Abstract
Bare sandy flats at and below low tide level of the Wadden Sea (eastern North Sea, European Atlantic) were observed in 2020 to have been invaded by an introduced grass-like alga, Vaucheria cf. velutina (Xanthophyceae). A dense algal turf accumulated and stabilized mud, where resident seniors of the lugworm Arenicola marina had reworked rippled sand. Algae and worms were incompatible. Initially, rising patches with algal turf alternated with bare pits where lugworms crowded. Their bioturbation inhibited young algae, while the felt of established algal rhizoids clogged feeding funnels of worm burrows. Eventually, a mosaic pattern of competitors gave way to a coherent algal turf without lugworms. Concomitantly, a rich small-sized benthic fauna took advantage of the novel algal turf. This exotic Vaucheria may have the potential for drastically altering the ecological web at the lower shore.
Highlights
In an increasingly interconnected human world, invasive alien species (IAS) transform ecological webs, on island and coastal ecosystems (Anton et al 2019; Bailey et al 2020; Pyšek et al 2020), which may entail high economic costs (Cuthbert et al 2021)
We argue that V. cf. velutina is not native, but constitutes a recent introduction: (1) the Wadden Sea region has been exceptionally well surveyed for species of Vaucheria in the past (i.e., Simons 1975; Polderman 1979a, b; Krieg et al 1988)
This study describes the beginning of a potential displacement of the most characteristic benthic species in the European Wadden Sea, the lugworm Arenicola marina, from the lower shore by an invasive alga forming dense turfs and accumulating mud where loose sand prevailed before
Summary
In an increasingly interconnected human world, invasive alien species (IAS) transform ecological webs, on island and coastal ecosystems (Anton et al 2019; Bailey et al 2020; Pyšek et al 2020), which may entail high economic costs (Cuthbert et al 2021). Fringing the eastern North Sea, the Wadden Sea comprises estuaries, salt marshes and extensive tidal flats, sheltered by sand bars and barrier islands (Reise et al 2010). The small filamentous algae of the genus Vaucheria (Xanthophyceae) were only known from uppermost shore, while none have been found around low tide level before until we found two near the island of Sylt in 2020 (Rybalka et al 2022). V. longicaulis may constitute a complex of hidden species (Christensen 1996) and is assumed to have been introduced
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