Plant species, inundation, and sediment grain size control the development of sediment stability in tidal marshes

Tidal marshes can contribute to nature-based shoreline protection by reducing the wave load onto the shore and reducing the erosion of the sediment bed. To implement such nature-based shoreline erosion protection requires the ability to quickly restore or create highly stable and erosion-resistant tidal marshes at places where they currently do not yet occur. Therefore, we aim to identify the drivers controlling the rate by which sediment stability builds up in young pioneer marshes. Sediment stability proxies were measured over age gradients spanning 18 years in six tidal marsh sites in the Western Scheldt estuary (SW Netherlands): Three were dominated by Spartina anglica, a densely growing pioneer species, and three by Scirpus maritimus, a less densely growing pioneer species. Our results showed that the presence of densely growing Spartina anglica increased sediment shear strength compared to the unvegetated tidal flat, while less densely growing Scirpus maritimus did not. This difference may be related to the contrasting clonal expansion strategies and related root densities of these two pioneer species. Sediment stability did not increase further beyond 6 years of coverage by Spartina anglica, implying that the observed effect of Spartina anglica on sediment stability occurs fast (<6 years). Furthermore, sediment stability often increased with decreasing inundation duration and sediment water content. This study shows that in order to create erosion-resistant sediment beds in future marsh restoration projects, the aim should be to create densely vegetated tidal marshes with well-draining, cohesive sediments at relatively high intertidal elevation. Although the development of erosion resistance takes time, our study demonstrates that in the case of densely growing Spartina anglica marshes, increased sediment bed stability can already be reached after 6 years. The ability of Spartina anglica marshes to increase sediment bed stability within 6 years, in combination with wave attenuation and sediment accretion, offers promising perspectives to implement marsh restoration projects as a nature-based shoreline protection strategy that can start to deliver its protective service within a reasonable amount of time.