Black and white slags are common by-products of the metallurgical industry, with white slags having limited recycling applications. In this study, different circular artificial reef (AR) designs were compared using AR Ecosystem Index Transformation (AREIT) metrics and structural stability tests. The three-layer “Puzzle Reef” demonstrated the highest structural integrity and was selected for deployment, which was performed on July 18th, 2023 at the OBSEA. Although extreme current conditions during October–November 2023 caused minor displacement of components, the structure remained intact overall. The reef's influence on the local fish community was evaluated through high-frequency photo monitoring (one image per min) from July 18th, 2023, to October 24th, 2024, using an AI-automated procedure for fish classification and counting. In parallel, colonizing assemblages were analysed in the laboratory from two reef components retrieved on December 16th, 2024. The visual census recorded 18 bony fish species and one cartilaginous fish family, totalling 1,228,915 individuals (ind.). Fish biodiversity and abundance increased over time, as did AR Multimeric Indices, indicating a positive ecological trend following AR deployment. These indices, together with AREIT, are proposed as valuable tools to inform global marine habitat policies. Biofouling analysis revealed a colonizing assemblage of 2930 ind.·m2 across 59 species, with a total dry biomass of 23.23 mg·m2. The assemblage, largely dominated by balanids, crustaceans, polychaetes, gastropods, and bivalves, was mostly composed of small-sized individuals with a mean biomass of 4.6 μg. Nevertheless, the Abundance-Biomass Comparisons curve indicated a stable and relatively mature community, characteristic of relatively low-disturbance conditions.

This research was conducted within the framework of the Spanish Government's “Severo Ochoa Centre Excellence” grant awarded to ICM-CSIC (CEX2019-000928-S) and the Research Unit Tecnoterra (ICM-CSIC/UPC). The authors extend their gratitude to the European Project Semester Students 2023 at EPSEVG-UPC –Emma Canavate, Kim Dehaan, Lucie Dzuirka, and Yassine El Hajji– for their contributions to the preliminary reef design. We also acknowledge financial support from the Plan Estatal de Investigación Científica y Técnica y de Innovación, under the 2021 call for Proyectos Estratégicos Orientados Transición Ecológica y Transición Digital 2021, which funded the SLAGREEF project (3D Slag Concrete Manufacturing Solutions for Marine Biotopes, TED2021-129760B-I00). The proposed data banking and analysis approaches were developed in alignment with the guidelines of the EU Projects DIGT4ECO (Digital Twin-Sustained 4D Ecological Monitoring of Restoration in Fishery-Depleted Areas; GAP-101112883), MERLIN (Demonstration of long-endurance intelligent multi-purpose autonomous vehicles for marine applications; GAP-01189796), and SUN-BIO (Sustainable and nature inclusive offshore energy with the parallel biodiversity flourishing, protection and monitoring; GAP-101157493). The authors extend their gratitude to Orcina for their kind support and for offering the use of the academic license OrcaFlex 6394 to the Universitat Politècnica de Catalunya. This study is also a contribution of DM and MPM to the Consolidated Research Group on Marine Benthic Ecology of the Generalitat de Catalunya (2021SGR00405).

With funding from the Spanish government through the "Severo Ochoa Centre of Excelence" accreditation (CEX2019-000928-S)

Peer reviewed