Sediment Exchange Across Coastal Barrier Landscapes Alters Ecosystem Extents

Abstract

Coastal barrier systems-barrier islands or spits and their associated salt marshes, bays, and coastal forestssupport rich ecosystems and faunal communities, buffer the impacts of storms on coastal regions, sequester carbon, and host culturally and economically important infrastructure (Barbier et al., 2011). The services provided by the various ecosystems found along barrier coastlines depend nonlinearly on their spatial extent (Barbier et al., 2008), which is especially sensitive to changes in relative sea-level rise (RSLR), storms, and sediment supply. In response to RSLR, barriers tend to migrate upward and landward primarily through the process of overwash, where sediment from the front (ocean side) of the barrier is swept to the back during storms. Marshes build soil vertically through physical and biological feedbacks that allow marshes to survive RSLR until a threshold rate (e.g., Kirwan & Megonigal, 2013;Morris et al., 2002) beyond which marshes drown and transition to subaqueous environments (e.g., Kirwan et al., 2010;Reed, 1995). Changes in marsh extent also occur horizontally via the competition between wave erosion at the marsh edge, which can lead to marsh loss even when a marsh is stable in the vertical dimension (Fagherazzi et al., 2013), and marsh progradation into the adjacent bay. Bay widening or narrowing occurs as direct consequence of marsh edge erosion or progradation, respectively. Coastal forest ecosystems lose extent, and marshes gain extent, from the progressive flooding of RSLR, as salinity and moisture stress lead to the decline of forest species at dynamic intertidal-subaerial margins (Fagherazzi et al., 2019;Kirwan & Gedan, 2019).