A pioneering longterm experiment on mesophotic macrofouling communities in the North Atlantic.

The mesophotic zone represents one of our planet's largest and least explored biomes. An increasing number of studies evidence the importance of macrofouling species in marine ecosystems, but information on these communities and the factors influencing their structures at mesophotic depths remain poor. This lack of understanding limits our ability to predict anthropogenic impacts or conduct restoration operations in the mesophotic and the lower boundary of the euphotic zones. In this study, we performed a 24-month experiment in a natural environment to investigate three factors influencing the macrobenthic community structure of the mesophotic and the euphotic lower boundary: depth, substrate orientation and substrate material. Using a manned submersible, several recruitment panels of two different materials were deployed at 100, 200 and 400 meters in vertical and horizontal positions. All three factors contributed to structuring the macrofouling communities, but depth and substrate orientation displayed the strongest effects. This study not only advances our understanding of lower boundary euphotic and mesophotic macrofouling communities but also establishes a foundation for future research and restoration efforts of mesophotic environments in the Madeira archipelago, where mesophotic habitats are amongst the least studied marine habitats in the Northeast Atlantic.