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Seagrasses are generally known as ecosystem engineers, as they reduce flow velocities in their canopies. In perennial subtidal meadows, this usually leads to increased net sedimentation rates and reduction of the grain size. The present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics. Sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat. Within the planting units (79shootsm⁻²) 4.7mm of sediment accreted, whereas in the most dense parts of these units (199shootsm⁻²) accretion amounted to 7.1mm. The silt fraction (<63μm) increased and the sand fraction (63–500μm) decreased in the eelgrass beds, which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass …