A field investigation of diffusion within a submerged plant canopy

Abstract

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.