VULNERABILITY ASSESSMENT OF THE FRINGING SALTMARSH OF NAPATREE LAGOON: WESTERLY, RI

Erosion of salt marshes is linked to many factors including; sea level rise, wave erosion and biological processes, and poses a risk of habitat loss for many species. Anecdotal observations of the fringing salt marsh at the Napatree Point Conservation Area (NPCA) in Watch Hill, Rhode Island is eroding, potentially jeopardizing an important ecosystem. The NPCA is a 2.4 km (1.5 mi) barrier spit separating Block Island Sound and Little Narragansett Bay, and is globally recognized as a sanctuary for endangered migratory shorebird species. The 260 m long fringing marsh examined here is narrow (12 m), and located within the 0.04 km² Napatree Lagoon. Multiple factors were assessed to determine the vulnerability of the marsh, including: elevation, platform stability, sediment composition, and crustacean burrowing. Previous studies of New England salt marshes have found that invasive crab species’ burrowing into the marsh can be responsible for marsh erosion. Monthly crab surveys in 2017/2018 indicated a population of green crabs (Carcinus maenas) live within the lagoon. Lateral burrows into the marsh were more common where the edge had the highest relief (0.59 m) and the edge is undercut, forming a ‘flap’. The undercut edge of the marsh could be caused by green crab burrowing, although that remains unknown. Fiddler Crabs (Uca) have also been captured burrowing on top of the marsh using time-lapse cameras, although the density of burrows (likely formed by Uca) on the marsh platform were low (n = 2 m^-2). The average elevation of the marsh platform based on RTK-GPS Elevation data from 2017 is 0.18 m NAVD88, compared to the elevation of MHW (0.36 m), suggesting the marsh platform is underwater during most high tides. Stakes were placed along the marsh edge and at two sites, the marsh has eroded backwards from the spike. The marsh stability determines its resilience to degradation from wave action. Stability and marsh composition data was gathered using an AgraTronx Soil Compaction Tester and Oakfield Soil Probe along 16 transects. The soil probes provided ~0.3 m cores of the marsh platform and indicated the marsh is composed of mostly sand with very little peat; most of the organic material was found in the northeast corner of the marsh.