Importance / Issues
The mean elevation of salt marsh surfaces must increase to keep pace with the annual rise in sea level and subsidence of salt marsh organic substrates. If the sedimentation rates in a salt marsh do not equal or exceed the net loss in elevation due to the steady increase in sea level and salt marsh subsidence, it will "drown." When a salt marsh "drowns," the surface of the marsh becomes sub-tidal which can cause drastic habitat changes such as the conversion of vegetated salt marsh to unvegetated mud flat.
Understanding changes in relative salt marsh elevation is important for interpreting changes in salt marsh vegetation communities. Salt marsh erosion and accretion are also important parameters for measuring the response of formerly impounded marshes to restoration of tidal influence, and will be particularly critical if the rate of sea level rise accelerates as predicted.
This project is also part of a worldwide effort to monitor sea level rise with sediment erosion tables (SETs) and cryogenic coring devices. These two techniques measure the amount of erosion and accretion on salt marsh surfaces.
Determine long term trends in salt marsh elevation at selected sites in NCBN parks and factors contributing to the observed changes (sediment deposition or erosion).
Assateague Island National Seashore (ASIS)
Cape Cod National Seashore (CACO)
Colonial National Historical Park (COLO)
Fire Island National Seashore (FIIS)
Gateway National Recreation Area (GATE)
Salt marsh sediment elevation change is measured using surface elevation tables (SETs). The SET instrument is a portable mechanical device (Figure 3) that provides high precision measurements of relative elevation change in wetland sediments or shallow water bottoms relative to the base of the rod SET (RSET) mark to which it is attached. The measurements are repeatable (each pin re-occupies the same point on the wetland surface at each sampling), can be made over long periods of time, and are of sufficiently high resolution to compare to long term sea level trends measured by tide gauges.
Sediment accretion is measured using marker horizons and either cryogenic corers or the "marsh plug" method. Marker Horizons (MH) are artificial soil layers established on the surface of the wetland or shallow water bottoms to measure vertical accretion. Numerous materials such as sand, feldspar, brick dust and glitter are used as marker horizons. Powdered white feldspar clay is the most common; its main advantages are that it is bright white and easily distinguishable from surrounding sediments, has a higher density than water, and forms a colloidal layer when wet (so it can be deposited as a contiguous layer even in open water). Cores are taken through this layer, and the thickness of the sediment (both mineral and organic) accumulated above the layer is measured as vertical accretion. Accretion rates are typically expressed as millimeters of accretion per year.