Assessing tidal salt marsh elevation along the coast of Maine, United States using unmanned aerial systems (UAS) and stable carbon isotopes

Elevation is a key control on the frequency and duration of flooding experienced by a salt marsh over the course of the tidal cycle, which in turn modulates the deposition of sediment onto the marsh surface. The amount of sediment deposited onto the marsh surface is an important factor in the development of the salt marsh and its ability to withstand sea level rise. Human interference in the form of agricultural practices (e.g., ditching and embayments) and mosquito control significantly altered the structure and function of salt marshes throughout New England with lasting impacts on marsh platform elevation and, consequently, the persistence of salt marshes in the face of sea level rise. This study establishes the present-day distribution of elevation and vegetation zones for a salt marsh in Maine, United States, and compares these baseline measurements to past estimates of elevation made using carbon stable isotopes (δ¹³C) measured in sediment cores. A LiDAR scan and a series of multispectral air photos were collected from a representative salt marsh in Maine (Cousins River Marsh, Yarmouth, ME). The LiDAR scan is processed to create a digital elevation model (DEM) of the marsh and the air photos are converted into a 2D digital model of the marsh platform. In New England salt marshes, an elevation-mediated gradient in vegetation exists across the marsh surface, with the most salt-tolerant species residing in lower-elevation areas. Different species of marsh grasses produce varying δ¹³C values, and once incorporated into the marsh peat, can potentially be used to identify changes in vegetation cover through time. Sediment cores collected from Cousins River are sub-sampled and analyzed for down-core variations in δ¹³C to assess salt marsh paleovegetation. Short-term radioisotopes ²¹⁰Pb and ¹³⁷Cs are used to produce age-depth models by integrating sedimentation over ~100 and ~70 years, respectively, and are correlated to stable carbon isotope results for an approximation of salt marsh elevation change. Results will inform our understanding of the relative influences of sea level rise and human-driven landscape alteration on salt marsh morphodynamics along the coast of Maine, with implications for salt marshes throughout New England.