Tidal marshes change dynamically in response to the deposition of mineral and organic sediment. The resulting evolution of marsh landscapes triggers changes in vegetation composition, which, in turn, feeds back on the rate of marsh accretion and the overall topography of the marsh. We hypothesize that feedbacks between constantly shifting sediments and marsh vegetation maintain biodiversity and marsh surface elevation in the absence of static physical gradients. We tested this hypothesis at Dyke Marsh Wildlife Preserve, a 24-ha freshwater tidal marsh on the western shore of the Potomac river, 13 km south of Washington, DC. Thirty-eight 1x1m plots were monitored for vegetation, sediment deposition and elevation from 2004 to 2012. We calculated elevation change in marsh surfaces using elevation surveys from 1992, 2005, 2011, and 2012, and quantified sediment deposition using bimonthly and seasonal-scale observations. The thirty-eight plots were classed into three elevation change categories - no change, accretion, and erosion – and correlated with changes in vegetation community composition.
Results/Conclusions
Changes in marsh surface elevation were spatially correlated. Northern portions of the marsh increased in elevation and southern parts lost ground to sea level rise. Still, considerable variability in elevation change occurred even within specific areas of the marsh. This variability is driven by sediment supply from the river and by differences in plant communities, which trap mineral sediments and produce their own organic sediments. We observed significant changes in community composition of marsh vegetation in the 38 permanent plots between 2004 and 2012. These shifts in vegetation appear to be more pronounced in areas of higher accretion. Impatiens capensis disappeared in areas that it was dominant in, whereas Schoenoplectus fluviatilis and Typha angustifolia invaded these areas. Thus, a shift from communities dominated by annual species to communities dominated by perennial species may be occurring with a loss in surface elevation. These changes, in turn, may drive differences in litter deposition, altering the balance between mineral and organic sediment deposition in the marsh and therefore ecogeomorphological dynamics.