Characterizing the relationship between taxonomic and functional diversity in benthic communities of coarse sediments in the English Channel

Trait-based approaches can complement taxonomic descriptors by revealing changes in functional organization under strong environmental filtering. However, in high-energy coarse-sediment habitats, how taxonomic and functional diversity covary, and how sieve mesh size influences the information obtained, remain poorly resolved. We investigated these questions in the English Channel, where high-energy coarse sediments dominate, and evaluated the additional information gained by retaining the 1–2 mm size fraction. Specifically, benthic macrofauna and sediments were sampled at 35 stations during four baseline surveys (autumn 2022 and 2023; winter 2023 and 2024) in the Bay of Seine, within areas targeted for offshore wind development. We quantified conventional taxonomic metrics and four functional diversity indices (FRic, FEve, FDiv, RaoQ) based on eight response traits, and compared patterns across seasons, sediment facies, and sieve mesh sizes (1 mm vs 2 mm). A total of 247,618 individuals (392 taxa; >1 mm) were identified. Sediment facies mainly affected functional dispersion, with lower RaoQ in gravel than in sandy gravel. Seasonal differences were expressed by higher biomass, FRic, and RaoQ in autumn than in winter. Taxonomic and functional β-diversity were strongly coupled (RV = 0.82), yet functional variation showed reduced amplitude, consistent with trait convergence under strong abiotic filtering. The 1–2 mm fraction added 10.8% taxa and about 30% individuals but only about 2% biomass; it minimally altered taxonomic patterns while substantially reshaping functional indices, indicating non-redundant functional information carried by small-bodied, trait-distinct taxa. These results support combining taxonomic and trait-based indicators for monitoring coarse-sediment benthos and suggest that finer sieving may improve the sensitivity of functional diagnoses.