Abstract The conservation and restoration of tidal marshes and their highly valued ecosystem services require in‐depth knowledge of the ecological processes facilitating or hindering marsh vegetation establishment on initially bare mudflats. While abiotic factors such as the impact of hydrodynamics and sediment bed dynamics have been extensively studied, the role of biotic interactions in marsh establishment is less understood. Here, we investigate how interactions among three key organism groups —benthic polychaetes ( Hediste diversicolor ), benthic filamentous algae ( Vaucheria sp.), and pioneer plant species ( Aster tripolium )— control early successional dynamics in brackish tidal marshes in a northwestern European estuary. Combining field monitoring, field transplantation experiments and lab mesocosm experiments, we show that polychaete predation substantially reduces plant seedling survival, thereby constraining vegetation establishment. However, mat‐forming filamentous algae, which preferentially establish adjacent to tidal drainage channels, play a pivotal role in mitigating these negative effects of polychaetes on plant seedlings. Algal mats had significantly lower polychaete densities and higher seedling survival compared to bare mudflats. Further, algae reduced polychaete‐induced seedling mortality, likely by limiting burrowing activity and potentially offering an alternative food source. Our findings highlight the critical role of benthos–algae–plant interactions in the early stages of tidal marsh development. These insights suggest that promoting filamentous algal mat development —for instance, by creating drainage channels or topographic heterogeneity— could enhance tidal marsh restoration success by facilitating pioneer vegetation establishment.
Hautekiet et al. (Wed,) studied this question.