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.