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This is the Accepted Version of a paper published in the journal: Marine Biodiversity

Beaman, Robin J., Bridge, Tom C.L., Lüter, Carsten, Reitner, Joachim, and Wörheide, Gert (2016) Spatial patterns in the distribution of benthic assemblages across a large depth gradient in the , Australia. Marine Biodiversity, 46 (4). pp. 795-808.

http://dx.doi.org/10.1007/s12526-015-0434-5

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1 Spatial patterns in the distribution of benthic assemblages across a large depth gradient in the 1 2 2 Coral Sea, Australia 3 4 3 5 6 4 1Robin J. Beaman* 7 8 5 2,3Tom C. L. Bridge 9 10 6 4Carsten Lüter 11 12 7 5Joachim Reitner 13 14 8 6Gert Wörheide* 15 16 9 17 18 10 1College of Science, Technology and Engineering, James Cook University, Cairns, Queensland, Australia 19 20 11 2ARC Centre of Excellence for Coral Studies, James Cook University, Townsville, Queensland, 21 22 12 Australia 23 24 13 3Australian Institute of Marine Science, PMB No. 3, Townsville, MC, Queensland, Australia 25 26 14 4Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany 27 28 15 5Faculty of Geosciences and Geography, University of Göttingen, Göttingen, Germany 29 30 16 6Department für Geo- und Umweltwissenschaften & GeoBio-Center, Ludwig-Maximilians-Universität 31 32 17 München, München, Germany 33 34 18 35 36 19 *Corresponding Authors: 37 38 20 Robin J. Beaman, 39 40 21 Ph. +61 7 4232 1693 41 42 22 Fax. +61 7 4232 1284. 43 44 23 E-mail. [email protected] 45 46 24 47 48 25 Gert Wörheide, 49 50 26 Ph. +49 89 2180 6718 51 52 27 E-mail. [email protected] 53 54 28 55 56 29 57 58 30 59 60 61 62 1 63 64 65 31 Acknowledgements 1 2 32 We thank the German Research Foundation (DFG) for the funding (projects Wo896/7 and Lu839/3, to 3 4 33 GW and CL respectively) and Marum (www.marum.de) for the ROV equipment supplied for the Deep 5 6 34 Down Under Expedition (www.deepdownunder.de). We also thank the captain, crew and scientific party 7 8 35 aboard the MV PMG Pride who undertook the sampling during the 2009 voyage. We thank Nicolas 9 10 36 Nowald for assistance with the ROV raw data and Andrew Gray-Spence for the Python program coding 11 12 37 used for the video data. Taxonomic help is gratefully acknowledged from Marc Eleaume, Richard 13 14 38 Fitzpatrick, Merrick Ekins, Phil Alderslade, Stephen Cairns, Justin Marshall, Vanessa Messmer and Rudi 15 16 39 Kuiter. We thank the two anonymous reviewers for helping to greatly improve the manuscript. 17 18 40 19 20 41 Abstract 21 22 42 The Queensland Plateau in the Coral Sea off north-eastern Australia supports numerous submerged and 23 24 43 emergent reefs. Osprey Reef is an emergent reef at the northern tip of the plateau ~1500 m in elevation. 25 26 44 Over such a large depth gradient, a wide range of abiotic factors (e.g. light, , substratum etc.) 27 28 45 are likely to influence benthic zonation. Despite the importance of understanding the biodiversity of 29 30 46 Australia’s Coral Sea, there is a lack of biological information on deep-water habitats below diving 31 32 47 depths. Here we used a deep-water ROV transect to capture video, still photos and live samples over a 33 34 48 depth range spanning 92 to 787 m at North Horn on Osprey Reef. Video analysis, combined with 35 36 49 bathymetry data, was used to identify the zones of geomorphology and the benthic assemblages along the 37 38 50 depth gradient. There were marked changes in the geomorphology and the substrate along this depth 39 40 51 gradient which likely influence the associated benthos. Cluster analysis indicated five benthic assemblage 41 42 52 groups, which showed clear zonation patterns and were generally predictable based on the depth and 43 44 53 sedimentary environment. These results are the first quantitative observations to such depths and confirm 45 46 54 that the waters of the Coral Sea support diverse benthic assemblages, ranging from shallow-water coral 47 48 55 reefs to mesophotic coral ecosystems, to deep-water azooxanthellate corals and sponge gardens. The 49 50 56 knowledge provided by our study can inform management plans for the Coral Sea Commonwealth 51 52 57 Marine Reserve that incorporate the deeper reef habitats and help to minimise future damage to these 53 54 58 marine ecosystems. 55 56 59 Keywords 57 58 60 59 Osprey Reef; benthic; zonation; geomorphology; mesophotic; coldwater coral; sponges 60 61 62 2 63 64 65 61 Introduction 1 2 62 The Coral Sea region off north-eastern Australia contains numerous pinnacles and modern reef complexes 3 4 63 that have formed on rifted continental blocks within the Queensland Plateau, the largest marginal plateau 5 6 64 on the Australian continental margin (Davies et al. 1989). Approximately the same size as the Bahama 7 8 65 Platform, this large (~165,000 km2) submerged carbonate platform formed when the Coral Sea Basin 9 10 66 opened in the Palaeocene about 60 Ma splitting off fragments of continental lithosphere (Mutter and 11 12 67 Karner 1980; Symonds et al. 1983). Subsidence and commencing in the Middle to Late Eocene 13 14 68 (42-37.5 Ma) led to its present median depth of about ~1100 m. The Queensland Plateau now supports 15 16 69 ~30 emergent and submerged reefs growing on the corners of these basement high points, some with 17 18 70 elevations over 1000 m above the surrounding plateau and basins. The emergent reefs include large banks 19 20 71 (over 100 km long) and smaller reefs and pinnacles (several km to 100 m wide), with living reef systems 21 22 72 occupying almost 15 percent of the surface of the Queensland Plateau. 23 24 73 25 26 74 development on the Queensland Plateau commenced growing when the Australian plate moved 27 28 75 into the tropics in the Early Miocene (22 Ma; Davies et al. 1988). In contrast, the major reef growth on 29 30 76 the neighbouring (~100 km to the west) commenced much later, between 452 and 365 31 32 77 ka (Webster and Davies 2003) and has been continually disrupted by the cyclic falling and rising of 33 34 78 eustatic sea-levels. Indeed, reef growth on the Queensland Plateau continued even during the Late 35 36 79 Miocene and Pleistocene lowstand sea-levels (120-200 m below present) when only the upper parts of the 37 38 80 coral reefs on the plateau were most likely exposed (Davies et al. 1989). The emergent reefs of the 39 40 81 Queensland Plateau were likely to be important refuges for coral reef taxa during these lower sea-levels, 41 42 82 and may also have provided the Great Barrier Reef with a source of propagules after catastrophic events, 43 44 83 a hypothesis recently supported by genetic data (Wörheide et al. 2002; van Oppen et al. 2011). Hence 45 46 84 these reefs, some of the largest in the world, should be afforded greater research attention. 