Andrew Heap, Rachel Przeslawski, Ralph Haese, and Justy Siwabessy, Geoscience Australia

Geoscience Australia scientists recently teamed up with researchers from the Australian Institute of Marine Science (AIMS) to carry out a seabed mapping survey on the Van Diemen Rise in the eastern Joseph Bonaparte Gulf off the coast of Australia’s Northern Territory. The survey was part of a three-year collaboration between the two organisations involving marine surveys to northern Australia using Geoscience Australia’s shallow water multibeam sonar system and AIMS’ research vessel RV Solander.

The eastern Joseph Bonaparte Gulf was selected because its seabed environments are representative of the shallow banks and shoals and intervening channels common across the whole Gulf and, more broadly, the northern Australian shelf.

Environments spanning the outer to inner shelf in the Gulf were mapped using the multibeam sonar system. This revealed a complex seabed characterised by shallow carbonate banks dominated by sponge gardens, deep channels, including one channel more than 200 metres deep, and sediment plains. There was also evidence of significant sediment transport across the banks.

The survey collected geological and biological samples as well as wave and tide current data and underwater video footage of seabed habitats. Epifaunal species richness was associated with geomorphology, with 141 sponge morphospecies identified on the banks, including reef-forming types such as lithistids, halichondrids and xestosponges (Fig. 1). Stony corals (Acropora sp.) were only observed in two places. A very high diversity of infauna occurred on the sediment plains, with >80 crustacea species from just seven sites. From the 45 mollusc species collected were three new species, a new Australian record, and a new distributional record.

Interestingly, water column and surface sediment measurements indicate that the marine mineral high-magnesium calcite which comprises coralline algae is very close to its saturation state. It is possible that this mineral could be lost over a time scale of decades if ocean acidification predictions prove correct, with attendant major changes in sediment composition. Other measurements showed that the water residence time in the Gulf increased onshore.

These data will help improve our understanding of seabed environments and habitats in the Joseph Bonaparte Gulf. A follow-up survey is planned for August 2010.

Photo:
Most common sponges recovered on the carbonate banks: a) Xestospongia sp. 1 (26 m), b) Ianthella sp. 2 (60 m depth), and c) Echinodictyum mesenterinum from (27 m depth). Scale bar = 10 cm.

Common sponges on carbonate banks. Photo: GA / AIMS