Orencio Duran Vinent, Ellen Herbert, Matthew L Kirwan

Salt marshes are simultaneously among the most valuable and vulnerable ecosystems in the world. We use a simplified formulation for sediment transport across marshes to explain why marshes are most vulnerable to sea level rise (SLR) in microtidal environments. We find inorganic sediment decay length scales with tidal range so that inorganic deposition is very low in the interior of microtidal marshes regardless of the suspended sediment concentration at marsh edge. We also find that drowning of interior marshes eventually leads to a runaway marsh loss due to the approximate scale invariance of inorganic deposition. Thus, organic accretion rather than inorganic accretion is the key factor determining microtidal marsh survival. In fact, because in many locations the rate of SLR is close to or exceeds a theoretical maximum organic accretion rate for tidal salt marshes, our results suggest impending drowning of global microtidal marshes regardless of local sediment supply.