Habitat-builders complexity boosts associated fauna functional trait richness (Zostera marina meadows, Baltic Sea)

Seagrass has been widely recognized as ecosystem engineers and habitat-builders increasing biodiversity and abundance of associated fauna. However, seagrass habitats are being degraded globally due to modification of coastline, terrestrial inputs, and climate change. While the effects of seagrass meadows vegetation density on associated macrofauna standing stocks and taxonomic richness have been widely investigated, the relationships with functional diversity aspects remain poorly understood. Here, we investigate if macrofauna standing stocks and richness indicators (both taxonomic and functional) are related to the habitat-builders presence and structural characteristics (density, biomass, diversity) in temperate Zostera marina Linnaeus, 1753 meadows (Baltic Sea). Relationships between faunal (density, biomass, species and functional trait composition and diversity), vegetation and sediment indicators were tested based on materials collected by scuba divers. Macrofauna density (ranging from 587.6 ± 188.2 to 4514.1 ± 977.8 ind. 0.1 m−2), biomass (ranging from 43.1 ± 28.3 to 13.6 ± 30.8 g 0.1 m−2) and species richness (ranging from 7 ± 1.4 to 20 ± 1.4) responded positively to seagrass presence regardless of meadows characteristics. Functional trait richness ranged from 203.7 ± 64.3 to 44.7 ± 41.1 and was enhanced only in presence of the greatest macrophyte diversity. Therefore, it is the diversity of meadows that boosts the functional richness of associated fauna. This implies that 1) monospecies seagrass meadows do not necessarily provide a higher number of functions than neighboring bare sediments in systems with low diversity such as Baltic Sea, 2) both taxonomic and functional indicators should be used to describe macrofauna communities. This result is an important message for conservation planning for systems with low diversity, as restoration of at least a few macrophytes species not only the dominant seagrass species could be considered to secure greater taxonomic and functional richness.