CAPE COD’S SALT MARSH DIEBACK PROBLEM- MARKED ELEVATION CHANGE, SHIFTS IN PLANT DISTRIBUTION AND EROSION STEMMING FROM HERBIVORY BY A NATIVE CRAB SPECIES

Denudation of salt marsh vegetation (aka salt marsh dieback) on Cape Cod, MA is driven by herbivory of a native, nocturnal crab species, Sesarma reticulatum. Large areas of bare sediments as well as rapid shifts in the distribution and abundance of salt marsh plants are occurring at the mouth of the Herring River in Wellfleet, where high densities of these crabs occur. Ground-based elevation surveys in 2011 and 2012 were conducted to assess annual marsh surface elevation change at this site. The results were also compared to a 2011 airborne LiDAR survey. Sediment accumulation rates associated with: 5 plant species, dead vegetation and bare areas were quantified. Finally, total suspended solids (TSS) concentrations below dieback areas as well as areas of intact vegetation were assessed. Estimates of the quantity of sediment lost varied widely according to the method of data collection; ground based surveys with real-time kinematic and total station equipment produced consistent and feasible loss rates. In contrast, the error associated with LiDAR was substantial even in this area of relatively sparse vegetation. Plant community type affected dieback erosion rates as measured by TSS with the maximum difference in TSS concentration five times higher at a dieback site than at a vegetated site. Shifts in plant distributions driven by elevation change, herbivory and sediment characteristics have occurred. Difference in the susceptibility to herbivory as well as sediment accumulation rates associated with different communities will synergistically lead to continued rapid change in the physical and biological characteristics of this site. The predicted consequences for the ecosystem function of this site and similar dieback sites in the region will be discussed.