Offshore Wind Farm Pile Foundations as Vertically Heterogeneous Habitats: Beta Diversity Partitioning and Functional Trait-Combination Richness of Epifaunal Communities

Ecological evidence for epifaunal communities on offshore wind farm structures remains geographically concentrated in temperate Northeast Atlantic systems, whereas comparable studies from the subtropical Northwest Pacific remain limited. We sampled upper, middle, and lower layers of five pile-supported structures in a shallow offshore wind farm in the northern South China Sea during spring and autumn, integrating taxon-specific wet biomass, occurrence data, environmental variables, beta-diversity partitioning, and eight functional traits. Taxon richness, functional trait-combination richness, and total wet biomass were lower in the upper layer than in the middle and lower layers, while wet-biomass-weighted Shannon diversity was lower in the upper than in the lower layer. Season and layer × season effects were not significant. Beta-diversity partitioning showed contributions of both turnover and nestedness-resultant dissimilarity, with relative contributions varying descriptively among seasons and layer contrasts. Measured environmental variables explained limited community variation, and the overall db-RDA was not significant, whereas variation partitioning identified the vertical layer as the only predictor group with a significant independent contribution. Global RLQ and FDR-corrected fourth-corner analyses did not support trait–environment coupling. These results extend evidence of vertical epifaunal differentiation to a geographically underrepresented subtropical Northwest Pacific offshore wind farm and support multi-layer sampling for biofouling assessment.