Competing biotic interactions between fibrous algae and macorbenthos shape pioneer vegetation establishment in tidal marshes

The conservation and restoration of tidal marshes and their highly valued ecosystem services require indepth
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.