Water depth affects the biodiversity and functional trait composition of Fucus vesiculosus-associated macroinvertebrate communities in the Baltic Sea

Eutrophication is one of the major anthropogenic drivers that extensively modifies coastal ecosystems. Bladderwrack Fucus vesiculosus (hereinafter Fucus ) is the main habitat-forming perennial macroalgae on non-tidal Baltic Sea rocky shores and provides essential habitats for many fish and macroinvertebrate species. Eutrophication has reduced Fucus depth distribution and led to the shallowing of Fucus -dominated habitats. Still, it remains unclear how this affects these littoral ecosystems. In this study, we examined changes in taxonomic and functional trait diversity of macroinvertebrates along a depth gradient ranging from 1 to 7 m in 4 locations. Our results revealed that both the number of taxa and functional richness increase with depth within the Fucus -dominated habitat. Community structure and functional trait composition changed significantly with depth. Deeper communities had a higher expression of traits linked to ecosystem stability; for example, long-lived and larger body size, and essential ecosystem functions such as nutrient cycling by detritivores. In contrast, shallow communities were characterized by traits linked to opportunistic taxa (e.g. high mobility and short lifespans). Our study highlights the importance of these threatened deep Fucus communities for coastal ecosystem functioning. Furthermore, to halt and reverse the trend of decreasing depth penetration of Fucus , efforts to counteract eutrophication are needed, and the remaining deep Fucus habitats should be protected.