The hidden reef made of giant algae doughnuts

A hidden reef exists behind the Great Barrier Reef—but it’s three times bigger than previously thought, constructed by algae, and made up of doughnut-shaped mounds.

Uncovering the true scale of the 6,000 km2 structure was made possible by airborne laser mapping technology LiDAR, provided by the Royal Australian Navy.

It has implications for the Great Barrier Reef’s habitat mapping and conservation zoning, as well as providing possible insights into past climates.

Halimeda is a common green alga made of living calcified segments and is responsible for the doughnut-shaped mounds, each 200–300 metres wide and up to 20 metres thick at the centre. When Halimeda die the calcified segments create limestone flakes that build up and over time create the reef-like mounds, called bioherms.

“While it was known that these structures existed in the northern Reef, their true shape and extent was unknown,” says lead author Mardi McNeil of QUT.

“This is the first time that Halimeda bioherms have been mapped in 3D. It raises questions into their role in carbon storage, providing habitat for reef creatures, and whether they’re susceptible to impacts from ocean acidification—since Halimeda is a calcifying organism.”

The team has begun analysing sediment cores taken from the mounds. Future work will include field surveys of the surface and sub-surface of the seafloor across the bioherms.

Researchers hope to learn what the bioherm’s sediments can reveal about reef evolution, past climates and environmental change on the Reef over a 10,000-year time-scale.

Banner image credit: Dr Emma Kennedy, UQ