Flow dynamics and sedimentation in Spartina alterniflora and Phragmites australis marshes of the Chesapeake Bay

Abstract

The introduction of invasive species such as Phragmites australis in the Chesapeake Bay has been viewed to be deleterious to habitat quality. Little is known, however, on the extent to which the replacement of Spartina alterniflora by Phragmites affects hydrodynamics and sediment trapping on the surface of impacted marshes. This study examined sediment deposition, sediment mobility, and flow conditions in adjacent Phragmites australis and Spartina alternifora marshes in Prospect Bay, Maryland, USA in order to determine if differences in plant morphology affect surficial flow properties and particle dispersion patterns. Measures of fine-scale flow dynamics, total suspended sediment (TSS) concentration, and particulate deposition were obtained at various distances from open water across the marsh surface over four sequential tidal cycles in Fall 1999. The hydrodynamic data indicate that both the gross and fine-scale properties of tidal flows were similar in both types of vegetation and that flow conditions were conductive to particle deposition. TSS concentrations did not differ between canopy types and decreased over time in both systems. There was no difference in TSS reduction over distance between Spartina and Phragmites. The sediment trap data indicate that maximum deposition occurs closer to open water in both Spartina and Phragmites and that the organic content of deposited matter increased with distance into the marsh interior. This study provides the first in situ, high resolution, over-marsh flow data for marshes dominated by Phragmites. The data provided herein suggest that differences in vegetative cover do not significantly affect flow regime, sediment transport, and sediment deposition patterns in the marsh systems examined.