Invading grass-like alga transforms rippled sand bars into bumpy muddy flats: arrival of a game changer in the Wadden Sea?

In the wake of biological globalization, translocated species of high bio-engineering capacity increasingly change bottom topography of sedimentary coasts. A Vaucheria taxon (Xanthophyceae) of unknown origin is spreading at the transition between intertidal and subtidal zones, while resident Vaucheria -species are confined to the upper shore in the Wadden Sea (European Atlantic). Near the island of Sylt, dense turfs of green filaments rapidly expanded over an area of 180 ha within 3 years. The unicellular filaments reach about 5 cm out of and 5 cm into the sediment. Felted rhizoids provide firm anchorage. Dry phytomass (up to 208 g m -2 ) was similar to that of intertidal seagrass beds. Residual filaments overwinter in the sediment and give rise to renewed growth in late spring. In addition, oospores germinate. Fine particles are trapped by the turf during summer, generating laminated cohesive mud. Muddy hummocks arise up to 20 cm above ambient sand flats, alternating with troughs but gradually merge into coherent and pertinacious plateaus of mud. This shift in bottom topography and sediment composition may potentially change the mud balance of tidal basins, and the capacity of tidal flats in catching up with accelerating sea-level rise.