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

Abstract

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.

High-Resolution Vertical Habitat Mapping of a Deep-Sea Cliff Offshore Greenland

“…These geomorphic features result from differential erosion processes affecting on the long term the geomorphology of the bedrock exposed. The terrain will in turn affect the benthic community composition by shaping local dynamics of the sediment and of the currents and by influencing the stability of the terrain (e.g., friability; Edinger et al, 2011;Robert et al, 2017Robert et al, , 2020. Finally, backscatter data acquired together with multibeam bathymetry at a resolution under a meter have potential in ecological modeling studies focusing on fine-scale influences of terrain heterogeneity usually captured with imagery (e.g., Wilborn et al, 2018;Corbera et al, 2019;De Clippele et al, 2019) or in spatial modeling of dynamic sedimentary processes (e.g., Huvenne et al, 2007;Lastras et al, 2011).…”

“…Deep-water vertical and overhanging cliffs are important marine habitats that often host diverse and abundant communities (Huvenne et al, 2011;Johnson et al, 2013;Morris et al, 2013;Robert et al, 2020), including ecosystem engineers such as coldwater corals and deep-sea sponges (Ramirez-Llodra et al, 2010). Vertical cliffs are often associated with broadscale geomorphic features, such as continental margin troughs (Edinger et al, 2011), canyons (Freiwald et al, 2009;Huvenne et al, 2011Huvenne et al, , 2012Gori et al, 2013;Brooke and Ross, 2014;Brooke et al, 2017;Robert et al, 2017;Pearman et al, 2020) and fjords (Haedrich and Gagnon, 1991;Gasbarro et al, 2018).…”

“…Ecological studies usually assume that habitats providing a variety of environmental niches also promote enhanced biodiversity (MacArthur and Wilson, 1967;Kohn and Leviten, 1976). Vertical habitats are known to be hotspots of biodiversity (Robert et al, 2017(Robert et al, , 2020 because of a number of interacting processes such as the complex smallscale topography defined by the geomorphology (Althaus et al, 2012;Carter et al, 2018), the depth (Stewart et al, 1985), hydrodynamics (Frederiksen et al, 1992;Mortensen et al, 2001;Kiriakoulakis et al, 2007;Lim et al, 2020;Pearman et al, 2020) and the substrate type (e.g., hard substrate for cold-water coral settlement; Gass and Roberts, 2006;Buhl-Mortensen et al, 2017;Davies et al, 2017).…”

Mortality, Population and Community Dynamics of the Glass Sponge Dominated Community “The Forest of the Weird” From the Ridge Seamount, Johnston Atoll, Pacific Ocean

“…With the advent of high-resolution ROV cameras and highpowered computers, it is now possible to reconstruct entire benthic communities using photogrammetry to centimeter or millimeter scale (Robert et al, 2017(Robert et al, , 2020Baker et al, 2019;Prado et al, 2019;Price et al, 2019), so providing greater accuracy and resolution than the meter scale of ROV location data alone (Kennedy et al, 2019). These 3D photogrammetric reconstructions avoid the problems of controlling for altitude that often accompany 2D photo-montages because the triangulation of points using photographs from multiple angles limits the possible errors-if there is not enough photographs to accurately reconstruct the area, then the 3D model will not be generated (see Robert et al, 2017 for further details).…”

Fine-Scale Heterogeneity of a Cold-Water Coral Reef and Its Influence on the Distribution of Associated Taxa

“…Nevertheless, photomosaics and 3D reconstructions have been used to visualise and characterise scleractinian cold-water coral habitats (Wheeler et al, 2005b,c;Lessard-Pilon et al, 2010;Vertino et al, 2010), derive their geomorphic properties (Robert et al, 2017(Robert et al, , 2019Price et al, 2019), detect colony size (Fabri et al, 2019) and create substrate/coral cover maps (Lim et al, 2017(Lim et al, , 2020Conti et al, 2019). However, so far these datasets have not been analysed with PPA, and thus relatively little is known about fine scale spatial organisation of reef building coral and reef inhabitants.…”