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Field measurements of bed elevation and related wave events were performed within a tidal marsh, on two cordgrass species, Spartina anglica (exotic) and Spartina maritima (native), in the Bay of Arcachon (SW France). Bed- and water-level time series were used to infer on the sediment behavior patterns from short to long term. A consistent response was found between the bed-level variation and the wave forcing, with erosion occurring during storms and accretion during low energy periods. Such behavior was observed within the two species, but the magnitude of bed-level variation was higher within the native than the exotic Spartina. These differences, in the order of millimeters, were explained by the opposite allocation of biomass of the two species. On the long term, the sedimentation/erosion patterns were dominated by episodic storm events. A general sediment deficit was observed on the site, suggested by an overall bed-level decrease registered within both species. However, further verification of within species variation needs to be considered when drawing conclusions. Despite possible qualitative limitations of the experimental design, due to single point survey, this work provides original and considerable field data to the understanding the different species ability to influence bed sediment stabilization and their potential to build marsh from the mudflat pioneer stage. Such information is valuable for coastal management in the context of global change

The Arcachon lagoon (Atlantic coast, SW France) is a mesotidal embayment where seagrass beds colonize the majority of intertidal areas. In recent years, the surface area of Zostera noltii meadows has considerably decreased, with possible consequences for the sediment balance of the lagoon. However, such interactions are poorly understood, and knowledge of the relationship between hydro-sedimentary processes and small intertidal meadows, such as Z. noltii, is limited.An intertidal mudflat, with variable meadow coverage, was studied during an annual survey. The study consisted in continuous high-frequency monitoring of bed altimetry, tidal elevation and waves. Sediment parameters and meadow characteristics were analyzed using samples collected monthly. Acoustic altimetry was validated as an efficient method to measure bed elevation in a vegetated environment, despite the presence of leaves under the transducer. The acoustic altimeter was also shown to have the potential to accurately estimate canopy height in a submerged environment. Our survey data showed centimetric bed accretion at all vegetated stations. Accretion was positively correlated with seasonal growth of themeadows. During seasonal degeneration, themeadow prevented erosion of the sea bed. These results highlight the important role of seagrasses as ecosystem engineers.