47 48 85 49 50 86 In 2012, the Australian Government proclaimed nearly 1 million square kilometres in the Coral Sea as a 51 52 87 marine reserve. The Coral Sea Commonwealth Marine Reserve extends over the Queensland Plateau, and 53 54 88 includes the large Diamond Islets, Diane Bank, Lihou, Flinders, Coringa-Herald, Holmes, Bougainville, 55 56 89 57 Shark and Osprey reefs. Australia’s marine reserves, including the Coral Sea Commonwealth Marine 58 90 59 Reserve, are currently under review and require information on key ecological features and their 60 61 62 3 63 64 65 91 conservation values. However, the remote location of the Coral Sea has resulted in a paucity of scientific 1 2 92 data on the deeper environments of this region (Ceccarelli et al. 2013). The majority of research that has 3 4 93 been conducted on benthic ecosystems in Australia’s Coral Sea has focussed on the shallow-water coral 5 6 94 reefs, generally in water depths of less than 20 m (Ceccarelli et al. 2013). More recent observations of the 7 8 95 mesophotic (30-150 m), disphotic (some light but not enough to sustain photosynthesis ~150 to 250 m) 9 10 96 and aphotic (no light and deeper than ~250 m) zones in the Coral Sea have indicated that these waters 11 12 97 also support diverse benthic communities (Bongaerts et al. 2011; Wörheide et al. 2011; Wörheide et al. 13 14 98 2012). These include a ‘relict' fauna of (Lüter et al. 2003), coralline sponges (Wörheide 15 16 99 2008; Karlińska-Batres and Wörheide 2013) and Hexactinellida (Dohrmann et al. 2011) discovered at 17 18 100 Osprey Reef. 19 20 101 21 22 102 Marine benthic communities commonly exhibit clear zonation along depth gradients from surface waters 23 24 103 to the deep-sea, attributed to various factors including substratum type, temperature, , water 25 26 104 chemistry, food and energy availability (Grassle et al. 1979; Cartes and Carrasson 2004; Carney 2005). 27 28 105 The emergent reefs of the Queensland Plateau therefore provide a habitat for benthic communities 29 30 106 spanning depths ranging from the sea surface to greater than 1000 m. Their steep flanks are likely to 31 32 107 support a diverse range of benthic communities, including shallow-water coral reefs, mesophotic coral 33 34 108 ecosystems (MCEs), and deep-water azooxanthellate corals and sponge gardens. Therefore, the flanks of 35 36 109 a Coral Sea reef could be expected to exhibit zonation in benthic assemblages when descending from the 37 38 110 shallow sunlit surface waters, through the twilight mesopelagic and disphotic zones, then into the deeper 39 40 111 and cooler aphotic waters. However, no studies have quantitatively examined the composition and 41 42 112 zonation of benthic communities on Australia’s Coral Sea reefs beyond diving depths. 43 44 113 45 46 114 In 2009, the Deep Down Under expedition (www.deepdownunder.de) to Australia's Coral Sea area 47 48 115 targeted Osprey Reef, which is situated at the northern tip of the Queensland Plateau (Fig. 1). The reef is 49 50 116 separated by about 125 km from the north-eastern Australia margin by the Queensland Trough and is 51 52 117 surrounded by clear, oceanic waters unaffected by continental sedimentation. Osprey Reef is steep-sided, 53 54 118 rising ~1500 m above the surrounding seafloor of the Queensland Plateau to the south-east and about 55 56 119 57 2500 m above the deeper Queensland Trough and Osprey Embayment to the north-west of the reef. The 58 120 59 shallow reef platform is 28 km long by 10 km wide, with a about 40 m deep enclosed by a near- 60 61 62 4 63 64 65 121 continuous rim rising to the sea surface. North-west Osprey Reef forms a distinct point known as ‘North 1 2 122 Horn’, which was chosen as the location of the present remotely operated underwater vehicle (ROV) 3 4 123 study (Fig. 2). 5 6 124 7 8 125 The surface water circulation in this region is dominated by the East Australian (EAC) which 9 10 126 forms on the Queensland Plateau from the South Equatorial Current (SEC; Church 1987). Around latitude 11 12 127 15° S, the EAC flows southward following the Great Barrier Reef shelf. Part of the SEC forms a 13 14 128 clockwise gyre called the Coral Sea Counter Current or Hiri Gyre which flows northward past Osprey 15 16 129 Reef and into the Gulf of Papua (Wolanski et al. 1995). Below these tropical surface waters are cooler 17 18 130 water mass properties indicative of Subantarctic Mode Water at a depth of about 600 m (SAMW; 19 20 131 Solokov and Rintoul 2000; Hartin et al. 2011) and of Antarctic Intermediate Water (AAIW; Solokov and 21 22 132 Rintoul 2000; Hartin et al. 2011) below 700 to 1000 m. SAMW and AAIW enter the Queensland Trough 23 24 133 from the south and flow northwards towards the Coral Sea Basin (Solokov and Rintoul 2000). The area is 25 26 134 also impacted by tropical cyclones with recent cyclones passing over Osprey Reef in May 2013 and 27 28 135 March 2015. 29 30 136 31 32 137 In this study, we have used a high-resolution digital elevation model (DEM), and the video and still 33 34 138 imagery collected by a deep-water Cherokee ROV, to identify the spatial patterns in geomorphology and 35 36 139 benthic assemblages found along a depth gradient spanning 787 m at Osprey Reef in the Coral Sea. We 37 38 140 show that Osprey Reef supports diverse benthic assemblages, and identify defined geomorphic zones 39 40 141 characterised by distinct benthic assemblage groups that were generally predictable based on the depth 41 42 142 and sedimentary environment. 43 44 143 45 46 144 Methods 47 48 145 Bathymetry data 49 50 146 Bathymetry data around North Horn were collected between 2006-2008 by the MV Undersea Explorer 51 52 147 using a Raymarine DSM300 singlebeam echosounder (dual 50/200 kHz) rated to about 1000 m depth. 53 54 148 Raw bathymetry and position data were imported to Caris HIPS/SIPS post-processing software, then 55 56 149 57 predicted and a modelled sound velocity were applied and edited to remove noise. The final xyz data 58 150 59 were imported to ESRI ArcGIS and combined with Royal Australian Navy airborne lidar bathymetry data 60 61 62 5 63 64 65 151 from the reef flat and lagoon at Osprey Reef. The ArcGIS application Topogrid was used to generate a 1 2 152 0.0001-arcdegree (~10 m) resolution ESRI grid for north-western Osprey Reef. The ESRI grid data were 3 4 153 also imported to QPS Fledermaus software for 3D visualisation. 5 6 154 7 8 155 ROV survey 9 10 156 ROV data were collected using the 34 m utility vessel MV PMG Pride during the Deep Down Under 11 12 157 expedition to the Queensland Plateau (www.deepdownunder.de). Imagery and specimens from benthic 13 14 158 ecosystems were collected using the Marum (www.marum.de) Cherokee ROV depth rated to 1000 m 15 16 159 with a of 450 kg and dimensions of 1.4 x 0.88 x 0.8 m. Propulsion uses 4 axial thrusters with a 17 18 160 vertical thrust of 45 kgf or a velocity of ~0.3 ms-1. A surface-controlled manipulator arm can collect 19 20 161 samples up to 7 kg. Optical devices include a Tritech Typhoon colour zoom videocamera (720 x 576 21 22 162 pixels), Kongsberg OE14 digital still camera (5 megapixel), a three red laser array (30 x 23 cm) and a 230 23 24 163 W spotlight. An IXSEA GAPS system provided USBL, INS and GPS absolute positioning of the ROV, 25 26 164 as well as the surface position of the vessel. 27 28 165 29 30 166 The ROV dive at North Horn was conducted from 11-12 December 2009 UTC over a period of 5 h 42 31 32 167 min, from the depths 787 to 92 m. Video data were streamed to the surface for recording as mpeg files on 33 34 168 MiniDV VCRs, together with the timestamp, position and depth overlays. This resulted in six mpeg files 35 36 169 each about one hour long. The still images were recorded internally on the ROV, timestamped and 37 38 170 downloaded to CD on recovery of the ROV. The ROV position and depth sensor data and timestamps 39 40 171 were recorded into CSV files at 5 s interval. 