The figure below-right shows te changes that have occurred in the relative surface elevations of control and fertilized salt marshes at North Inlet, SC. Fertilized marsh sites demonstrated a sudden increase in elevation relative to the controls in the second year of the experiment during 1977. This is due to the increased trapping of sediments on the fertilized plots where biomass density is far greater than on control sites. Between 1997 and 2000, the elevations of fertilized and control sites both increased at rates that exceeded the long-term rate of rise of mean relative sea level (ca. 3.4 mm/yr, see figure below). Marsh surface elevation in fertilized plots increased at a greater rate than in control plots, which is consistent with the idea that the trapping of sediment is enhanced by the vegetation. See Morris, J.T., P.V. Sundareshwar, C.T. Nietch, B. Kjerfve, D.R. Cahoon. 2002 Responses of coastal wetlands to rising sea level. Ecology.
The marsh surface is not able to precisely track interannual changes in mean sea level. Changes in the marsh surface lag behind and are less dynamic than changes in relative mean sea level. Feedbacks among sea level, biomass density, and sedimentation rate constantly readjust the marsh surface toward an equilibrium with relative sea level. These feedbacks include a positive effect of plant biomass on the sedimentation rate and a positive effect of sea-level rise on plant growth. In any marsh there is an optimum elevation of the marsh surface. The actual marsh surface may be greater than or less than the optimum. Below is a photo of a 'marsh organ' in Louisiana that is designed to determine the location of the optimum elevation relative to mean sea level.
