Assessing the environmental status of cold-water coral reefs: BARCS, a novel multi-parametric index

Cold-water coral (CWC) frameworks are complex biogenic structures that sustain high biodiversity and provide essential ecosystem services in the deep sea. These vulnerable ecosystems are increasingly threatened by anthropogenic activities, particularly bottom fishing. While much research has addressed the origin, distribution, and ecology of CWC reefs, little attention has been devoted to the standardised assessment of their conservation status. Within the European Union, the Marine Strategy Framework Directive (MSFD) requires member states to evaluate the environmental status of marine ecosystems. However, monitoring efforts largely focus on coastal waters, with limited initiatives targeting the deep sea. Here, we present the BAthyal coral Reefs Conservation Status (BARCS) index, a multi-parametric tool designed to evaluate and monitor deep-sea scleractinian-dominated ecosystems. The BARCS index integrates two components: a Structure Index (SI), which assesses reef complexity, demographic parameters of habitat-forming scleractinians, and associated species richness; and an Impact Index (II), which quantifies anthropogenic pressure based on sedimentation, marine litter, and entanglement. Together, these yield ten metrics that can be reliably derived from remotely operated vehicle (ROV) footage. The index was tested at eleven Mediterranean sites (across the Gulf of Lions and Ligurian, Tyrrhenian, Ionian, and Adriatic seas), encompassing diverse ecological settings and levels of human impact. Results demonstrate its ability to discriminate among environmental conditions and to disentangle natural variability from anthropogenic stressors. Validation against independent proxies of human impact further supports its robustness. The BARCS index thus represents the first standardised tool for assessing and monitoring bathyal CWC reefs in line with MSFD requirements.