THE CRITICAL LEADING EDGE OF GULF OF MAINE SALT MARSHES – INTERFACE WITH FRESHWATER WETLANDS

Salt marshes are dynamically important ecosystems for a myriad of reasons including: serving as nurseries; providing a buffer for coastal uplands by shielding and protecting against direct wave attack and storm surges; retaining fine-grained sediment delivered by rivers; and filtering nutrients and pollutants from runoff following storms, all of which thereby reduce impact on the environment. Since sea level exerts strong control over system dynamics in salt marshes, reading the stratigraphic record allows stronger predictions of future changes at the edge of salt marsh (SM) and freshwater wetlands (FW). Downeast coastal Maine has a setting unusual in the lower 48 states of extensive bogs, some of which are intersected by the leading edge of salt marshes. This research focuses on the past behavior of the SM-FW transition zone in several of these marshes in response to sea-level rise, in an effort to predict how current and future trends of sea-level rise affect these sensitive ecosystems. Additionally, this research aims to determine the mechanisms driving change at the leading edge of the salt marsh transgression. We undertook an intensive field campaign during May-September 2011 to collect ground-penetrating radar (GPR) data and multiple Dutch cores along planned transects normal to the SM-FW boundary. The GPR revealed general substrate stratigraphy and guided coring and sampling transects to establish overall stratigraphy and rates of change (sediment accumulation). The interface of FW and SM at depth corresponds with a blanking of the GPR signal by salt water. Dutch cores generally penetrate SM over FW peat, and refuse in glaciomarine mud or sand. Stratigraphic sections using cores and radiometric dating enable reconstruction of changes in SM-FW transition zones over the past several decades to centuries.