Substrate and seasonality shape invertebrate assemblages fouling marine plastics: Implications for dispersal potential

The increasing accumulation of plastics in marine environments poses major threats to biodiversity, including the facilitation of species dispersal through rafting, which may lead to biological invasions. We examined how plastic type and seasonality influence faunal fouling assemblages on beached plastics along the Barcelona coastline (Western Mediterranean), and assessed plastic use by Spirorbinae, the most abundant taxon. Between November 2023 and April 2024, six sampling campaigns yielded 8045 plastic items, 207 of which presented 1343 fouling specimens from 24 taxa across four phyla (Annelida, Bryozoa, Arthropoda, and Mollusca). Serpulidae was the most prevalent fouling taxon. Three bryozoan species (Aetea longicollis, Schizoporella unicornis, and Cellepora pumicosa) were recorded on plastics for the first time, and three non-indigenous species (Celleporaria brunnea, Arbopercula angulata, and Watersipora subtorquata) were also identified, alongside the cryptogenic Bugula neritina. Plastic properties influenced the fouling assemblages, with higher diversity indices observed on larger and hard plastics. We found evidence of non-random use of highly buoyant foamed plastics by Spirorbinae, which may enhance dispersal, particularly given their short planktonic larval duration. Seasonal effects on species composition were minimal: taxa restricted to a single season were generally rare, while common taxa occurred consistently throughout the study period. However, seasonal variation was detected in plastic characteristics, with foamed plastics more abundant in spring. Overall, our findings highlight the complex interplay between substrate traits, species-specific plastic use, and seasonal changes in plastic availability in shaping fouling communities, thereby improving our understanding of how plastics may facilitate the spread of non-indigenous species.