Structure and diversity of invertebrate communities in the presence and absence of canopy-forming Fucus vesiculosus in the Baltic Sea

Canopy-forming seaweeds are regarded as important habitat-formers in shallow coastal zones and their present decline or disappearance in many coastal areas can potentially affect diversity and abundance of associated species. Here, we evaluate potential effects of large-scale disappearance of the dominant canopy-forming species Fucus vesiculosus L. on vegetation-associated invertebrates in the Baltic Sea. We compare the macroalgal and invertebrate assemblages between sites where F. vesiculosus has disappeared over a larger area and adjacent sites with intact Fucus vegetation. The species richness of animals did not differ between sites with and without F. vesiculosus and no species were clearly confined to sites with Fucus. However, total animal abundance and biomass were generally lower when F. vesiculosus was absent and the animal assemblage at sites without Fucus differed consistently from adjacent Fucus sites. Since the assemblage of epiphytic and turf-forming macroalgae also differed with the presence of Fucus, the differences in invertebrate assemblages can be explained either by direct effects of the Fucus plants or by altered abundance of other macroalgal species. Based on these observations we suggest that large-scale disappearance of F. vesiculosus in the Baltic Sea may result in a changed composition and biomass of vegetation-associated invertebrates, with potential effects on higher trophic levels.