When shelter is not a refuge: Reporting microplastic abundance in European (France) Sabellaria alveolata reef ecosystems

The coastal environment is increasingly impacted by microplastic (MP) pollution, potentially affecting temperate biogenic reefs constructed by sedentary, gregarious organisms such as the polychaete Sabellaria alveolata , able to transform soft sandy sediments into firm, three-dimensional bioconstructions. However, the extent of this threat remains poorly documented. This study provides a comparative assessment of MP contamination in S. alveolata reefs and in adjacent coastal sediments at two sites along the French Atlantic coast: Champeaux and Douarnenez. The objective was to evaluate how local environmental conditions and reef structural characteristics influence MP accumulation. The results revealed mean MP concentrations ranging from 0.81 to 1.73 items g⁻¹ across both sites, with fibers largely dominating the assemblage (95%), compared to fragments and other MP morphologies. At the Champeaux site, MP abundance in reef samples (~1.73 items g⁻¹) was markedly higher than in the surrounding sediments (~0.81 items g⁻¹), with significantly greater concentrations detected in the deeper reef layers (10–15 cm) compared to surface layers (0-5 cm). In contrast, MP distribution at the Douarnenez site was more homogeneous, with comparable abundances between reef (~1.46 items g⁻¹) and sediment (~1.18 items g⁻¹) samples. The different accumulation patterns observed in the two sites probably mirror the complex interplay between local environmental forces and reef structure. We hypothesized that at Champeaux, the larger and more stable reef structure coupled with the local hydrodynamism, have favored MP entrapment in the reef, which has continued over time. At Douarnenez, where the reef is smaller and more dynamic and local hydrodynamism is different, due to the peculiar coastal morphology, MPs accumulate more uniformly with abundances in the reef mirroring those in the surrounding sediments. Overall, this study highlights the active role of S. alveolata reefs in coastal plastic dynamics, functioning locally as long-term sinks for MPs and potentially increasing exposure risks for organisms living on or within the reef structure.