Morphodynamics in Mid-Atlantic and New England saltmarshes: ecophysical processes and implications with sea-level rise
Wilson, Carol A
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This dissertation reports on the physical, biological, and biogeochemical processes in saltmarshes of Santee Delta, South Carolina, and Boston Harbor and Plum Island Estuary in Massachusetts to evaluate and quantify their dynamic response to sealevel rise. This includes comprehensive ecophysical analyses along the continuum of low to high marsh settings (South Carolina and Massachusetts, respectively), in addition to presenting information on historical saltmarsh evolution, recent anthropogenic alteration, and future sustainability. In the Santee Delta, South Carolina, relative sea-level rise is resulting in rapid creek extension on low marsh platforms of Spartina alterniflora. Measurements of redox potential, pH, belowground biomass, and soil strength reveal that crab colonization and bioturbation from the crab Sesarma reticulatum facilitate creek extension by altering the geotechnical and geochemical properties of the soil. Oxidized conditions in the upper 10-15 cm of the marsh induced by burrowing causes enhanced degradation of belowground biomass, which reduces the structural integrity of the soil and lowers elevation. This process ultimately increases the erosion potential of the sediment in creek head areas. In Plum Island Estuary, Massachusetts, creek extension is similarly occurring into localized depressions on high marsh platforms dominated by Spartina patens. Coring and stratigraphic analyses, elevation and vegetation surveys, and accretion rates on marsh platforms and in re-vegetating pools suggest the marshes are in dynamic equilibrium with sea-level rise. Degradation of organic matter, loss in elevation and pool formation is counterbalanced by creek incision, drainage, rapid revegetation, and regain in elevation. These marshes appear to be stable with respect to sea-level rise, and the recent increase in pool formation is linked to changes in drainage density. Finally, saltmarsh evolution throughout the Holocene is investigated in Boston Harbor. Cores, stratigraphic analysis and radiocarbon dating indicate that the marshes on Thompson and Peddocks Islands developed ~2-4 thousand years ago when rates of sea level rise decelerated after glacial melting. A reduction in sediment supply from drumlin bluff stabilization and anthropogenic diking has greatly impacted these marshes in the past century, evidenced by transition from high to low marsh vegetation. Their sustainability with accelerating rates of sea-level rise is questionable.
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