Assessing Long-Term Trends in Lateral Salt-Marsh Shoreline Change along a US East Coast Latitudinal Gradient

Marshes are valuable intertidal habitats that respond to changes in their environment, and their perimeters can rapidly advance or retreat over time. This study used the analyzing moving boundaries using R (AMBUR) tool kit to measure approximately 70 years of edge change at salt marshes within three Long-Term Ecological Research sites along the U.S. East Coast: Georgia Coastal Ecosystems (GCE), Virginia Coast Reserve (VCR), and Plum Island Ecosystems (PIE). At each site, changes were assessed at the open-fetch marsh outer perimeter as well as throughout interior channels of varying sizes. At the open-fetch marsh outer perimeter, both the PIE and VCR study marshes exhibited significant net retreat, with the fastest rates in areas exposed to high fetch where wave action is strong, whereas the GCE marsh exhibited significant net advance. Changes in the sinuous interior channels were smaller, with channels often retreating on one edge but were balanced by advance on the opposite bank. When advance and retreat in the interior channels were considered along with the outer perimeter, the GCE and VCR study marshes exhibited dynamic stability in which overall marsh edge showed no significant net change, and the overall rate of marsh retreat at PIE, although still significant with respect to the uncertainty of the analysis, was considerably reduced. This study demonstrates the importance of assessing shoreline changes throughout the marsh, as rates of retreat and advance at the open-fetch marsh perimeter may differ greatly from those in the interior, and not be indicative of the overall change in marsh edge.