MORPHOLOGICAL CONTROLS ON MAINE'S SALT MARSHES: DYNAMIC SALT MARSH POOLS, GROUNDWATER, AND SEA-LEVEL RISE

Salt marshes are critical components of coastal systems. Morphological changes to salt marshes may alter their extent and function, resulting in significant modifications of ecosystem services. Many factors and interactions between factors, influence salt marsh morphology, including: sediment supply, excess nutrients, sea-level rise, and biotic interactions, among others. This study examines the surficial dynamics of six salt marshes along Maine’s coast. We focus on salt marsh pools, shallow, water-filled depressions that are common and substantial features of Maine marshes. Analyses of time-series aerial photographs since the 1960s show that some pools in Maine are: (1) formed over decadal time periods, (2) dynamic, altering their shape and size, (3) interactive with tidal creeks, draining and re-vegetating, and (4) very large, and these pools tend to form in interior marsh sections or along the upland-marsh boundary. Overall, Maine marshes have lost vegetated marsh area to pools, but not at the dramatic rates or amounts observed in places like the mid-Atlantic or Gulf Coast. While sea-level rise has been credited with the growth of pools in other locales and Maine marshes are experiencing rising sea level at a rate of approximately 2 mm per year, it may not be the only driver of pool morphologies in Maine marshes. Groundwater has received little attention for its potential control of north-temperate salt marsh morphology, despite the resemblance of salt marsh pools to open-water features from other groundwater-controlled landscapes, like flarks found in northern peatlands. Preliminary data from shallow (about 1 m depth) and deep (about 2.5 m depth) groundwater wells at Grand Marsh, Gouldsboro, ME show that horizontal movements of shallow and deep groundwater generally align with measurements of pool orientation and vertical flow paths reveal up-welling of shallow groundwater to open-water features, suggesting a connection between groundwater and the surficial expressions of pools. Our results demonstrate that Maine marshes are dynamic, that pools are important drivers of surficial change in these systems, and that pools themselves are likely responsive to multiple stressors, including sea-level rise and groundwater flows.