Vegetation Change on a Northeast Tidal Marsh: Interaction of Sea‐Level Rise and Marsh Accretion

Abstract

Increasing rates of relative sea—level rise (RSL) have been linked to coastal wetland losses along the Gulf of Mexico and elsewhere. While such losses have yet to be reported for New England tidal marshes, rapidly rising RSL may still be affecting these systems. Studies of the Wequetequock—Pawcatuck tidal marshes over four decades have documented dramatic changes in vegetation that appear to be related primarily to differential rates of marsh accretion and sea—level rise. Other environmental factors such as sediment supply and anthropogenic modifications of the system may be involved as well. When initially studied in 1947—1948 the high marsh supported a Juncus gerardi—Spartina patens belting pattern typical of many New England salt marshes. On the most of the marsh complex the former Juncus belt has been replaced by forbs, primarily Triglochin maritima, while the former S. patens high marsh is now a complex of vegetation types–stunted Spartina alterniflora, Distichlis Spicata, forbs, and relic stands of S. patens. These changes are documented by vegetation sampling that closely followed the 1947—1948 methods and by peat core analysis. Marsh elevations were determined by leveling, and the mean surface elevation of areas where the vegetation has changed is significantly lower than that of areas still supporting the earlier pattern (4.6 vs. 13.9 cm above mean tide level). The differences in surface elevation reflect differences in accretion of marsh peat. Calculations based on sandy overwash layers deposited during historically recorded storms as well as on experimentally placed marker horizons of known age indicate that stable areas have been accreting at the rate of local sea—level rise, 2.0—2.5 mm/yr at least since 1938; changed areas have accreted at about one half that rate. Lower surface elevations result in greater frequency and duration of tidal flooding, and thus in increased peat saturation, salinity, and sulfide concentrations, and in decreased redox potential, as directly measured over the growing season at both changed and stable sites. It is proposed that these edaphic changes have combined to favor establishment of a wetter, more open vegetation type dominated by to distinctive communities–Stunted S. alterniflora and forbs. Changes documented on the Wequetequock—Pawcatuck system have been observed on the other Long Island Sound marshes and may serve as a model for the potential effects of seal—level rise on New England tidal salt marshes.

Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise

“…5). Our documentation of changes from marsh hay or spike grass to cordgrass at both Rumstick Cove and Nag Creek Marshes and similar observations at numerous other sites in the region (13)(14)(15) indicate that these changes reflect at least a regional alteration of salt-marsh community composition. The relative increase in mineral matter within the cordgrass-dominated peat indicates that more frequent flooding, not nutrient additions, is responsible for the landward migration of cordgrass.…”

“…Core RUM3 was taken from the edge of one of these pannes, where cordgrass remains become dominant at 11 cm depth. This panne development is similar to cordgrassdominated panne development documented in the last 60 years at Barn Island, Connecticut (13). The transition to cordgrass in core RUM1 predates the 1900 horizon derived from Pb-210 data.…”

“…High marsh, dominated by marsh hay, spike grass, and black rush, exists at about mean high water and accretes vertically and laterally over submerging upland and low-marsh sediments. Several studies have noted a recent reversal of this long-term trend in marsh-community dynamics by documenting sediments dominated by cordgrass remains overlying high-marsh sediments at several sites in southern New England (13)(14)(15). Two of these studies (13,14) attributed the recent migration of cordgrass onto the high-marsh surface to the combined influence of changes in geomorphology associated with the September 21, 1938, New England Hurricane and increased rates of SLR.…”

Spartina maritima influence on the dynamics of the phosphorus sedimentary cycle in a warm temperate estuary (Mondego estuary, Portugal)

“…Vale et al, 1990;Warren and Niering, 1993;Christensen & Andersen, 1996;Sá nchez et al, 2001;Flindt et al, 2002).…”

Salt marsh stabilization affects algal primary producers at the marsh edge

“…In the mid-Atlantic region of the U.S., the rate of sea level rise over the past century (2.4-4.4 mm/year) exceeded the global average rate of 1.7 mm/year, affecting the elevation, function and distribution of shoreline habitats as well as restoration projects (Nixon 1980;Warren and Niering 1993;Donnelly and Bertness 2001;CCSP 2009). An increasingly common management response to sea level rise is to construct hard structures to protect shrinking shoreline habitats (Benoit et al 2007).…”