AN EXAMINATION OF INCREASED EROSION AND RELEASE OF CARBON FROM A CAPE COD SALT MARSH SYSTEM

Over the past six decades, the salt marshes of Wellfleet, MA below the Herring River dike have experienced significant vegetation loss, termed salt marsh dieback. Dieback occurs both in Spartina alterniflora stands along tidal creek banks and in patches throughout the low marsh, and in Spartina patens along the seaward edge of the high marsh. Loss of S. alterniflora appears to be caused by herbivory of the purple marsh crab Sesarma reticulatum while the exact source of S. patens dieback remains unclear. Due to the loss of healthy root material to stabilize the peat, dieback areas exhibit greater erosion than healthy areas at comparable elevations. This study investigates the effect of vegetative dieback on sedimentation and erosion on the marsh surface.

Dieback areas experience increased sediment suspension and erosion, resulting in the release of organic carbon from salt marsh peat to the estuarine system. Experiments and observations include: sedimentation and erosion patterns within a tidal creekshed; volumetric calculation of material eroded from dieback sites; sediment deposition in healthy versus dieback regions of the low marsh; and total suspended solids in the water column during flood versus ebb tidal flow, spring versus mean high tides, healthy versus devegetated sites, in high marsh versus low marsh. These related observations indicate significant loss of material from the marsh surface. Levels of suspended sediment in healthy high and low marsh locations during the flood tide are 34% higher on average than during the ebb tide, as healthy vegetation enhances sediment deposition. However, dieback sites in both high and low marsh had roughly 58% more sediment in suspension during the ebb tide than during the flood tide, suggesting increased sediment erosion. Further, low marsh sites below large high marsh dieback areas had approximately 4.5 times more suspended sediment than their counterparts below healthy high marsh, consistent with significant erosion from the high marsh dieback sites. Volumetric calculations of sediment lost from ten of the surveyed dieback patches based on field measurements indicate erosion and export of 81,641 L of peat material, correlating to an approximate release of 11,668 kg of C from the marsh back into the tidal system, based on mean bulk density and loss-on-ignition analyses.