41 42 172 43 44 173 Data analysis 45 46 174 The ROV navigation data for Longitude, Latitude, Depth, Year, Month, Day, Hour, Min, Sec were 47 48 175 recorded continuously to an Excel spreadsheet for the video data. These records were parsed to show only 49 50 176 whole-minute positions, resulting in the availability of 343 x one-min records. A Cherry SPOS keyboard 51 52 177 (Model G86-63400) provided up to 144 keys programmed using a classification scheme of physical and 53 54 178 biological parameters to append a Characterization sample to the spreadsheet (Table 1). This 55 56 179 57 characterization method is similar to the underwater video analysis conducted by Anderson et al. (2007) 58 180 59 and Post et al. (2011) for analysis of benthic survey data collected in other regions. The classification 60 61 62 6 63 64 65 181 scheme in Table 1 is for generic use in the deep-sea with the main headings of: Primary substrate (>50%), 1 2 182 Secondary substrate (>25%), Features, Relief, Bedforms, Biological cover, Lebensspuren, Biota and 3 4 183 Additional. Lebensspuren refers to the ‘life traces’ from bioturbating animals (Przeslawski et al. 2012). 5 6 184 7 8 185 The video files were viewed for a 15 second period at each whole minute, then classified using the 9 10 186 parameters in Table 1. For every one-min sample, both the Primary and Secondary substrate were 11 12 187 recorded into an Excel spreadsheet, in addition to the Relief type, Biological cover and any biota 13 14 188 observed. Biological cover was a qualitative measure: barren has no biota observed; low 10%; medium 15 16 189 10-50%; and high >50%. The keyboard provided a relatively quick method of recording the classification 17 18 190 by adding an abbreviation code into the Characterization cell for each one-min sample. The higher- 19 20 191 resolution still images provided assistance to classify the physical environment and biota where the video 21 22 192 data were unclear. The physical specimens collected by the ROV and now archived at the Queensland 23 24 193 Museum in Brisbane, Australia, provided additional confirmation of the taxon classifications. 25 26 194 27 28 195 Video data were converted to a presence/absence matrix using a Python program script, with each cell 29 30 196 assigned either 1 or 0 under the column headings representing every classification type shown in Table 1. 31 32 197 This presence/absence matrix was imported into a point shapefile for visualisation in ArcMap 10.1 to 33 34 198 show the positions and observed parameters of each one-min sample (Excel spreadsheet of the data 35 36 199 matrix is given in Online Resource 1). 37 38 200 39 40 201 Cluster analysis 41 42 202 Patterns in the distribution of the benthic biota data were examined using multivariate techniques in 43 44 203 Primer v6 (Clarke and Gorley 2006). The cluster analysis data excluded the demersal fish, pelagic shark 45 46 204 and the biological cover variables. The Primary substrate and Relief types were added as factors for each 47 48 205 1-min sample in the Primer worksheet. The presence-absence data were converted to a Bray-Curtis 49 50 206 similarity resemblance matrix to quantify the similarity among samples. The resemblance matrix was 51 52 207 analysed using group-averaged cluster analysis and the Bray-Curtis similarity displayed as a Principal 53 54 208 Coordinates (PCO) plot, with symbols visualised as five groups using Bray-Curtis similarity clustered at 55 56 209 57 45% similarity. SIMPER analysis was conducted to identify the main contributing taxa for each benthic 58 210 59 assemblage group, shown in Table 2. Additionally, an abiotic matrix was created using the Primary 60 61 62 7 63 64 65 211 substrate and Relief records, together with depths binned as arbitrarily coded depth ranges of 150 m (e.g. 1 2 212 <150, 150-300, 300-450, 450-600, 600-750 and >750 m). With a total ROV sample range of 695 m, from 3 4 213 the deepest record at 787 m to end of the dive in 92 m, the 150 m bins were considered an optimal bin 5 6 214 size for abiotic analysis. The variables (Substrate, Relief and Depth) were initially ranked, and then biota 7 8 215 and abiotic matching conducted using BIOENV analysis with Euclidean Distance against the biota 9 10 216 resemblance matrix. 11 12 217 13 14 218 Results 15 16 219 Geomorphology 17 18 220 The bathymetry mapping resulted in a 10 m-resolution grid with dimensions of about 13.8 km in the 19 20 221 north-south direction, 9.7 km in the east-west direction, with a 3D surface area of 117.0 km2 and depths to 21 22 222 932 m (Fig. 2). North Horn is the name of the prominent reef flat at the northern tip of Osprey Reef, up to 23 24 223 3 km wide at its greatest extent, which thins to about 700 m towards The Entrance, a narrow cleft through 25 26 224 the reef flat into the extensive lagoon behind. The ROV transect followed a gently curving line from a 27 28 225 depth of 787 m up towards North Horn. The geomorphology below North Horn shows the reef wall to a 29 30 226 depth of 30 m, dropping steeply with the slope gradient increasing from 20° to 50° with depth (Fig. 3a 31 32 227 and b). Then a precipitous cliff, with a gradient of 60° to 70°, occurs from 30 to 130 m depth. This near- 33 34 228 vertical cliff is pockmarked with caves and overhangs between 110 and 130 m. Calcareous sand streams 35 36 229 downward through narrow clefts and between rock protrusions, draping over surfaces with a suitably 37 38 230 gentle gradient. 39 40 231 41 42 232 From 130 m to between about 250 to 300 m, the slope eases to a generally consistent 30° to 40° gradient, 43 44 233 ending at a narrow (~50 m wide) shoulder that can be traced almost around the study area. Below this 45 46 234 narrow shoulder extending to depths of about 350 to 450 m, the slope becomes highly variable and 47 48 235 topographically rugose, with gradients ranging between 28° to 55°. This geomorphic rough zone reflects a 49 50 236 variable substrate of steep rock walls, interspersed with large boulders and narrow gravel-filled gullies 51 52 237 (Fig. 3a and b). Below this rough zone from about 450 m, the slope becomes noticeably less steep as a 53 54 238 broad sandy apron extends out around the base of Osprey Reef, with gradients easing from about 35° to 55 56 239 57 10° at the limit of the dataset in 932 m. Occurring sporadically within this broad sand-covered apron are 58 240 59 exposed vertical rock faces, several metres to 10s of m in scale, largely sediment-free and heavily Fe-Mn 60 61 62 8 63 64 65 241 stained (Fig. 3a and b). Several canyons also incise into and extend downward through the apron with 1 2 242 their heads starting around 450 m at the upper limit of the apron. Within these canyons, loose boulders 3 4 243 and gravel were observed that were not as heavily Fe-Mn stained as the in-situ rock walls found at 5 6 244 equivalent depths, and also contrasting against the blanket of coarse white sand found on the main apron. 7 8 245 9 10 246 Benthic assemblages 11 12 247 Cluster analysis of the presence/absence data indicated five benthic assemblage groups at 45% similarity. 13 14 248 In general, the PCO1 axis is associated with a gradient from a hard to soft substrate and the PCO2 axis 15 16 249 reflects a deep to shallow gradient (Fig. 4). Together, the first two PCO axes explain 58.5% of the 17 18 250 variability in taxa distribution. The BIOENV results indicated that the environmental variables: substrate 19 20 251 type and depth, were significantly correlated (p < 0.001) to the observed distribution of benthos, 21 22 252 explaining 37% of total variation. Table 2 shows the SIMPER results of the main contributing taxa for 23 24 253 each of the five benthic assemblage groups. 