The hanging gardens of Blanes Canyon, Northwestern Mediterranean Sea

Submarine canyons are ubiquitous geomorphic features found intercepting the continental margins. As such, they provide environmental conditions suitable for many suspension feeding organisms, as they settle on steep rocky canyon walls, whilst taking advantage of increased currents that bring suspended organic matter and food. Additionally, demersal fishing grounds can be found surrounding submarine canyons where it can negatively affect species inhabiting these environments, including vulnerable ecosystems such as cold-water corals (CWCs). In order to understand the impacts of demersal fisheries in CWC communities, we first need to understand their distribution, species composition or vulnerability. Blanes Canyon is an example of a submarine canyon surrounded by demersal fishing grounds, where baseline knowledge on CWCs currently lacks. This study contributes to filling these knowledge gaps by using a dense grid of ROV transects along the canyon, high resolution bathymetry data and CTD surveys, which altogether provide a quantitative description of megabenthic assemblages. Blanes Canyon hosts at least 12 CWC species within 450-1300 m depth range, mainly inhabiting the steep canyon walls. Different assemblages of CWC species were found. Desmophyllum dianthus was the most abundant species, found throughout the entire canyon. Colonial scleractinian species ( Desmophyllum pertusum and Madrepora oculata ) were found in the canyon head but were lacking in the eastern canyon branch, where octocorals ( Muriceides lepida ) and black corals ( Leiopathes glaberrima ) were prevailing. Detailed CTD survey indicated that nepheloid layers (bottom and intermediate) were found at the same depth range as the megabenthic communities, since they provide suspension feeders with particulate organic matter (POM). Overall, this study confirms Blanes Canyon as a CWC habitat, providing densities and spatial distribution of different megabenthic species, along with information of their environmental niches. • Blanes Canyon hosts cold water corals (CWCs) its canyon walls • 94 morhospecies were found, 12 were CWCs that accounted for 79% of all records • Most abundant species was Desmophyllum dianthus • The upper canyon head is dominated by colonial scleractinian species • East canyon branch is dominated by octocorals and black corals