Relationships between ecosystem properties and sea-level rise vulnerability of tidal wetlands of the U.S. Mid-Atlantic.

Tidal wetlands in the Mid-Atlantic, USA, are experiencing high rates of relative sea level rise, and it is unclear whether they will be resilient in the face of future flooding increases. In a previous study, we found 80% of our study areas in tidal freshwater and salt marshes in the Delaware Estuary and Barnegat Bay had elevation change rates lower than the 19-year increase in mean sea level. Here, we examine relationships between marsh elevation dynamics and abiotic and biotic parameters in order to assess their utility as indicators of vulnerability to relative sea level rise. We further apply a range of marsh vulnerability indicators including elevation change rates to evaluate their ability to corroborate marsh habitat change over the last 30 years. Of the field measurements, soil bulk density and belowground plant biomass were among the strongest predictors of elevation change and accretion dynamics across all marsh types and settings. Both tidal freshwater and salt marshes tended to have higher rates of elevation increase and surface accretion in areas where soil bulk density and live belowground biomass were higher. Nine of the ten marshes experienced a net loss of area from the 1970s to 2015 ranging from 0.05 to 14%. Although tidal freshwater marshes were low in elevation and experienced variable elevation change rates, marsh area loss was low. Conversely, salt marshes closest to the coast and perched high in the tidal frame with a higher degree of human modification tended to experience the greatest marsh loss, which incorporated anthropogenic impacts and edge erosion. Thus, our regional assessment points to the need for a comprehensive understanding of factors that influence marsh resilience including human modifications and geomorphic settings.

Retreating marsh shoreline creates hotspots of high-marsh plant diversity.

Marsh edge retreat by wave erosion, an ubiquitous process along estuaries, could affect vegetation dynamics in ways that differ from well-established elevation-driven interactions. Along the marshes of Delaware Bay (USA) we show that species composition from marsh edge to interior is driven by gradients in wave stress, bed elevation, and sediment deposition. At the marsh edge, large wave stress allows only short-statured species. Approximately 17m landward, decreasing wave stress and increasing deposition cause the formation of a ridge. There, high marsh fugitive and shrub species prevails. Both the marsh edge and the ridge retreat synchronously by several meters per year causing wave energy and deposition to change rapidly. Yet, the whole ecogeomorphologic profile translates landward in a dynamic equilibrium, where the low marsh replaces the high marsh ridge community and the high marsh ridge community replaces the mid-marsh grasses on the marsh plain. A plant competition model shows that the disturbances associated with sediment deposition are necessary for the high marsh species to outcompete the mid-marsh grasses during rapid transgression. Marsh retreat creates a moving framework of physical gradients and disturbances that promote the co-existence of over ten different species adjacent to the marsh edge in an otherwise species-poor landscape.