metabarcoding of eDNA from tidal pools on artificial and natural oyster reefs
The amount of humanmade structures in the worlds' oceans is tremendous and provides a settlement opportunity for a broad range of species. The ecological value of adding hard substrate from a restoration point of view is increasingly recognized and nature-based solutions  are integrated in new marine construction works to reduce erosion, enhance biodiversity or providing other ecosystem services.  Artificial reefs are widely studied, yet predicting their community composition compared to natural reefs remains challenging.  This study compared the community composition of a 13-year-old, humanmade intertidal Pacific oyster reef to a 27-year-old intertidal Pacific oyster reef of natural origin in the Oosterschelde Estuary in the Netherlands. Using eDNA metabarcoding of water samples and morphological identifications of hand-collected animals from tidal pools, we analyzed the communities and their biological traits at three time points. Additionally, we compared the species and biological traits detected by the two methods.  Both reefs showed similar community composition, ecological traits, and prevalence of non-indigenous species (NIS). The artificial reef did not appear to facilitate the spread of NIS more than the natural reef at this stage of development. Notably, eDNA detected a broader range of ecological traits, including highly mobile species, while hand-collection of animals better identified burrowing and tube-dwelling organisms. We conclude that after 13 years, the artificial reef closely resembled the natural one, demonstrating successful ecological integration. These findings can inform nature-based construction and emphasize the need for tailored long-term monitoring based on target taxa and associated traits.