25 26 254 27 28 255 The following group assemblages are described from shallow (92 m upper limit) to deepest (787 m lower 29 30 256 limit). The shallowest assemblage observed is the Group A mesophotic assemblage. Typically found 31 32 257 above 150 m, this assemblage is dominated by a relatively high benthic coverage of large and dense 33 34 258 gorgonian fans (Heliania spp., Annella spp.), sea whips (Junceella spp.), soft coral colonies 35 36 259 (Chironephthya sp., Dendronephthya spp.) and sponges (Fig. 3c and f). A reduced coverage of smaller 37 38 260 stylasterid corals, soft corals and bamboo coral (Lepidisis spp.) were found inside the caves and 39 40 261 overhangs between 110 and 130 m (Fig. 5a and b). Samples within this group show a relatively high 41 42 262 average similarity (SIMPER = 57.57%). Towards the deeper section of this zone, below the caves, 43 44 263 benthic coverage reduced further and became more depauperate with large gorgonian fans situated mostly 45 46 264 on prominent rock ridges, together with occasional large soft coral colonies and isolated bamboo corals 47 48 265 (Keratoisis spp.). 49 50 266 51 52 267 Group B is a relatively low coverage to barren, disphotic (150 to ~250 m) assemblage lying below the 53 54 268 mesophotic group and occurs at the lower limits of light irradiance, with encrusting red algae observed in 55 56 269 57 smaller patches on rocks only to ~200 m. This assemblage first appeared on the narrow (50 m across), 58 270 59 gently sloping shoulder lying directly above the rough geomorphic zone, which stretches from about ~250 60 61 62 9 63 64 65 271 to 450 m (Fig. 5c and d). The assemblage is dominated by black corals (Sibopathes spp.), hydro corals 1 2 272 (Stylaster spp.) and soft corals (Chironephthya spp.), together with crinoids clinging to the colonies (Fig. 3 4 273 3c and f). SIMPER analysis shows a high average similarity of 75.10%. The substrate is mostly a thin 5 6 274 veneer of sand with small patches of exposed rock and boulders providing attachment surfaces for the 7 8 275 sessile fauna. The attachment of the larger coral colonies are more focussed on prominent vertical rock 9 10 276 ridges rather than on the flatter, sand-draped rock between the ridges (Fig. 5c). 11 12 277 13 14 278 Group C is a Lebensspuren (life traces) assemblage, typically found at depths below about 450 m within 15 16 279 the gently-sloping apron zone (Fig. 6b and d). However, this group was not exclusively confined to these 17 18 280 deeper depths as Lebensspuren were also observed in isolated patches of soft sediment within the gullies 19 20 281 and on ledges within the rough geomorphic zone, which separates the apron from the disphotic and 21 22 282 mesopelagic zones above (Fig. 3c and d). Lebensspuren were observed within the rough zone as shallow 23 24 283 as 342 m. Typically, the Lebensspuren were characterised by numerous tracks, small pits and occasional 25 26 284 large mounds left by the infauna and mobile fauna in these sandy habitats. This group occurred where the 27 28 285 primary substrate is mostly all sand, which tends to be the predominant substrate of the gently-sloping 29 30 286 apron zone. The only live fauna observed on the sand during the ROV transect were the occasional 31 32 287 echinoid or ophiuroid roaming over the sandy surface, or isolated bamboo corals (Keratoisis spp.) poking 33 34 288 through sand. This group showed a high average similarity of 71.95%. 35 36 289 37 38 290 Group D is a low to medium cover coldwater assemblage within the aphotic zone below about 450 m on 39 40 291 the apron (Fig. 6a, b and f). This assemblage is characterised by the hexactinellid sponge (Psilocalyx 41 42 292 wilsoni) and isolated bamboo coral colonies (Lepidisis spp.), together with scattered holothurians, small 43 44 293 shrimp and echiuran worms (Fig. 3c and f). SIMPER analysis shows only a low average similarity of 45 46 294 5.85% between the samples, which can be explained by a large number of taxa observed in low 47 48 295 abundance, for example the glass sponges were only at one site at 501 m. Typically, the sessile benthos 49 50 296 were found attached to the boulders found in the canyon axes or on relatively small vertical rock faces 51 52 297 with a low (<1 m) to moderate (1-3 m) relief that protrude through the sandy apron. Living biota typically 53 54 298 occurred on hard primary substrate, but was also found where sand formed a thin veneer over rock, 55 56 299 57 allowing colonisation by sessile biota. 58 300 59 60 61 62 10 63 64 65 301 Group E is a relatively high biological cover coldwater assemblage found below about 450 m within the 1 2 302 apron zone, occurring where isolated and relatively large (10s of m in scale) vertical rock walls protrude 3 4 303 through the surrounding sand (Fig. 6c, d and e). The heavily Fe-Mn stained rock provides a substratum 5 6 304 for the attachment of dense sessile fauna. This assemblage is dominated by infaunal polychaetes, bamboo 7 8 305 corals (Isididae), worm shells (Vermetidae), stalked crinoids, and large colonies of golden corals 9 10 306 (Chrysogorgia spp.) and occasional precious corals (Corallium sp.; Fig. 3c and f). Squat lobsters and 11 12 307 other crustaceans were also observed on the rock substrate and up on the coral colonies. The average 13 14 308 similarity among samples was a relatively high 53.47 %. 15 16 309 17 18 310 Demersal and pelagic fauna 19 20 311 While the focus of the ROV transect was on the substrate and sessile benthos, other demersal and pelagic 21 22 312 fauna were also recorded. Chambered (Nautilus pompilius) were observed in the deeper apron 23 24 313 zone at 501, 545, 580, 602, 612 and 625 m hovering or traveling over the seabed with no clear affinity to 25 26 314 any particular substrate or relief (Fig. 3e). Grey cut-throat eels (Synaphobranchus affinis) were observed 27 28 315 lying within nets attached to the vertical rock walls at 628 m. Coral reef fish were first observed in the 29 30 316 rough geomorphic zone, despite a lack of benthic fauna. A silvertip shark (Carcharhinas albimarginatus) 31 32 317 and a flowery cod (Epinephelus fuscoguttatus) were observed at 317 and 342 m, respectively. Oblique- 33 34 318 banded snapper (Pristipomoides zonatus) and the anthid (Odontanthias tapui) were observed swimming 35 36 319 near bare rock overhangs at a depth of 283 m. This anthid was previously known only from the central 37 38 320 East Pacific, representing a substantial range extension into Australian waters. Typical coral reef fish 39 40 321 became more numerous above ~250 m, coinciding with the appearance of the narrow shoulder found 41 42 322 above the rough zone. Fish density increased again in the shallower waters of the overhanging caves and 43 44 323 reef wall found above ~150 m. 45 46 324 47 48 325 Discussion 49 50 326 Patterns of depth zonation 51 52 327 Benthic assemblages at North Horn exhibited clearly defined patterns of depth zonation from 92 to 787 m 53 54 328 water depth. We identified five benthic groups, the occurrence of which was determined by substrate type 55 56 329 57 (sand versus exposed rock) in addition to depth. The most significant faunal break occurred at around 450 58 330 59 m depth, with coldwater fauna below this depth. The accumulation of sediments in areas of low slope 60 61 62 11 63 64 65 331 generally restricted the occurrence of sessile benthos to steep slopes, vertical walls or overhangs. 1 2 332 However, allochthonous sediments advected downslope appear to provide an important habitat for 3 4 333 infauna and some detritivorous taxa, evidenced by the Lebensspuren, which did not occur on steeper 5 6 334 slopes in association with sessile benthic fauna. 