Twenty-year record of salt marsh elevation dynamics in response to sea-level rise and storm-driven barrier island geomorphic processes: Fire Island, New York, USA

Twenty years of surface elevation table and marker horizon monitoring at three sites along the Fire Island (New York, USA) barrier island indicates that rates of marsh surface elevation change (Watch Hill, 4.4 mm year⁻¹; Hospital Point, 3.5 mm year⁻¹; Great Gun, − 0.3 mm year⁻¹) were lower than the rate of monthly mean sea-level rise during the 2002–2022 monitoring period (5.1 mm year⁻¹, NOAA Sandy Hook, NJ, water level station). The Great Gun monitoring site, with an elevation deficit relative to sea-level rise, shallow subsidence (surface accretion > marsh elevation rate), low elevation capital, prolonged marsh surface flooding, and declining vegetation cover, displays characteristics common to deteriorating marshes. The submergence trend was not as evident at the other monitoring sites, but with low tidal range (0.4 m) and projections of accelerated sea-level rise, sustainability is questioned if marsh elevation change continues to lag behind the local rate of relative sea-level rise. Hurricane Sandy occurred during the monitoring period (October 2012), creating a new inlet located about 300 m from one of the monitoring sites. Surprisingly, no immediate signals of deposition or erosion were noted from the marker horizon sampling. Overwash sand deposits on the marsh surface were extensive along Fire Island, although not reaching the monitoring sites, and will likely provide opportunities for future salt marsh growth, as will the flood-tide delta created by the inlet. Projecting the future of barrier island salt marshes under a regime of accelerated sea-level rise and episodic storms requires knowledge of marsh elevation and accretion processes and geomorphic dynamics.