(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1997-09)
Tarrell, Alvin E.
Saltwater marshes and wetlands are important buffers at the land-sea interface. Among
the most biologically active ecosystems on Earth, natural and man-made wetlands are
important interceptors of pollutants and nutrients bound for the coastal ocean. The
transport, dilution, and deposition processes occurring within the marsh are key factors in
determining this interception, and these are in turn determined largely by tidally driven
flows as influenced by marsh vegetation and other physical characteristics. Vegetation
type and density are of primary importance in these processes, both for pollutant/nutrient
uptake concerns and in determining hydrodynamic characteristics of the marsh.
This study examines the effect of vegetation density and ambient flow on diffusivity
within a tidal marsh canopy, specifically Spartina alterniflora. Vegetation densities from
0-1.4% stem coverage and flows from 2-12 cm/sec were investigated using Rhodamine
WT tracer, with resultant measured diffusivities ranging from approximately 0.5-3.0
cm2/sec. Diffusivity was found to be a strong function of ambient current, but a much
weaker function of vegetation density. Presence of vegetation caused transverse and
vertical diffusivities to be essentially isotropic over all vegetation densities, as opposed to
the order of magnitude difference found in earlier non-vegetated studies. Only slight
vegetation coverage was found to be necessary to produce this isotropy, with little
additional change as stem density increased.