7 8 335 9 10 336 Shallower than ~250 m depth, benthic assemblages were composed of a tropical fauna, whereas depths 11 12 337 below ~450 m are comprised of coldwater taxa. Intermediate depths of ~250 to 450 m, found within the 13 14 338 rough geomorphic zone, were depauperate in biodiversity despite the occurrence of hard substratum 15 16 339 suitable for colonisation by sessile benthos. The CSIRO Atlas of Regional Seas (CARS2009; Dunn and 17 18 340 Ridgeway 2002) climatology profile for north-western Osprey Reef shows the temperature at 450 m is 19 20 341 10°C, dropping steadily to 4°C at 1000 m (Fig. 7). The cooler water masses at these depths first approach 21 22 342 Subantarctic Mode Water (SAMW) below a depth of about 600 m, then Antarctic Intermediate Water 23 24 343 (AAIW) below 700 to 1000 m. The change from tropical to coldwater assemblages in this depth range 25 26 344 clearly reflects the change in water mass properties. The depauperate fauna between ~250 to 450 m may 27 28 345 be caused by fluctuations in the depth of the within this depth range. The CARS climate 29 30 346 profile shows a thermocline around 200 m (Fig. 7). However, the depth of the thermocline around oceanic 31 32 347 can vary substantially depending on factors such as the El Nino Southern Oscillation (Colin 2009). 33 34 348 A deepening of the thermocline to ~450 m depth could preclude colonisation by coldwater taxa above 35 36 349 these depths, while a shallower thermocline could define a lower depth limit for tropical fauna (Kahng et 37 38 350 al. 2012). Fluctuations in temperature within the rough zone due to the vertical movement of the 39 40 351 thermocline may be too great to allow either tropical or coldwater taxa to survive permanently here. 41 42 352 43 44 353 The mesophotic assemblage occurred from the shallow limit of the transect at 92 m to a depth of around 45 46 354 150 m. Mesophotic communities supported abundant octocorals but few hard corals below 100 m, 47 48 355 consistent with previous observations from the Coral Sea (Sarano and Pichon 1988; Bongaerts et al. 49 50 356 2011) and the shelf-edge of the Great Barrier Reef (Bridge et al. 2011; Bridge et al. 2012). The lack of 51 52 357 hard corals at Osprey Reef and elsewhere in the Coral Sea at lower mesophotic depths is somewhat 53 54 358 surprising given the exceptionally high water clarity and the greater abundance of hard corals at depths 55 56 359 57 >100 m at other locations in the Indo-Pacific (Kahng and Maragos 2006; Kahng and Kelley 2007). The 58 360 59 occurrence of benthic taxa may be influenced by the topography of the reef slope, particularly the caves 60 61 62 12 63 64 65 361 and overhangs observed between 110 and 130 m, which likely reflect lowstand sea level at the Last 1 2 362 Glacial Maximum (Lambeck and Chappell 2001). The inside walls of these caves are mostly bare of 3 4 363 sediment, being too steep to accumulate sand, and the lower light inside result in a noticeably reduced 5 6 364 benthic cover and smaller coral colonies compared to the higher benthic cover and larger colonies just 7 8 365 outside. The large gorgonian corals (Annella spp.) found around the 100 m depth are also the preferred 9 10 366 habitat for pygmy seahorses (Hippocampus denise; Nishikawa et al. 2011). The ROV transect was 11 12 367 terminated at a depth of 92 m, below the depth where the mesophotic assemblage typically transitions into 13 14 368 a phototroph-dominated shallow-water community. However, a previous submersible dive by Sarano and 15 16 369 Pichon (1988) suggests that assemblages dominated by typical shallow-water corals transition into the 17 18 370 mesophotic octocoral-dominated community at ~65 m. This transition depth is supported by SCUBA 19 20 371 observations by one of the authors (TB), and is also consistent with the transition from phototroph- to 21 22 372 heterotroph-dominated assemblages on the Great Barrier Reef outer-shelf (Bridge et al. 2011). 23 24 373 25 26 374 Below 150 m, we observed an increase in the abundance of fine sediments covering all non-vertical 27 28 375 surfaces. The isolated patches of black corals, stylasterids and soft corals that did occur in the disphotic 29 30 376 zone were found only on vertical rock ridges or where the underlying rock is exposed through a veneer of 31 32 377 soft sediment. On the adjacent Great Barrier Reef shelf edge, a study of mesophotic communities on 33 34 378 sediment-covered, submerged reefs also found octocorals were concentrated in areas least likely to 35 36 379 accumulate sediment, along steep walls and the edges of reef crests (Bridge et al. 2011). Accumulation of 37 38 380 sediments in regions of low slope suggests the blanket of sediment is an important factor in limiting the 39 40 381 occurrence of sessile benthos. Although these sediments occurred throughout the ROV transect, the 41 42 382 presence of coarse reef material and white sand at shallow depths suggests these sediments are derived 43 44 383 from the reef flat and upper forereef slope and advected downslope. In addition to the influence of 45 46 384 downwelling sediments, depth zonation in the disphotic zone was also influenced by declining light 47 48 385 irradiance. The deepest observed phototrophic taxa were small patches of encrusting red algae observed at 49 50 386 depths of ~200 m, suggesting this depth represents the deepest point where photosynthesis remains 51 52 387 possible. 53 54 388 55 56 389 57 Below 450 m, patches of dark, heavily FeMn-stained hard substratum protruding through the sand were 58 390 59 densely covered with sessile and motile fauna. This habitat supported diverse benthic communities, 60 61 62 13 63 64 65 391 including the first record of precious corals found in tropical Australian waters (Wörheide et al. 2011; 1 2 392 Wörheide et al. 2012), and other ‘relict’ fauna such as sea lilies, brachiopods (Lüter et al. 2003), coralline 3 4 393 sponges (Wörheide 2008; Karlińska-Batres and Wörheide 2013) and glass sponges (Dohrmann et al. 5 6 394 2011). These rocks walls are also preferred by the grey cut-throat eels seen lying within nets attached to 7 8 395 the rock. This study provides the first description of such communities in Australia’s Coral Sea, 9 10 396 highlighting the dearth of information on deep-sea biodiversity. 11 12 397 13 14 398 In contrast to the diverse coldwater communities occurring on hard substratum, the majority of habitat 15 16 399 below 450 m depth was gently-sloping and covered in coarse white sand. Although depauperate in sessile 17 18 400 benthos, these habitats contained extensive Lebensspuren indicative of an abundant infaunal community. 19 20 401 The abundance of Lebensspuren points to a relatively stable physical environment with low near-seabed 21 22 402 currents and an absence of observed sediment slides required to preserve these life traces. In addition to 23 24 403 the Lebensspuren communities below 450 m, we observed echinoderm and large echiuran worms of 25 26 404 several metres in length in gravel-filled gullies in the rough geomorphic zone and in the larger canyon 27 28 405 axes at 724 m (Fig. 5e and f). These detritivores appear to prefer these more dynamic habitats that 29 30 406 typically funnel gravel and larger boulders downslope, and likely rely on the nutrients provided from the 31 32 407 shallows. The Coral Sea is a period of high cyclonic activity, and these deep-water detritivore 33 34 408 communities may partly rely on material transported by cyclones from the shallow reef into deeper 35 36 409 waters. 