Artificial reefs do increase secondary biomass production: mechanisms evidenced by stable isotopes

Artificial reefs (ARs) are used worldwide as a tool to manage and restore marine coastal ecosystems and to support small-scale fisheries, as increases in fish biomass around them commonly occur. Whether ARs actually produce biomass, or only attract fish from natural zones, is strongly debated. Using stable isotope ratios to elucidate the trophic organization of the largest Mediterranean artificial reef system, the present work demonstrates that the studied ARs effectively support biomass production, as invertebrate species directly depended on locally produced organic matter (OM). OM of pelagic origin was the main source of matter due to the predominance of filter-feeder organisms on the ARs, while benthic primary production was of secondary importance. Isotopic ratios of fishes confirmed the importance of the ARs as a food supplier. Their position in the trophic network was consistent with the hypothesis proposing the effective ability of ARs to increase fish biomass through production mechanisms. Carbon and nitrogen stable isotope ratios provided a basis for achieving an integrative view of trophic relationships and food web functioning of ARs. This work constitutes a baseline for future work on efficient management of coastal zones, including natural and artificial reefs.