Salt Marsh Elevation

Monitoring Objectives

• Determine the rate of salt marsh accretion or subsidence
• Determine trends in soil salinity

Background
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 accretion 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 and other estuarine ecosystem components. 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.

Monitoring Approach
The Southeast Coast Network will be deploying Rod Surface Elevation Table (RSET) monitoring stations at parks with significant saltmarsh wetland resources to measure the relative effects of surface and subsurface processes on salt marsh elevation change using or modifying the Louisiana Department of Natural Resources protocol (Folse and West, 2004). The RSET works on the same principle as the Surface Elevation Table (Cahoon et al. 2002) but permits the determination of elevation change occurring over different portions of the sediment profile because it can be attached to bench marks that are driven to both deeper and shallower depths than the SET. A minimum of three RSET stations will be deployed in each park through a spatially-balanced sampling design following the methods of Folse and West. Elevation measurements will be conducted annually at each station using the National Geodetic Survey's Procedures for Connecting SET Bench Marks to the NSRS (Geoghegan et al. 2009).

When attached to a bench mark rod, the RSET provides a constant reference plane in space from which the distance to the sediment surface can be measured by means of pins lowered to the sediment surface. Repeated measurements of elevation can be made with high precision because the orientation of the table in space remains fixed for each sampling. Fixed elevation stations will be co-located with fixed plots for salt marsh vegetation monitoring.

In addition, soil salinity and salt marsh accretion will be measured at all points where Vegetation Community monitoring occurs within network salt marshes.

Parks Where Protocol Will be Implemented
Cape Hatteras National Seashore (CAHA), Cape Lookout National Seashore (CALO) , Fort Pulaski National Monument (FOPU), Fort Frederica National Monument (FOFR), Cumberland Island National Seashore (CUIS), Timucuan Ecological and Historic Preserve (TIMU) , Fort Matanzas National Monument (FOMA) and Canaveral National Seashore (CANA)

Principal Investigator
Tony Curtis, Coastal Ecologist