First ROV Exploration of the Perth Canyon: Canyon Setting, Faunal Observations, and Anthropogenic Impacts

Abstract

This study represents the first ROV-based exploration of the Perth Canyon, a prominent submarine valley system in the southeast Indian Ocean offshore Fremantle (Perth), Western Australia. This multi-disciplinary study characterizes the canyon topography, hydrography, anthropogenic impacts, and provides a general overview of the fauna and habitats encountered during the cruise. ROV surveys and sample collections, with a specific focus on deep-sea corals, were conducted at six sites extending from the head to the mouth of the canyon. Multi-beam maps of the canyon topography show near vertical cliff walls, scarps, and broad terraces. Biostratigraphic analyses of the canyon lithologies indicate Late Paleocene to Late Oligocene depositional ages within upper bathyal depths (200-700 m). The video footage has revealed a quiescent 'fossil canyon' system with sporadic, localized concentrations of mega-and macrobenthos (∼680-1,800 m), which include corals, sponges, molluscs, echinoderms, crustaceans, brachiopods, and worms, as well as plankton and nekton (fish species). Solitary (Desmophyllum dianthus, Caryophyllia sp., Vaughanella sp., and Polymyces sp.) and colonial (Solenosmilia variabilis) scleractinians were sporadically distributed along the walls and under overhangs within the canyon valleys and along its rim. Gorgonian, bamboo, and proteinaceous corals were present, with live Corallium often hosting a diverse community of organisms. Extensive coral graveyards, discovered at two disparate sites between ∼690-720 m and 1,560-1,790 m, comprise colonial (S. variabilis) and solitary (D. dianthus) scleractinians that flourished during the last ice age (∼18 ka to 33 ka BP). ROV sampling (674-1,815 m) spanned intermediate (Antarctic Intermediate Water) and deep waters (Upper Circumpolar Deep Water) with temperatures from ∼2.5 to 6 • C. Seawater CTD profiles of these waters show consistent physical and chemical conditions at equivalent depths between dive sites. Frontiers in Marine Science | www.frontiersin.org 1 April 2019 | Volume 6 | Article 173 Trotter et al. Perth Canyon Deep-Water Coral FaunaTheir carbonate chemistry indicate supersaturation ( calcite ∼ 1.3-2.2) with respect to calcite, but mild saturation to undersaturation ( aragonite ∼ 0.8-1.4) of aragonite; notably some scleractinians were found living below the aragonite saturation horizon (∼1,000 m). Seawater δ 13 C and nuclear bomb produced 14 C compositions decrease in the upper canyon waters by up to ∼0.8 (<800 m) and 95 (<500 m), respectively, relative to measurements taken nearby in 1978, reflecting the ingress of anthropogenic carbon into upper intermediate waters.

Solenosmilia variabilis-bearing cold-water coral mounds off Brazil

“…Hebbeln et al 2014), on the continental margin off South America in the South Atlantic (Viana et al 1998;Sumida et al 2004) and have also been reported from the Indian Ocean (e.g. Western Australia, Trotter et al 2019) and the NE Pacific (e.g. Lumsden et al 2007).…”

“…In general, S. variabilis occurrences have been reported from the Atlantic, Pacific and Indian Ocean (Fig. 1, Zibrowius 1973Zibrowius , 1980Cairns 1995;Freiwald et al 2017;Trotter et al 2019) and are often associated with D. pertusum occurrences, but at greater water depth (Zibrowius 1980;Zibrowius, pers. comm.…”

Are Non-reef Habitats as Important to Benthic Diversity and Composition as Coral Reef and Rubble Habitats in Submarine Canyons? Analysis of Controls on Benthic Megafauna Distribution in the Porcupine Bank Canyon, NE Atlantic

“…Henry and Roberts (2007) recognized that vertical habitat heterogeneity significantly influences benthic fauna diversity. In addition, depth and other geomorphic canyon features have been found to affect the spatial distribution of benthic fauna (De Mol et al, 2011;Kenchington et al, 2014;Fanelli et al, 2018;Trotter et al, 2019).…”

“…They can be connected to landward river systems (Huang et al, 2014), terrestrially disconnected or tectonically controlled (Greene et al, 1991;Liu et al, 1993;Lo Iacono et al, 2014). Canyons have complex topographies and morphologies that affect hydrographic, oceanographic and biological processes (Allen et al, 2001;Bosley et al, 2004;Trotter et al, 2019). They are conduits for organic matter, nutrients and sediment transport to the abyssal zone (de Stigter et al, 2007;Kiriakoulakis et al, 2011;Mazzini et al, 2012;Puig et al, 2013) as well as litter and organic pollutants (de Jesus Mendes et al, 2011;Mordecai et al, 2011;Schlining et al, 2013;Tubau et al, 2015).…”

Distribution of deep-water scleractinian and stylasterid corals across abiotic environmental gradients on three seamounts in the Anegada Passage

“…However, live scleractinian reefs were recently discovered on seamounts in the North Pacific at Ω arag values as low as 0.71 ( Baco et al, 2017 ) and as low as 0.81 off Southern California ( Gómez et al, 2018 ). Additionally, scleractinians have been observed below the ASH in the Indian Ocean ( Trotter et al, 2019 ) and South Atlantic Ocean ( Barbosa, Davies & Sumida, 2020 ). These revelations indicate that, under the right conditions, scleractinian corals can persist in undersaturated waters ( Baco et al, 2017 ).…”