37 38 410 39 40 411 With up to a further 500-1500 m in elevation to the base of Osprey Reef, at ~1500 m on the Queensland 41 42 412 Plateau or ~2500 m on the Queensland Trough side, we expect that additional benthic assemblage groups 43 44 413 could be found. Deep-water scleractinian and alcyonacea habitat-forming corals are widely distributed 45 46 414 throughout the southwest Pacific region (Bostock et al. 2015) and on the southern Tasmanian margin 47 48 415 (Thresher et al. 2014). The coldwater scleractinian species: Solenosmilia variabilis, Enallopsammia 49 50 416 rostrata and Madrepora oculata, are relatively cosmopolitan (Davies and Guinotte 2011), and while they 51 52 417 were not seen in the ROV transect, Enallopsammia sp. and Madrepora sp. have been found as skeletal 53 54 418 remains in a dredge sample 390 km south of Osprey Reef at 1170 m in the Queensland Trough, associated 55 56 419 57 with a debris field of an undersea landslide (Beaman and Webster 2008). Around New Zealand, species 58 420 59 of Solenosmilia, Enallopsammia and Madrepora have peak depth ranges of between 1000-1400 m, 600- 60 61 62 14 63 64 65 421 1600 m and 1000-1200 m respectively, with AAIW providing optimum living conditions for scleractinian 1 2 422 corals (Bostock et al. 2015). On the southern Tasmanian margin, Enallopsammia sp. are observed on 3 4 423 to ~<1000 m, with Solenosmilia sp. observed 1000 to <1500 m (Thresher et al. 2014). One 5 6 424 might expect that in the deeper waters around Osprey Reef, also living within AAIW, scleractinian corals 7 8 425 will also be present. Future work is required to study if these deeper coldwater fauna exist, in addition to 9 10 426 collecting detailed oceanographic samples. 11 12 427 13 14 428 Implications for management 15 16 429 The Coral Sea Commonwealth Marine Reserve, proclaimed by the Australian Government in 2012, 17 18 430 encompasses nearly 1 million square kilometres and is currently under review. This review requires 19 20 431 information on key ecological features and their conservation values, hence this research at Osprey Reef 21 22 432 is timely. Currently, the no take zone at north-western Osprey Reef extends only to the shallow reef edge, 23 24 433 providing little protection for the deeper assemblages described here (Bridge et al. 2013), or indeed 25 26 434 protection against fishing of the resident reef sharks, which attract a sustainable dive tourism industry 27 28 435 (Barnett et al. 2012). Many of the Coral Sea’s emergent reefs, which host a significant proportion of the 29 30 436 region’s biodiversity, have received no additional protection within the new reserve (Bridge et al. 2015). 31 32 437 Similar to Osprey Reef, other reefs within the reserve have no take zones that extend only to the reef 33 34 438 edge, with limited protection for the deeper reef habitats on the outer reef slope known to support diverse 35 36 439 ecological communities (Bongaerts et al. 2011; Bridge et al. 2013; Englebert et al. 2014). 37 38 440 39 40 441 Our results confirm that the deep-sea habitats of Osprey Reef support diverse benthic communities which 41 42 442 can be used as a baseline to expand research against comparable depth zones on similar emergent reefs 43 44 443 across the Queensland Plateau. Although this work is the result of only a single transect, there is a 45 46 444 generally good concordance of samples within the groups. We expect that the overall depth-related 47 48 445 patterns of the benthic zonation observed at Osprey Reef are broadly applicable to the other emergent 49 50 446 reefs on the Queensland Plateau as they share a similar geological history, geomorphology and physical 51 52 447 . Future research should aim to extend and test these observations at the other 29 emergent 53 54 448 reefs on the plateau. With a total perimeter length of 1765 km around their summits, these reefs have 55 56 449 57 potentially large areas of suitable habitat for diverse benthic communities extending into the depths 58 450 59 around their flanks. The current review of the Coral Sea Commonwealth Marine Reserve is an 60 61 62 15 63 64 65 451 opportunity to adopt management plans that incorporate the deeper reef habitats within zones which help 1 2 452 to minimise future damage to these marine ecosystems. 3 4 453 5 6 454 References 7 8 455 Anderson TJ, Chochrane GR, Roberts DA, Chezar H, Hatcher G (2007) A rapid method to characterize 9 456 seabed habitats and associated macro-organisms. In: Todd BJ, Greene HG (eds) Marine 10 457 Geological and Benthic Habitat Mapping. Special Publication 47. Geological Association of 11 458 Canada, St John's, Canada, pp 71-80 12 13 459 Barnett A, Abrantes KG, Semour J, Fitzpatrick R (2012) Residency and spatial use by reef sharks of an 14 460 isolated and its implications for conservation. PLOS ONE 7:e36574. 15 461 doi:10.1371/journal.pone.0036574 16 17 462 Beaman RJ, Webster JM (2008) Gloria Knolls: A new coldwater coral habitat on the Great Barrier Reef 18 463 margin, Australia. In: Neil H, Tracey D (eds) 4th International Symposium on Deep-Sea Corals, 19 464 1-5 December 2008, Wellington, New Zealand. p 248 20 21 465 Bongaerts P, Bridge TCL, Kline DI, Muir PR, Wallace CC, Beaman RJ, Hoegh-Guldberg O (2011) 22 466 Mesophotic coral ecosystems on the walls of Coral Sea atolls. Coral Reefs 30:335. 23 467 doi:10.1007/s00338-011-0725-7 24 25 468 Bostock HC et al. (2015) The carbonate mineralogy and distribution of habitat-forming deep-sea corals in 26 469 the southwest pacific region. Deep-Sea Res I 100:88-104. doi:10.1016/j.dsr.2015.02.008 27 28 470 Bridge TCL, Done TJ, Beaman RJ, Friedman A, Williams SB, Pizarro O, Webster JM (2011) 29 471 Topography, substratum and benthic macrofaunal relationships on a tropical mesophotic shelf 30 472 margin, central Great Barrier Reef, Australia. 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Coral Reef Research Foundation, Koror, Palau 1 2 494 Davies AJ, Guinotte JM (2011) Global habitat suitability for framework-forming cold-water corals. PLOS 3 495 ONE 6:e18483. doi:10.1371/ journal.pone.0018483 4 5 496 Davies PJ, Symonds PA, Feary DA, Pigram CJ (1988) Facies models in exploration - the carbonate 6 497 platforms of north-east Australia. APEA Journal 28:123-143 7 8 498 Davies PJ, Symonds PA, Feary DA, Pigram CJ (1989) The evolution of the carbonate platforms of 9 499 northeast Australia. In: Crevello PD, Wilson JL, Sarg JF, Read JF (eds) Controls on Carbonate 10 500 Platform and Basin Development, vol Special Publication 44. SEPM Society of Economic 11 501 Paleontologists and Mineralogists, Tulsa, U.S.A., pp 233-258. doi:10.2110/pec.89.44.0233 12 13 502 Dohrmann M, Göcke C, Janussen D, Reitner J, Lüter C, Wörheide G (2011) Systematics and spicule 14 503 evolution in dictyonal sponges (Hexactinellida: Sceptrulophora) with description of two new 15 504 species. Zoological Journal of the Linnean Society 163:1003-1025. doi:10.1111/j.1096- 16 505 3642.2011.00753.x 17 18 506 Dunn JR, Ridgeway KR (2002) Mapping ocean properties in regions of complex topography. Deep-Sea 19 507 Res I 49:591-604. doi:10.1016/S0967-0637(01)00069-3 20 21 508 Englebert N, Bongaerts P, Muir P, Hay KB, Hoegh-Guldberg O (2014) Deepest zooxanthellate corals of 22 509 the Great Barrier Reef and Coral Sea. Marine Biodiversity 45:1-2. doi:10.1007/s12526-014- 23 510 0221-8 24 25 511 Grassle JF, Sanders HL, Smith WK (1979) Faunal changes with depth in the deep-sea benthos. Ambio 26 512 Special Report 6:47-50 27 28 513 Hartin CA, Fine RA, Sloyan BM, Talley LD, Chereskin TK, Happell J (2011) Formation rates of 29 514 Subantarctic mode water and Antarctic intermediate water within the South Pacific. Deep-Sea 30 515 Res I 58:524-534. doi:10.1016/j.dsr.2011.02.010 31 32 516 33 Kahng SE, Kelley CD (2007) Vertical zonation of habitat forming benthic species on a deep 34 517 photosynthetic reef (50–140 m) in the Au’au Channel, Hawaii. Coral Reefs 26:679-687. 