level is just below the marsh platform and decreases during high tides or storm surges. Modified after Tonelli et al. (2010) These physical and biotic characteristics determine erosion resistance and exposure to wave activity.
Aboveground vegetation slows flow velocities, traps sediment, and attenuates waves and turbulence (Christiansen et al., 2000;Leonard and Croft, 2006;Mudd et al., 2010;Riffe et al., 2011). At the same time, belowground roots and rhizomes help to stabilize marsh sediment (Coops et al., 1996;Micheli and Kirchner, 2002;Sean McLoughlin, pers. comm., 2013) and play an important role in reducing erosion. Edge stability is a function of the binding capacity of the root system to sediment, which is determined by the biomass, length, diameter, and tensile strength of the roots (van Eerdt, 1985).
Root strength typically decreases with depth, making marsh edges susceptible to undercutting. Excessive nutrients can also weaken creek banks and marsh boundaries, triggering slumping and lateral erosion. In fact, high nutrient levels increase aboveground leaf biomass, decrease the dense, belowground biomass of bank-stabilizing roots, and increase microbial decomposition of organic matter, leading to weaker, more porous soil (Deegan et al., 2012). Sediment shear strength increases as the ratio of root biomass to sediment mass increases, and marshes with dense root mats are generally more resistant to erosion from wave attacks and tidal currents (van Eerdt, 1985;Allen, 1989;Micheli and Kirchner, 2002;Watts et al., 2003). However, Feagin et al. (2009) failed to find a relationship between belowground biomass and edge erosion, and attributed erosion resistance to sediment characteristics, including bulk density, percent sand, water content, and organic matter. Their results suggest that above a threshold bulk density of 0.9 g cm -3 , increases in the fractions of very coarse sand and bulk density lead to higher erodibility. In contrast, McLoughlin (2010) found a strong inverse correlation between bulk density, fraction of sand, and erosion rate. Lessconsolidated sediment is more easily eroded than firmer, muddier sediment, and edges with sandy sediment are typically more susceptible to undercutting from wave action than those with finergrained sediment (Allen, 1989).
The abundance and composition of invertebrates in marshes, including burrowing crabs and bivalves, also influence marsh edge resistance to erosion (McLoughlin, 2010). Dense, interconnected crab burrows, which can reach densities as high as 700 m -2 along some marsh edges, decrease sediment shear strength and increase permeability and water content, ultimately reducing soil strength and erosion resistance (Allen and Curran, 1974;Montague, 1980;Escapa et al., 2008). On the other hand, the presence of bivalves such as the ribbed mussel Guekensia demissa may stabilize marsh edges and reduce erosion rates by both slowing wave and current velocities and binding sediment to the root mat (Bertness, 1984).
Intertidal oyster reefs adjacent to marsh edges may similarly reduce wave energy and erosion rates (Meyer et al., 1997;Piazza et al., 2005;Scyphers et al., 2011). Within the Virginia Coast a par aDigm ShiFt: marSheS aS NONequiliBrium l aNDScape S There is strong evidence that salt marshes are very resilient to increases in sea level (Kirwan et al., 2010). An increase in sea level results in more flooding of the marsh surface, and, therefore, there is more time for sediment to settle on the platform (Reed, 1995;Temmerman et al., 2005). This feedback keeps the marsh  In fact, high inputs of sediment can counteract very fast rates of sea level rise (Yang et al., 2001). If the rate at which waves and currents are removing sediment from the marsh boundary is higher than the rate at which sediment is provided by rivers and by the adjacent sea or continental shelf, the marsh will enter into an erosive state, and this state can be irreversible even in absence of sea level rise (Mariotti and Fagherazzi, 2013 (Mariotti and Fagherazzi, 2013).
While our findings are readily applicable to coastal areas with substantial river inputs, they also apply to fringing marshes, in which the ocean is the sediment source. Again, a marsh can expand even in presence of sea level rise if sediment supply and organogenic accumulation are large enough to offset drowning and lateral erosion (e.g., Redfield, 1965).