Salt marshes are dynamic environments, increasing in vertical elevation and migrating, often landward, as sea level rises. With sea level rise greater than marsh elevation increase, marshes can be submerged, marsh soils become waterlogged, and plant growth becomes stressed, often resulting in conversion of vegetation-dominated marsh to mudflat or open water habitat. Given that the rate of sea level rise is expected to accelerate over the next century, it is important to understand the processes that control marsh development. More specifically, the objectives of this project were to quantify vertical marsh elevation change in relation to recent rates of sea-level rise and to investigate factors or processes that are most influential in controlling the development and maintenance of Fire Island salt marshes.
The 50-km Fire Island is located on the south shore of Long Island (NY). The island is bordered by Moriches Inlet to the east and Fire Island Inlet to the west, exchanging with the Atlantic Ocean and the back-barrier lagoon system of Great South Bay and Moriches Bay. Spartina alterniflora dominated salt marsh occurs along the bay shoreline of Fire Island. Three marsh areas were selected for study; Great Gun Meadows, Hospital Point, and Watch Hill, 2.5 km, 12 km, and 20 km, respectively, from Moriches Inlet.
Surface Elevation Tables (SETs), in conjunction with feldspar marker horizons, were used to evaluate recent (2002 to 2007) relationships between marsh surface elevation change and rates of relative sea level rise and understand the surface and subsurface processes that influence marsh elevation change. The elevation of a salt marsh is controlled by sediment accretion and organic matter build-up, resulting in increases in elevation, while the subsurface processes of sediment compaction/subsidence and organic matter decomposition, as well as erosion of surface sediments contribute to elevation loss. The surface accretionary processes are monitored by repeated sampling of artificial marker horizon plots and marsh surface elevation is correspondingly monitored with the surface elevation table. SETs and marker horizons were established at the three marsh areas in August 2002, with monitoring proceeding for a 58 month period to May 2007. SET and marker horizon monitoring is planned to continue for the long-term. Marsh surface elevation change is compared to sea level rise to determine if the marsh is keeping pace with sea level.
All three sites
reveal an elevation deficit when compared to sea level rise; the marshes
appear to not be keeping pace with rates of sea level rise. Based on
the SET elevation monitoring during the 58 month study period, elevation
change of the marsh surface ranged from an increase of 2.04 mm y-1 and
2.08 mm y-1 at Hospital Point and Watch Hill, respectively, to an elevation
decline of -1.05 mm y-1 at Great Gun. Records of relative sea level
over the past 60 to 100 years from NOAA water level stations in the
vicinity of Great South Bay/Moriches Bay (Montauk Point and Battery,
NY; Sandy Hook, NJ) ranged from 2.52 mm y-1 to 3.79 mm y-1, all greater
than measured marsh elevation changes. However, it is noted that the
marsh surface elevation trend determined for Great Gun may not be representative
of the larger Great Gun marsh because the SET monitoring may have occurred
in a portion of marsh where a natural marsh drainage was forming.
Based on the chronology from radiometric dating of the cores, it was estimated that salt marsh development was initiated at the Great Gun site around 1766 AD and Hospital Point near 1778, coinciding with establishment of nearby Hallets (1788) and Smiths (1773) inlets, respectively. The role of storm-induced inlets and barrier island overwash events in the bayward transport of sediment, flood tidal delta formation, and marsh development is well-known. Also related to inlets, at the Great Gun marsh there was a clear correlation between the opening of Moriches Inlet in 1931 and the abrupt termination of salt marsh peat development, replaced by deposition of inorganic sediment. It is likely that the tidal range increased substantially at Great Gun with the opening of the new inlet, the existing Spartina marsh was inundated and converted to tidal flat, then with subsequent re-development of marsh to the present as hydrologic conditions became favourable for vegetation to thrive. Inlets and associated flood tidal deltas represent a fundamental process supporting the establishment of back-barrier salt marsh habitat. The long-term maintenance of salt marshes at Fire Island seems tightly coupled to preservation of inlet processes.
The Fire Island marsh study sites are in an elevation deficit relative to the long-term rate of sea-level rise at Sandy Hook. This deficit trend could continue for the long-term, there could be a pulse of sediment delivered to the marsh surface during a future storm event, or the marshes may have the capacity to periodically adjust, over the long-term, during episodes of low rates of sea-level rise. If the observed elevation deficit continues, it is likely that the Fire Island marshes will become wetter, areas of high marsh Spartina patens may convert to Spartina alterniflora, and open water habitat may increase. There could also be a landward encroachment of marshes to upland areas, a natural process of marsh development in response to sea level rise, assuming that cultural development (e.g., bulkheads) will not impede this migration. Given that the global rate of sea-level rise is expected to accelerate over the next century, and that some marshes in the northeast show evidence of submergence, it is especially important to continue monitoring of marsh elevation changes in response to sea-level rise.
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