Eelgrass meadows harbor more macrofaunal species but bare sediments can be as functionally diverse

Seagrass meadows are important ecosystem engineers, providing habitat for a range of marine organisms which sustain many ecosystem functions. Due to global loss of seagrass, conservation and restoration incentives are rapidly increasing. However, it is not well understood how environmental setting affects the structural and functional diversity of macrofaunal communities in eelgrass meadows (Zostera marina), and to what extent their importance as macrofaunal habitat is affected by abiotic variables across spatial scales. In the present study, we explored macrofaunal communities in eelgrass meadows and adjacent bare habitats across contrasting environments in Southern Sweden. We assessed taxonomic and functional diversity in concert and evaluated environmental properties as explanatory variables for the observed differences. Our results showed that eelgrass meadows supported a significantly higher taxonomic richness and abundance compared to adjacent bare sediments whereas differences in structural and functional diversity was mainly a function of site and highly influenced by dominance of the polychaete Pygospio elegans. Interestingly, functional redundancy was higher in brackish compared to marine sites, illustrating the effect of generalist species that cover a wide range of functional traits. Abiotic sediment variables did not explain the variation in structural or functional diversity, but were largely overshadowed by the effect of eelgrass and its belowground biomass. From a restoration perspective, our results imply that the positive effects of eelgrass restoration are similar across environments and that the presence of eelgrass is the most important factor in structuring benthic communities, but the effects on functional diversity and redundancy need further attention.