Spartina-dominated salt marshes (about 250-ha) fringe much of the Great South Bay back-barrier portions of Fire Island National Seashore (FIIS), a 50-km barrier island.  There are no quantitative studies to document if these salt marshes are keeping pace with sea level; however, recent data assembled by the NY Department of Environmental Control suggests that back-barrier salt marshes to the west of Fire Island (Jamaica Bay) are being lost by erosion and submergence processes at an alarming rate.  Salt marshes associated with Fire Island National Seashore provide essential nursery habitat for estuarine fishes, shorebirds/waterbirds, and other trophic groups.  

The US Army Corps of Engineers, in a draft Environmental Impact Statement (1999), has proposed several alternative projects aimed at stabilizing the Fire Island barrier island (e.g., immediate closure of any storm-caused breaches through the island, artificial dune creation on the ocean beach, beach nourishment of the ocean shoreline, etc.).  The intent is to reduce storm-induced flooding of Great South Bay and protect human development from flooding and storm damage. Some of these activities are presently underway (e.g., interim plan for breach closure).  Great South Bay marshes have evolved over the past several millennia in association with a dynamic barrier system, characterized by episodic sediment delivery to the Bay by overwash and inlet processes.  At present, we don’t have an adequate information base to predict how salt marshes in Great South Bay, and particularly along the Fire Island shoreline, will respond to barrier island stabilization efforts. 

The purpose of this project is to evaluate the historic and present response of FIIS salt marshes to sea level rise and other factors relevant to salt marsh development. 

·        Are FIIS salt marshes currently keeping pace with sea level?

·        What factors are most influential in controlling the development and maintenance of salt marshes at FIIS? (e.g., sea level, storm events, proximity to inlets or overwash areas, subsurface decomposition processes, etc.). 

By addressing these questions, it will be possible to make informed predictions as to the future of salt marshes within Great South Bay and predictions on the response of salt marshes to proposed barrier island manipulations.

The 50-km Fire Island is bounded by Fire Island Inlet to the west and Moriches Inlet to the east (Fig. 1).  It is hypothesized that salt marshes in the vicinity of these inlets are influenced by a different set of factors or magnitude of factors than marshes that are located away from inlets.  Tidal range, tidal currents, and the potential for sediment transport are greater near inlets and these are all factors that are especially important in controlling marsh development. Also, marshes near portions of the barrier island with historic overwash episodes (e.g., Old Inlet portion of FIIS) will reflect different marsh development processes than those remote from overwash events.  This produces a gradient of conditions that will be used in the project study design.  Marsh development processes will be evaluated along a continuum from inlets to areas away from inlets and from historic overwash areas to areas remote from overwash.  Field sites include salt marshes at Watch Hill (remote from inlet), Hospital Point (historic overwash) and Smith County Park at Moriches Inlet (near inlet).  

The study will rely on two principal methods:  1) Surface Elevation Table (SET) and soil marker horizons to evaluate the relationship among marsh elevation, sedimentation, and sea level rise and understand processes controlling sediment elevation changes, and 2) sediment cores to evaluate historic responses of marsh development to barrier island dynamics and storms. Three SET-accretion monitoring stations were established at each of the three marsh areas, where sediment elevation and accretion are being monitored every three to four months.  Sediment cores were collected in the vicinity the SET-accretion monitoring stations at each marsh area for evaluation of historic accretion rates using cesium 137 and lead-210 methods.