Cold-water coral assemblages on vertical walls from the Northeast Atlantic

Aim: In this study, we assess patterns of cold-water coral assemblages observed on deep-sea vertical walls. Similar to their shallow-water counterparts, vertical and overhanging walls in the deep sea can host highly diverse communities, but because of their geometry, these habitats are generally overlooked and remain poorly known. These vertical habitats are however of particular interest, because they can protect vulnerable coral ecosystems from trawling activities. As such, it is important to understand their ecology and assess their global importance. Location: Vertical walls on complex geomorphic features, in particular walls of the Rockall Bank Slope Failure Escarpment, Whittard and Explorer Canyons, Northeast Atlantic. Methods: Video analysis of remotely operated vehicle transects carried out at five sites is used to investigate differences in species composition and diversity across walls and to compare those to nearby cold-water coral sites on flat terrain. A high-resolution photogrammetric reconstruction is further employed to examine whether wall complexity plays a role in promoting niche differentiation at very fine spatial scales. Results: The investigated walls showed differences in species assemblage both across walls and in comparison to flat sites, with the fine-scale heterogeneity engendered by walls allowing niche differentiation between closely related taxa. Main Conclusions: Vertical walls represent an important cold-water coral habitat with differences in species composition across walls within a region, illustrating their role in driving diversity patterns. Based on publicly available bathymetric datasets and a catalogue of broad-scale terrain features, globally over 8,000 features are likely to have vertical walls and cold-water corals, which highlight the need to consider deep-sea vertical habitats in current conservation efforts.