Abstract
The topographical and hydrological complexity of submarine canyons, coupled with high substratum heterogeneity, make them ideal environments for cold-water coral (CWC) habitats. These habitats, including reefs, are thought to provide important functions for many organisms. The canyons incising the continental slope of the Bay of Biscay have distinct morphological differences from the north to the south. CWCs have been reported from this basin in the late 19th century; however, little is known about their present-day distribution, diversity and environmental drivers in the canyons. In this study, the characteristics and distribution of CWC habitats in the submarine canyons of the Bay of Biscay are investigated. Twenty-four canyons and three locations between adjacent canyons were sampled using a Remotely Operated Vehicle (ROV) or a towed camera system. Acquired images were annotated for habitat type (using the CoralFISH classification system), substrate cover and coral identification. Furthermore, the influence of hydrological factors and geomorphology on the CWC distribution was investigated. Eleven coral habitats, formed by 62 morphotypes of scleractinians, gorgonians, antipatharians and seapens, inhabiting hard and/or soft substrate, were observed. The distribution patterns were heterogenous at regional and local scales; the south Bay of Biscay and the southeastern flank favored soft substrate habitats. Biogenic and hard substrate habitats supported higher coral diversities than soft substrate habitats and had similar species compositions. A higher coral species turnover characterized soft substrate habitats. Substrate type was the most important driver of the patterns in both distribution and composition. Observations of coral reefs on steeper areas in the canyons and coral rubble on flatter areas on the interfluve/upper slope, support the hypothesis that canyons serve as refuges, being less accessible to trawling, although natural causes may also contribute to the explanation of this distribution pattern. The results of this study fed into a proposal of a Natura 2000 network in the Bay of Biscay where management plans are rare.
Highlights
Submarine canyons incise many continental shelves and slopes around the world (Harris and Whiteway, 2011)
Of the 14,874 analyzed images, coral habitats were recorded on 4,191 images, of which 2,350 images were selected for substrate cover measurements and species identification (“subset” images)
Le Danois (1948) described both scleractinian and sand/mud facies on the continental margin of the Bay of Biscay as one of the first studies in this basin. He observed (i) aggregations of the seapens K. stelliferum, Umbellula spp., and Pennatula spp. emerging from muddy bottoms between 500 and 1,000 m depth, in the north and south of the basin, (ii) scleractinian facies formed by the reef-forming species L. pertusa, M. oculata, and S. variabilis on the Celtic and Armorican margins, and (iii) several gorgonians, antipatharians and solitary scleractinians that were associated with this scleractinian facies including some species that are observed in the present study, e.g., N. versluysi and Antipathes dichotoma
Summary
Submarine canyons incise many continental shelves and slopes around the world (Harris and Whiteway, 2011) These physiographical features have a complex, heterogeneous topography that creates specific hydrological processes, such as accelerated (bottom) currents, internal waves and dense shelf water cascading (De Leo et al, 2010; Harris and Whiteway, 2011). The orders that meet this definition are Scleractinia (stony corals), Alcyonacea (soft corals, including gorgonians), Antipatharia (black corals), Pennatulacea (seapens) as well as the hydrozoan family Stylasteridae (hydrocorals) (Cairns, 2007) Most of these CWCs need hard substrate to settle, with the exception of most seapens, some scleractinians (predominantly solitary) and some gorgonian species (Roberts et al, 2009). CWCs are filter-feeders that rely on currents to deliver food particles (Wagner et al, 2012) and vertical migration of zooplankton (Carlier et al, 2009; Mienis et al, 2012; Wagner et al, 2012; Hebbeln et al, 2014)
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