SALT MARSH LOSS: DROWNING OR RETREAT?

Understanding the mechanisms responsible for salt marsh deterioration is a key issue for society. Marsh drowning, occurring when vertical accretion on the marsh platform cannot keep pace with sea level rise, is often viewed as the main driver for salt marsh loss. However, while salt marshes can reach a stable 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. Marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is larger than the input of sediment from nearby rivers or from the continental shelf. A simple model predicts that with low and moderate rates of sea level rise (5 mm/yr), retreat is a more likely marsh loss modality than drowning. In addition, conditions associated with marsh retreat increase the system resilience by transferring sediment on the marsh platform, in accordance with measurements of high sedimentation rates in rapidly retreating marshes and during storms. We conclude 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.