ROV transect reveals the mesopelagic Osprey Reef, Coral Sea, Australia
May 14, 2014

Authors: Beaman, R.J., Lüter, C., Reitner, J., Wörheide, G.

Year: 2014

Publication: GEOHAB 2014, 5-9 May 2014 GEOHAB Marine Geological and Biological Habitat Mapping, Lorne, Australia


When Australia’s Coral Sea Reserve was proclaimed in Nov 2012, Osprey Reef was highlighted as the jewel in the crown of the new system of ocean reserves. This oceanic seamount lies at the northern end of the Queensland Plateau and is regularly used for dive adventure tourism due to the beautiful coral and prolific sharks found there. For many years, these vessels have also carried scientists to this remote site and we installed software to automatically capture singlebeam echosounder data. The crowd-sourced bathymetry data were used to create a 10 m-resolution 3D gridded surface of north-west Osprey Reef to a depth of 932 m. A steep cliff lies between 30 to 130 m, then a narrow shoulder with less gradient occurs to about 250 to 300 m. Below the shoulder, the seabed becomes steeper again forming a rough zone to depths of about 320 to 420 m. A broad apron then extends out around the base of Osprey Reef which is incised with canyons.

In Dec 2009, the Deep Down Under expedition, a German-Australian collaboration, brought a 1000 m-rated Cherokee ROV with a manipulator arm to collect relict fauna in the mesopelagic zone. The 3D map helped guide the ROV dive below North Horn at Osprey Reef to a depth of 787 m. Over five hours of georeferenced video imagery were analysed at 1 min intervals using a programmable keyboard with up to 144 keys available for a classification scheme, using the headings: Primary substrate (>50%), Secondary substrate (>25%), Features, Relief, Bedforms, Biological cover, Lebensspuren (life traces) and Biota. The resulting presence/absence matrix was converted to a point shapefile for viewing in ArcGIS, thus providing a dense record of the physical and biological character of the ROV traverse in relation to the 3D bathymetry grid

We discovered a strong relationship between taxon assemblages and the physical substrate and relief, and observe a major change in benthos assemblages around 500 m depth. On the deeper apron within the axis of a prominent canyon, we found mostly barren, loose boulders and gravel, with large (>2 m long) echiuran worms scavenging within this disturbed area. Up on the canyon wall away from the canyon axis, carbonate sand blankets the seafloor, providing a soft sediment environment for Lebensspuren tracks and pits, and the occasional echinoid and large spider crab. A dramatic contrast occurs wherever a rock wall protrudes through the sand. These walls have a high coverage of precious, golden and bamboo corals, together with epibenthic crustaceans and crinoids. Nautilus shells also appear in the video, drawn to the ROV’s lights.

Above the apron in depths less than 500 m, the rough geomorphic zone has only minor sessile benthos, even on the obviously stable rock walls. Motile fauna are the occasional echiuran worm and ophiuroids found in areas of disturbed seabed. Around 250 m, the transect over the narrow shoulder reveals an increase in benthic coverage with a lower mesophotic assemblage of stylasterid hydrocorals and black corals. The first coral fish and sharks appear here. Higher up, larger soft coral colonies, stylasterids and seawhips appear, mostly along the protruding edges of rocks. Around 150 m, the seabed becomes very steep where large overhanging caves and tunnels appear likely related to previous lowstand sea-levels. Above 125 m, an upper mesophotic assemblage of dense soft corals, seawhips and gorgonian fans complete the transect.

The ROV video analysis is a valuable record of a deep Coral Sea reef which has seldom had such exploration at these depths. For the wider Coral Sea Reserve, the implications for managers are that the patterns observed here are broadly applicable to the other seamounts on the Queensland Plateau as their geomorphology and oceanography are similar. Together with the new records of taxa, such as the precious corals, Osprey Reef may justifiably be called the (deep) jewel in the crown of Australia’s ocean reserves.

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