Dynamics of surface elevation and microtopography in different zones of a coastal Phragmites wetland

Surface elevation and microtopography of coastal wetlands are important attributes with respect to the adaptation to sea level change. While the total increase in elevation determines if a wetland can keep up with sea level rise, microtopography impacts, inter alia, hydrological patterns, which again drive surface elevation changes. We analyzed the dynamics of surface morphology during the course of one year at five locations along a transect from land (interior zone) to sea (fringe zone) in a coastal Phragmites wetland at the Darss-Zingst Bodden Chain (Southern Baltic Sea) using a modified Surface-Elevation Table. Landwards this wetland is confined by a dyke while the seaward boundary of the wetland is formed by a micro-cliff. A small strip between the interior and the fringe zone is characterized by small basins (basin zone). Surface elevation changes in the wetland interior are mainly driven by its own productivity. Litter accumulation by the dense Phragmites stands is high, but a combination of high organic sediments and water scarcity led to compaction and counteracted vertical accretion. While the surface elevations of the measurement location in the basin zone and one of the locations in the interior zone remained fairly stable (intra-annual changes<+/- 0.8 mm), the other location in the interior showed a drastic decrease in surface elevation (-3.3 cm year(-1)). This large deviation may have been caused by the high spatial variability of sediment organic matter. The micro-cliff eroded constantly (3.7 cm year(-1)); its steepness and surface roughness increased making it more vulnerable to the forces of the sea. The micro-cliff protects the wetland interior from flooding but at the same time suppresses allogenic sediment input. The simple approach to use freely available water level data to predict morphological changes in the fringe zone did not yield significant model results. However, the strong intra-annual variability in elevation change especially in the fringe zone showed that short-term changes may often mask long-term trends. Although we cannot predict long-term developments from our relatively short-term measurements, our results indicate that wetland evolution in such squeezed environments is threatened. In dyked wetlands sediment supply from the land is suppressed and lateral erosion at the seaward edge leads to marsh retreat. None of the measuring locations is currently keeping up with local sea level rise. (C) 2016 Elsevier B.V. All rights reserved.