Unveiling deep-sea habitats of the Southern Ocean-facing submarine canyons of southwestern Australia

We present the outcomes of the first deep-sea remotely operated vehicle study of previously unexplored submarine canyon systems along the southwest Australian continental margin. This was conducted around: (1) the Bremer Marine Park; (2) the Mount Gabi seamount and nearby slope-shelf margin at the interface of the Southern and Indian oceans; with new information from (3) the Perth Canyon Marine Park located in the SE Indian Ocean. These canyons differ from many explored around the world in having no connectivity to continental river systems, thus little detrital input, with the Bremer systems and Mount Gabi facing the Southern Ocean which plays a key role in the global ocean circulation and climate systems. Such studies in the vast deep waters around the Australian continent are rare given the lack of local ROV capability available for research, thus little is known about these environments. Using the resources of the Schmidt Ocean Institute, we characterised the submarine topography from highresolution bathymetric mapping, geology, physical and chemical oceanography, and provide an overview of these environments and fauna observed and collected. We show that these Southern Ocean-influenced environments incorporate South Indian Central Water, Subantarctic Mode Water, Antarctic Intermediate Water, and Upper and Lower Circumpolar Deep Water, with Antarctic Bottom Water present in deep water just south of the Bremer canyon systems. The richness in megabenthos, especially along the steep, rocky substrates of the canyon heads and walls around the Bremer canyon systems, contrasts to the comparatively depauperate fauna of the more northerly Perth Canyon. Various corals serve as important substrates for a range of other species and often exhibit particular faunal associations. Especially notable are distinct ecological zones including a bryozoan and sponge-dominated (animal) forest on the shelf edge, spectacular coral gardens along canyon margins, and the occurrence of solitary scleractinians well below the aragonite saturation horizon. Subfossil coral deposits were discovered across all three study areas, reflecting periodic waxing and waning of deep-water Scleractinia throughout this southwest region. Extensive pre-modern assemblages at Mount Gabi contrast markedly with the sparse populations of living species and suggest that it might have once been a major coral ‘hotspot’, or whether they reflect long-term coral aggregations is yet to be determined. Nevertheless, stark differences in both living and past coral distribution patterns across our study sites point to at least localised fluctuations in Southern Ocean-derived nutrient and/or oxygen supplies to these deep-sea communities.