Carbonate banks and terraces of the Van Diemen Rise and the Sahul Shelf, and pinnacles of the Bonaparte Basin (North and North-west marine regions) are Key Ecological Features (KEFs) represented in the Oceanic Shoals Commonwealth Marine Reserve (CMR).
ABOVE AND BELOW: Benthic biological material was sampled with epibenthic sleds. Twenty-nine sponge species collected are new to science, with as many as 100 potential new species yet to be confirmed. Sediment-dwelling animals were highly diverse with 266 observed species, including newly discovered species of sea spider, squat lobster and worm.
These features, with their abrupt bathymetry presenting a range of substrates, aspects and depths, were considered potential biodiversity hotspots, and merited further investigation. This project explored and discovered new marine biodiversity information, with a ship-based expedition in the RV Solander to previously unsampled carbonate banks and pinnacles in the western part of the Oceanic Shoals CMR. The information is being integrated with existing data to better understand biodiversity distribution and sensitivity in this area of active offshore exploration and development.
The collaborative nature of this project capitalised on the unique expertise of the research partners. A 25-day voyage involving the Australian Institute of Marine Science, Geoscience Australia, the Museum and Art Gallery of the Northern Territory, and the University of Western Australia targeted three rectangular areas. Each rectangle covered approximately 200 km² of seabed and contained one or more banks or pinnacle features and contrasting non-KEF habitats. Multibeam sonar and other acoustic tools were used to create high-resolution seabed maps and benthic biological material was sampled with epibenthic sleds. The project also observed seabed habitats and fish communities with cameras including towed video (and associated stills cameras) and benthic and pelagic baited video stations.
Video: Oceanic Shoals – Commonwealth Marine Reserve, Timor Sea 2012 – Pelagic baited cameras
The Western Oceanic Shoals CMR contains more banks and pinnacles than previously thought, with high resolution mapping in four survey areas revealing 41 banks and pinnacles covering an area of 152 km², an increase from 105 km² (~33%). This finding suggested that the number of banks and pinnacles is likely to be significantly greater throughout the CMR than previously had been indicated by available charter or the bathymetric data in the 250 m grid national bathymetric database.
In general, open, low-relief areas of continental shelf supported very low abundances of macrobenthos. The sampled biotas were dominated by sponges, with soft corals the next dominant group, and much lower numbers of other major taxa. Sponges and soft corals were mostly associated with the sides and plateau areas of banks and pinnacles. Benthic biodiversity and abundance on banks and pinnacles decreased with water depth and across the transition, from the hard substrate of banks to soft sediment plains.
The high diversity of sponges, in particular on raised geomorphic features (banks, pinnacles, ridges, terraces) compared to subdued features (plains, valleys), adds to the growing awareness that Australia is a global diversity hotspot for sponges. Distinct regional faunas, including differences from east to west across the Oceanic Shoals CMR, and high levels of endemism are evident.
Shallower banks that rose to within 45 m of the sea surface supported greater biodiversity, including isolated hard corals of limited diversity. It is noteworthy that other studies of more seaward, clearer water shoals in the same CMR, undertaken in 2014 by AIMS, found much more abundant and diverse hard coral communities more similar to shelf edge shoals. This indicates the CMR boundary has captured a broad range of shoal and pinnacle features and associated environmental conditions, and the diversity of biota associated with these physical environmental gradients.
Bathymetry of the Joseph Bonaparte Gulf and surrounding areas
IMAGE: The Oceanic Shoals CMR contains components of three KEFs considered regionally important for biodiversity or ecosystem function and integrity: the carbonate banks and terraces of the Sahul Shelf and the Van Diemen Rise, and the pinnacles of the Bonaparte Basin. (Visualised depths fall between 0 and 300 m.) Image: Geoscience Australia.
Tidal currents are important in shaping the seabed by scouring holes into soft sediments around the base of banks and pinnacles and by extending the length of pockmarks. Levels of suspended sediment (turbidity) appear higher in the western part of the CMR than the east, with some smaller pinnacles partly buried by sediment.
Demersal fish communities appear to correlate with the spatial patterns observed for the benthic biodiversity, occurring in larger and more diverse communities on the shallower, less turbid banks. Given that water clarity limited the utility of the demersal, camera-based methods deployed, better sampling of fish diversity across more turbid, mid-shelf regions of the CMR is likely to require extractive sampling such as trawls and traps. The surveyed area also supports a wide range of pelagic animals, with 32 species observed: 11 shark species, black marlin, barracuda, Olive Ridley turtle, sea snakes and orca.
New knowledge and opportunities
Biological samples collected in the Oceanic Shoals CMR clearly demonstrate the region’s high biodiversity values. The abrupt bathymetric features such as pinnacles and shoals appear to be locations of species and biomass accumulation for macrobenthos, and other demersal and pelagic species relative to the broader shelf environs. The greater abundance of shoals than previously thought and the variety of their sizes, depths and shelf positions indicates a complex mosaic of biodiversity hotspots.
The research voyage extended understanding of biodiversity patterns on shoals into more turbid mid-shelf environs than previously assessed. The results confirm the importance of these features in supporting diversity, and elaborate the role of light reaching the seabed in shaping the dominant benthos. They also support the notion that as turbidity increases, the depth at which autotrophic species are able to dominate the seabed decreases. In the CMR this may be particularly the case for habitat-engineering species such as hard corals and large macroalgae. An opportunity exists to develop predictive models – based on data for depth, water quality and shelf position – to better understand the nature of benthic communities on the majority of shoal and pinnacle features yet to be surveyed.
Outputs and outcomes
This project exercised strong, national, multidisciplinary collaboration in marine science to build a greater understanding of marine connectivity, both within the CMR and across the broader region. It produced high-resolution bathymetry, geochemical and geophysical data, biological collections, and a new qualitative model of the KEFs of the Oceanic Shoals CMR. The results confirmed the value of the KEFs in terms of the biodiversity they host and their regional significance. They were communicated in scientific and non-technical formats, to raise the understanding of marine biodiversity in Australia’s poorly known north and north-western waters, (including the potential effects of fishing, and oil and gas exploration and extraction).