Vast Halimeda ‘Bioherm Reef’ Discovered Behind the Great Barrier Reef

Scientists from James Cook University, University of Sydney and Queensland University of Technology have just discovered a vast Halimeda reef hiding in plain sight.

Halimeda is a green macroalgae composed of calcified leaf-like segments. As these segments die, they accumulate over millennia as thick mounds called bioherms. The newly discovered Halimeda reef consists of a living layer of Halimeda algae, which covers the bioherms, and the reef covers an area of 6095 km2, on the landward side of the northern Great Barrier Reef.

Although the Halimeda habitat was previously known to GBR scientists, this new study shows the reefs are three times larger and more complex than previously thought.

Scientists used high-resolution airborne LiDAR and multibeam bathymetry datasets to gain a more accurate calculation of the bioherms. Previous studies of the reef used sediment grabs, coring and seismic profiling, and the distribution was estimated at 2000 km2 using interpolation from echosounder profiles.

Halemida Reef — McNeil, M. A., Webster, J. M., Beaman, R. J., and Graham, T. L., 2016, New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef, Australia: Coral Reefs. [GRG]

“We’ve known about these geological structures in the northern Great Barrier Reef since the 1970s and 80s,” says JCU’s Dr. Robin Beama, “but never before has the true nature of their shape, size and vast scale been revealed.”

Seminal literature describes the Halemida habitat as ridges and troughs running parallel to the reef. While these new findings show the reef as having a complex reticulate morphology with three distinct morphological sub-types: reticulate, annulate, and undulate.

Conservation

The vast expanse of living Halimeda on top of the bioherms, and the structural complexity of the bioherms themselves, create a significant inter-reef habitat.

Although not much is known about their exact role in the GBR ecosystem, their value regarding biodiversity, nursery refugia, and role in larval dispersal and patch reef connectivity may be comparable to that of seagrass meadows and mangrove habitats.

The Halimeda ghost pipefish, Solenostomus halimeda. Photo Vincent Chalias

Mardi McNeil from Queensland University of Technology and lead author on the new research paper, said the extent of Halimeda bioherms is vast.

“We’ve now mapped over 6000 square kilometers. That’s three times the previously estimated size, spanning from the Torres Strait to just north of Port Douglas. They clearly form a significant inter-reef habitat which covers an area greater than the adjacent coral reefs.” [James Cook University]

Nicole Helgason

Nicole Helgason is a professional scuba diver and can often be found photographing corals and sharing her passion about coral reefs around the world. Nicole is Reef Builders Event Manager and is responsible for running ReefStock shows in Denver and Sydney. Nicole is also a manager for Reef Builders social media accounts and a weekly contributor to the Reef Builders site. Nicole has a Bachelors degree in Coastal Geography from the University of Victoria, and is originally from Vancouver Canada.