INTERACTIONS BETWEEN SEA-LEVEL RISE AND GROUNDWATER INFLUENCE SURFICIAL SALT MARSH MORPHOLOGY IN MAINE

At the surface, salt marshes link terrestrial, coastal ecosystems with the nearshore, ocean environment; in the subsurface, groundwater links all three of these environments. Most groundwater studies in salt marshes focus on the ecotones between these environments (e.g., upland transitions, tidal-exchange effects) or on the movement of groundwater through the ecosystem without consideration of effects on the marsh itself (e.g., transport of land-derived nutrients and contaminants to the nearshore system). Salt pools (shallow, water-filled depressions common to north-temperate salt marshes) resemble open-water features from other groundwater-controlled landscapes, like flarks found in northern peatlands. In 14 locations in Grand Marsh, Gouldsboro, ME, we installed shallow (about 1 m depth) and deep (about 2.5 m depth) groundwater wells equipped with 30 cm long screens to investigate how groundwater flow paths relate to surficial pool morphology, specifically pool necks (distinct, elongate, open-water features that extend from the main pool body). Horizontal movements of shallow and deep groundwater generally align with measurements of neck orientation; vertical flow paths reveal up-welling of shallow groundwater to open-water features, suggesting a connection between groundwater and the surficial expressions of pool and neck morphologies. Grand Marsh is also experiencing rising sea level at a rate of approximately 2 mm per year. Analyses of time-series aerial photography from 1966-2009 indicate that pool area has increased over the time period examined, but that pools, necks, and tidal creeks may interact to drain and re-vegetate pools, providing a mechanism for marsh resilience. This study is the first in Maine to suggest that interactions between sea-level rise, coastal land-use change, and groundwater shape surficial marsh morphology and is one example of a human-geological legacy that is likely influencing north-temperate coastal settings.