Is the salt marsh vegetation a determining factor in the sedimentation processes?

Abstract

Many authors have referred to the impor- tant role of vegetation in the consolidation of salt marsh sediments, but experiments previously carried out by us have shown results that do not always agree with these statements. In other words, the type of salt marsh surface coverage is not the main factor that contributes to the consolidation of sediments. To test this hypothesis different Portuguese salt marsh stations (species/unvegetated areas) from two sites, Tagus estuary (Corroios and Pancas) and Ria de Aveiro (Barra and Verdemilho), were compared to evaluate their influence on suspended matter deposition on the salt marsh surface. A short-term sedimentation study was performed within stands of Spartina maritima, Halimione portulacoides, Sarco- cornia perennis subsp. perennis and unvegetated areas, by analysing the deposition of sediment material on nylon filters anchored to the marsh surface. Numerical results obtained from hydrody- namic models coupled to a Lagrangean module implemented for the Ria de Aveiro and the Tagus Estuary, namely the root-mean square velocity (Vrms) and residual velocity of tides, were also used. Average sedimentation rates (mean value between the different surface cover in a salt marsh) showed a seasonal trend more or less defined but with signif- icantly different values between sites and salt marshes. Sedimentation rates varied between marshes: there are significant differences between Pancas and the other three marshes, but only signif- icant differences in sedimentation rates between Spartina and Sarcocornia. Despite the important role of vegetation in the consolidation of salt marsh sediments, our results suggest that, the position of stations and related abiotic conditions in the salt marshes are determining factors of variation to take into account in the studies related with the stabilization and survival of salt marshes facing sea level rise.