35 518 doi:10.1007/s00338-007-0253-7 36 37 519 Kahng SE, Maragos JE (2006) The deepest zooxanthellate, scleractinian corals in the world? Coral Reefs 38 520 25:254. doi:10.1007/s00338-006-0098-5 39 40 521 Kahng SE, Wagner D, Lantz C, Vetter O, Gove J, Merrifield M (2012) Temperature related depth limits 41 522 of warm-water corals. In: 12th Int. Coral Reef Symposium, 9C Ecology of mesophotic coral 42 523 reefs, Cairns, Australia. pp 1-5. 43 44 524 Karlińska-Batres K, Wörheide G (2013) Microbial diversity in the coralline sponge Vaceletia crypta. 45 525 Antonie van Leeuwenhoek 103:1041-1056. doi:10.1007/s10482-013-9884-6 46 47 526 Lambeck K, Chappell J (2001) Sea level change through the last glacial cycle. 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J Mar Res 58:223-268. 13 547 doi:10.1357/002224000321511151 14 15 548 Symonds PA, Davies PJ, Parisi A (1983) Structure and stratigraphy of the central Great Barrier Reef. 16 549 BMR Journal of Australian Geology and Geophysics 8:277-291 17 18 550 Thresher R et al. (2014) Strong depth-related zonation of megabenthos on a rocky continental margin 19 551 (~700–4000 m) off southern Tasmania, Australia. PLOS ONE 9:1-17. 20 552 doi:10.1371/journal.pone.0085872 21 22 553 van Oppen MJH, Peplow L, M., Kininmonth S, Berkelmans R (2011) Historical and contemporary 23 554 factors shape the population genetic structure of the broadcast spawning coral, 24 555 millepora, on the Great Barrier Reef. Molecular Ecology 20:4899-4914. doi:10.1111/j.1365- 25 556 294X.2011.05328.x 26 27 557 Webster JM, Davies PJ (2003) Coral variation in two deep drill cores: significance for the Pleistocene 28 558 development of the Great Barrier Reef. Sediment Geol 159:61-80 29 30 559 Wolanski E, Norro A, King B (1995) Water circulation in the Gulf of Papua. Continental Shelf Research 31 560 15:185-212 32 33 34 561 Wörheide G (2008) A hypercalcified sponge with soft relatives: Vaceletia is a keratose demosponge. 35 562 Molecular Phylogenetics and Evolution 47:433-438. doi:10.1016/j.ympev.2008.01.021 36 37 563 Wörheide G, Hooper JNA, Degnan BM (2002) Phylogeography of western Pacific Leucetta 38 564 ‘chagosensis’ (Porifera: Calcarea) from ribosomal DNA sequences: implications for population 39 565 history and conservation of the Great Barrier Reef World Heritage Area (Australia). Molecular 40 566 Ecology 11:1753-1768. doi:10.1046/j.1365-294X.2002.01570.x 41 42 567 Wörheide G, Vargas S, Lüter C, Reitner J (2011) Precious coral and rock sponge gardens on the deep 43 568 aphotic fore-reef of Osprey Reef (Coral Sea, Australia). Coral Reefs 30:901. 44 569 doi:10.1007/s00338-011-0802-y 45 46 570 Wörheide G, Vargas S, Lüter C, Reitner J (2012) Erratum to: Precious coral and rock sponge gardens on 47 571 the deep aphotic fore-reef of Osprey Reef (Coral Sea, Australia). Coral Reefs 31:299. 48 572 doi:10.1007/s00338-011-0830-7 49 50 573 51 52 574 53 54 575 55 56 576 57 58 577 59 60 578 61 62 18 63 64 65 579 Figure and Table captions 1 Fig. 1 2 580 Regional-scale map of the Osprey Reef study area in relation to the north-eastern Australia margin 3 581 and the Queensland Plateau. Osprey Reef rises about 1500 m above the surrounding Queensland Plateau 4 582 seafloor 5 583 6 584 Fig. 2 7 3D-view of north-western Osprey Reef. The yellow line indicates the ROV transect below North 585 8 Horn. The dive initially started in the water column and video recording commenced in 787 m at the 586 9 seabed. White labels are geomorphic zones discussed in main text. Depth contours shown at 100 m 10 587 intervals 11 588 12 589 Fig. 3 Plan-views of the complete ROV transect: (a) Primary substrate; (b) Relief; (c) Biological cover; 13 590 (d) Lebensspuren; (e) Pelagics; (f) selected Corals. Depth contours shown at 20 m intervals 14 591 15 16 592 Fig. 4 PCA ordination plot showing similarity between sites based on the composition of the benthic taxa. 17 593 Vectors are overlain showing the dominant taxa within groups (see Table 1). Group A is a mesophotic 18 594 assemblage. Group B is a disphotic assemblage. Group C is a Lebensspuren (life traces) assemblage. 19 595 Group D is a low cover, coldwater assemblage. Group E is a high cover, coldwater assemblage. In 20 596 general, the PCO1 axis shows a hard to soft substrate and the PCO2 axis reflects a deep to shallow 21 597 gradient. Together, PCO1 and PCO2 account for 58.5% of the taxa distribution 22 598 23 24 599 Fig. 5 Representative still images from ROV transect shallower than 450 m: (a) 118 m rock and sand 25 600 substrate with high relief. Rock surfaces with a high cover of Junceella spp. sea whips, Heliania spp. 26 601 gorgonian fans, sponges and red algae; (b) 134 m rock substrate with rock wall relief. Prominent caves 27 602 fringed with small stylasterid, Chironephthya spp., Dendronephthya spp. corals, sponges, and red algae; 28 603 (c) 200 m rock and gravel substrate with high relief. Edges of rock walls have clusters of Chironephthya 29 604 spp. and other large gorgonian fans; (d) 257 m rock and gravel substrate with high relief. Some upper 30 605 surfaces and edges of rock have patches of stylasterid, antipatheria, and other soft corals; (e) 433 m rock 31 606 and sand substrate with high relief. Motile echinoderms, such as brittlestars, are found on gravel between 32 607 33 boulders. No corals observed; and (f) 456 m rock and gravel substrate with moderate relief. Minor FeMn 608 34 oxide staining on rock in canyon axis with echiuran worm draped on gravel 609 35 36 610 Fig. 6 Representative still images from ROV transect deeper than 450 m: (a) 501 m rock and sand 37 611 substrate with moderate relief. Clusters of Psilocalyx wilsoni hexactinellid sponges on rock face; (b) 513 38 612 m sand substrate with low relief. Dense Lebensspuren in sand with occasional Keratoisis spp. and 39 613 Lepidisis spp. bamboo corals; (c) 603 m rock and sand substrate with rock wall relief. Heavy FeMn oxide 40 614 staining on rock with golden, precious and bamboo corals, sponges, gastropods, polychaetes, 41 615 42 echinoderms, and crustacea; (d) 625 m rock and sand substrate with rock wall relief. Dense Lebensspuren 616 43 pits and tracks in sand. Heavy FeMn oxide staining on rock with high cover of Chrysogorgia spp. golden 617 44 corals and Corallium sp. precious corals, echinoderms, polychaetes, and crustacea; (e) 714 m sand and 618 45 rock substrate with moderate relief. Heavy FeMn oxide staining on rock with large bamboo coral and 46 619 crustacean; and (f) 758 m boulder and gravel substrate with moderate relief. Light FeMn oxide staining 47 620 on boulder and small colonies of Lepidisis spp. bamboo corals 48 621 49 622 Fig. 7 CSIRO Atlas of Regional Seas (CARS) climatology profile for north-west Osprey Reef 50 623 51 52 624 Table 1 Underwater video classification scheme of physical and biological parameters 53 625 54 55 626 Table 2 SIMPER results of the main taxa contributing to the five benthic assemblage groups using the 56 627 Bray-Curtis similarity measure 57 58 628 Electronic Supplementary Material 59 60 629 Excel spreadsheet of the video data matrix 61 62 19 63 64 65 Figure1 100 km N Plateau Queensland Australia 147°E 0 Osprey Reef Fig. 2 Trough Queensland 146°E Osprey Embayment Cooktown 145°E

