Reef hidden behind Great Barrier Reef could hold secrets in its history

Marine scientists working with laser data have revealed a vast reef behind the world-famous Great Barrier Reef, three times larger and its structure is differently shaped than previously thought. The reef isn’t made of corals, but of algae called Halimeda bioherms. This is the first time anyone has mapped a reef of this type to such detail, anywhere in the world.

Halimeda bioherms — living calcified segments forming a community 10000 years old covering 6000 kilometers squared.

Lead author on the new research paper published in Coral Reefs (the Journal of the International Society for Reef Studies) about their findings, Mardi McNeil from Queensland University of Technology (QUT) said the extent of the geological structures uncovered was vast.

Using LiDAR* technology the Royal Australian Navy assisted the researchers to gather data and map over 6000 square kilometres, three times the previously estimated size. The area spanning from the Torres Strait to just north of Port Douglas forms a significant inter-reef habitat which covers an area greater than the adjacent coral reefs.

The sheer size, extent and complexity of the strange donut-shaped circular mounds and hollows completely surprised us. As the data resolution was increased, the more we were amazed.

Researchers mapped over 6000 km squared of Halimeda bioherm distribution(left).

This is the first time that the Halimeda bioherms — reef-like structures built of limestone flakes — have been revealed in 3D. The marine scientists realised immediately previous scientific interpretations of the bioherms needed to be re-assessed.

What, for example, is the significance of this vast inter-reef habitat in terms of the sheer volume of calcium carbonate; their role in carbon storage; as habitat for biological communities; and in terms of effectively managing the Great Barrier Reef with respect to habitat mapping, boundaries, and conservation zoning.

Donut shaped reef a previously unknown structurl feature

The deeper seafloor behind the familiar coral reefs amazed the researchers. High-resolution seafloor data provided by LiDAR-equipped aircraft revealed great fields of the unusual donut-shaped circular mounds, each 200–300 metres across and up to 10 metres deep at the centre.

Scientists have known about these geological structures in the northern Great Barrier Reef since the 1970s and 80s, but never before has the true nature of their shape, size and vast scale been revealed.

What is the reef made of and what can it tell us?

Halimeda bioherms are created by the growth of Halimeda, a common green algae composed of living calcified segments. These form small limestone flakes on death, looking much like white cornflakes. Over time these flakes build up into large reef-like mounds, or bioherms.

Revelations about the extent of the bioherm field make questions over its vulnerability to climate change even more pressing. As a calcifying organism, Halimeda may be susceptible to ocean acidification and warming.

Bigger questions yet to be addressed

Future research would require sediment coring, sub-surface geophysical surveys, and employing autonomous underwater vehicle technologies to unravel the physical, chemical and biological processes of the structures.

Researchers need to investigate whether the Halimeda bioherms been impacted, and if so to what extent.

Their next steps will be to investigate what the 10–20 metre thick sediments of the bioherms can tell us about past climate and environmental change on the Great Barrier Reef over this 10,000 year time-scale.

Researcher Mardi McNeil also sees the need to identify the finer-scale pattern of modern marine life found within and around the bioherms now that the researchers understand their true shape.

Contact Mardi McNeill at QUT for more information on this amazing research. This research was originally published in Coral Reefs, Journal of the International Society for Reef Studies.

For more information, go to QUT Research.

*Laser Airborne Depth Sounder (LADS) uses red and green LiDAR technology to rapidly scan the seafloor to depths of about 50 metres, generating a dense grid of depth data points. The bathymetry data are then used by the Australian Hydrographic Service (AHS) to revise the nautical charts used by mariners.

* The Weapons Research Establishment first developed LADS airborne LiDAR bathymetry in Australia in 1972. The navy LADS Flight unit is based out of Cairns for year-round surveying of Australia’s shallow waters.

*Less than 15% of the Earth’s oceans deeper than about 200 metres have been mapped using modern surveying techniques.