Abstract

Cold-water coral (CWC) reefs are complex structural habitats that are considered biodiversity “hotspots” in deep-sea environments and are subject to several climate and anthropogenic threats. As three-dimensional structural habitats, there is a need for robust and accessible technologies to enable more accurate reef assessments. Photogrammetry derived from remotely operated vehicle video data is an effective and non-destructive method that creates high-resolution reconstructions of CWC habitats. Here, three classification workflows [Multiscale Geometrical Classification (MGC), Colour and Geometrical Classification (CGC) and Object-Based Image Classification(OBIA)] are presented and applied to photogrammetric reconstructions of CWC habitats in the Porcupine Bank Canyon, NE Atlantic. In total, six point clouds, orthomosaics, and digital elevation models, generated from structure-from-motion photogrammetry, are used to evaluate each classification workflow. Our results show that 3D Multiscale Geometrical Classification outperforms the Colour and Geometrical Classification method. However, each method has advantages for specific applications pertinent to the wider marine scientific community. Results suggest that advancing from commonly employed 2D image analysis techniques to 3D photogrammetric classification methods is advantageous and provides a more realistic representation of CWC habitat composition.

Highlights

  • Azooxanthallate scleractinian corals, such as Lophelia pertusa and Madrepora oculata, are recognised by their three-dimensional branching morphology and framework building capacity (Mortensen et al, 1995; Roberts, 2002; Jonsson et al, 2004; Costello et al, 2005; Wheeler et al, 2005a, 2007b; Gass and Roberts, 2006; Frontiers in Marine Science | www.frontiersin.org de Oliveira et al.Guinan et al, 2009)

  • Since 2016, the Porcupine Bank Canyon (PBC) has been designated as a special area of conservation (SAC) (n◦003001) by the European Union Habitats Directive (2016), and no fishing or other exploratory activities are allowed in the area

  • 1–Multiscale Geometrical Classification (MGC) An MGC approach was utilised in this study to perform a binary classification of our 3D Cold-water coral (CWC) reef reconstructions

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Summary

Introduction

Azooxanthallate scleractinian corals, such as Lophelia pertusa (synonymised to Desmophyllum pertusum in Addamo et al, 2016) and Madrepora oculata, are recognised by their three-dimensional branching morphology and framework building capacity (Mortensen et al, 1995; Roberts, 2002; Jonsson et al, 2004; Costello et al, 2005; Wheeler et al, 2005a, 2007b; Gass and Roberts, 2006; Frontiers in Marine Science | www.frontiersin.org de Oliveira et al.Guinan et al, 2009). These cold-water coral (CWC) species can form structural habitats such as small coral patches (Wilson, 1979a), reefs (Masson et al, 2003; Roberts et al, 2006b; Victorero et al, 2016; Lim et al, 2018), and giant carbonate mounds (Hovland and Thomsen, 1997; Mienis et al, 2006; Wheeler et al, 2007a; Freiwald et al, 2011; Huvenne et al, 2011) that can reach up to 400 m above the seabed. The presence of reef-forming CWC colonies has been documented in a range of settings from fjords (Fosså et al, 2006) to continental shelfs, slopes (Wilson, 1979b; Mortensen et al, 1995; Wheeler et al, 2005c; Leverette and Metaxas, 2006; Mienis et al, 2006) to seamounts and submarine canyons (Huvenne et al, 2011; Appah et al, 2020) throughout the North Atlantic, Indian, and Pacific oceans and the Mediterranean Sea (de Mol et al, 2005; Freiwald and Roberts, 2005; Wheeler et al, 2007a,b; Roberts et al, 2009; Freiwald et al, 2011; Lim et al, 2018). It is essential to understand these environments and to assign priority areas for protection (Ferrari et al, 2018; Appah et al, 2020)

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