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polychaete bamboo coral 20 gastropod golden coral crinoid stalked precious coral brittlestar tracks anemone pits mounds

urchin 0 3D sponge hydro coral black coral

PCO2 (25.8% of total variation) gorgonian whip gorgonian fan -20 soft coral

shallow

hard soft -40

-40 -20 0 20 40 PCO1 (32.7% of total variation) Figure5

a. b. stylasterids Junceella

Heliania Dendronephthya Chironephthya d a c b f e

c. d.

Chironephthya

Stylaster e. f.

echiuran worm

brittlestar Figure6

a. b.

Keratoisis

b a c Psilocalyx wilsoni d Lebensspuren e pits, tracks, mounds f c. d.

Corallium

sea lily Chrysogorgia Lebensspuren pits, tracks

Chrysogorgia

Corallium e. f. decorator crab

F. Isididae Lepidisis Figure7 Temperature (°C) 0 5 10 15 20 25 30 0

500

1000 Salinity Temperature

(m) Depth 1500

2000

2500 34.4 34.6 34.8 35.0 35.2 35.4 35.6 35.8 Salinity Table1

Type Definition Abbreviation Primary substrate >50%

rock* exposed bedrock s1_rock boulder* >25 cm loose material s1_bldr gravel* 2 mm-25 cm loose material s1_grav sand* lighter colour, grains visible to naked eye s1_sand mud darker than sand, grains not visible s1_mud Secondary substrate >25%

rock* exposed bedrock s2_rock boulder* >25 cm loose material s2_bldr gravel* 2 mm-25 cm loose material s2_grav sand* lighter colour, grains visible to naked eye s2_sand mud* darker than sand, grains not visible s2_mud Features

shell hash finely (~2 mm) broken shell material shl_hash sediment clouds sediment suspended from the bottom sed_cloud iceberg scour - recent sharp morphology scour_rec iceberg scour - relict subdued features bounded by ridges scour_rel glacial till unsorted glacial sediment,fine to coarse textures till vents seafloor vents vents manganese nodules manganese concretions mang_nod Relief

rock wall* vertical wall with slope angle >80° r_wall high relief* >3 m relief r_high mod relief* 1-3 m relief r_mod low relief* <1 m relief r_low flat relief 0 m relief r_flat Bedforms

hummocky irregular bedform, >50% terrain r_hum sand waves wave-like bedforms in sediment sand_wave sand ripples ripple-like bedforms in sediment ripples subtle waves bedforms less clearly defined sub_waves sand ribbons linear sheets of sand sand_rib Biological cover

barren* no biota on sediment surface barren low* <10% low_cov medium* 10-50% med_cov high* >50% high_cov Lebensspuren

craters depression in surface sediment >10 cm wide leb_cratr pits* depression in surface sediment <10 cm wide leb_pit shell shell remains, e.g. midden leb_shl burrows holes penetrate surface leb_bur tracks* tracks visible on surface leb_trk mounds* mounds of sediment leb_mnd Biota

3D sponge* Class Demospongiae sp_3d: encrusting sponge Class Demospongiae sp_encrst glass sponge* Class Hexactinellida sp_glass hard coral colonial Order Scleractinia h_cor_col hard coral fragment Order Scleractinia h_cor_frg hard coral solitary* Order Scleractinia h_cor_sol soft coral* Order Alcyonacea soft_cor black coral* Order Antipatharia blk_cor bamboo coral* Order Alcyonacea bam_cor golden coral* Order Alcyonacea gold_cor precious coral* Order Alcyonacea prec_cor hydroid colonial* Order Hydroidea hydr_col hydro coral* Order Anthoathecata hydr_cor gorgonian fan* Order Gorgonacea gorg_fan gorgonian whip* Order Gorgonacea whip sea pen Order Pennatulacea seapen anemone on substrate* Order Actiniaria anem_sub anemone on invert Order Actiniaria anem_invt jellyfish Class Scyphozoa jellyfish crinoid stalked* Class Crinoidea crin_stlk crinoid on substrate* Class Crinoidea crin_sub crinoid on invert* Class Crinoidea crin_invt basket star Class Ophiuroidea bskt_star brittle star on substrate* Class Ophiuroidea brit_sub brittle star on invert* Class Ophiuroidea brit_invt holothurian on substrate* Class Holothuroidea holo_sub holothurian on invert Class Holothuroidea holo_invt holothurian infaunal Class Holothuroidea holo_inf urchin* Class Echinoidea urchin pencil urchin* Class Echinoidea urch_pen sea star on substrate Class Asteroidea star_sub sea star on invert Class Asteroidea star_invt crustacea other Subphylum Crustacea crust_oth shrimp* Subclass Eucarida shrimp lobster* Order Decapoda lobster crab* Order Decapoda crab spider crab* Order Decapoda spid_crab hermit crab Order Decapoda herm_crab mantis shrimp Order Stomatopoda man_shrmp sea spider Class Pycnogonida spider barnacle sessile Class Cirripedia barn_sess barnacle stalked Class Cirripedia barn_stlk Phylum Brachiopoda brachiop mollusc other any other mollusc molsc_oth bivalve Class Bivalvia bivalve gastropod* Class Gastropoda gastrpd slug Class Gastropoda slug octopus* Class Cephalopoda oci squid Class Cephalopoda squid nautilus* Class Cephalopoda nautilus pteropod Class Gastropoda pteropod worm other* any other worm worm_oth ribbon worm Phylum Nemertea worm_rib polychaete mobile Class Polychaeta polych_mb polychaete infaunal* Class Polychaeta plych_inf bryozoa Phylum Bryozoa bryo tunicate colonial Class Ascidiacea tun_col tunicate solitary Class Ascidiacea tun_sol fish* any fish species fish shark* Class Chondrichthyes shark ray or skate Class Chondrichthyes ray unkown sessile invert unknown sessile invertebrate attached to seabed ses_invrt unknown motile invert unknown motile invertebrate near seabed mot_invrt Additional

missed missed record if classifying in real time missed interface* boundary between two substrata interface undefined* seabed not visible undefined

*observed in this study.

Table2

Group Taxa Av. Abundance Av. Similarity Contribution% Cumulative%

A soft coral 1.00 34.72 60.31 60.31 gorgonian fan 0.66 11.53 20.02 80.33

3D sponge 0.48 5.38 9.35 89.68

gorgonian whip 0.38 3.70 6.43 96.12

B black coral 1.00 34.80 46.33 46.33 crinoid on invertebrate 0.86 25.00 33.29 79.62

soft coral 0.57 7.65 10.19 89.81

hydro coral 0.57 7.65 10.19 100.00

C pits 1.00 40.65 56.50 56.50 tracks 0.86 27.28 37.92 94.42

mounds 0.29 2.74 3.81 98.23

D worm other 0.12 1.58 27.06 27.06 glass sponge 0.13 1.28 21.84 48.90

bamboo coral 0.13 1.15 19.74 68.64

holothurian on substrate 0.10 0.97 16.54 85.18

shrimp 0.11 0.65 11.08 96.26

E polychaete infaunal 0.95 26.67 49.88 49.88 bamboo coral 0.75 12.45 23.28 73.16

gastropod 0.67 9.36 17.51 90.67

golden coral 0.32 1.70 3.18 93.85

crinoid stalked 0.25 0.94 1.75 95.61

Online Resource 1

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