Australian scientists have finally mapped an astonishing geological feature located right next to the northern Great Barrier Reef. The surprising reef-like structures are known as bioherms - huge fields of circular mounds formed by dead, calcified algae of the Halimeda genus.
These common green algae, which look a lot like a Christmas cactus, contain large amounts of calcium. When they die, what’s left behind are piles of limestone flakes, which form into large bioherms over thousands of years.
Even though these reef-like structures have not entirely escaped scientists’ attention before, this is the first time they have acquired extensive survey data to study them. The results were published last week in the journal Coral Reefs.
“Scientists have known about the existence of Halimeda bioherms on the Great Barrier Reef since the 1970’s and 80’s,” says lead author of the study Mardi McNeil from Queensland University of Technology (QUT). “It is only with modern survey techniques such as multibeam echosounder and aircraft mounted LiDAR that we have been able to see what the bioherms really look like.”
These high-tech survey data were collected by the Royal Australian Navy and provided by the Australian Hydrographic Service as part of a much larger geoscience project intended to map the GBR seafloor in the highest resolution yet. But it has also lead to unexpected high-level detail on Halimeda bioherms.
“We realised immediately that previous scientific interpretations of the bioherms needed to be re-assessed in light of this new information,” adds McNeil.
The team of researchers from James Cook University (JCU), University of Sydney, and QUT were astonished by the sheer extent of these structures, much greater than previously thought.
“As the data resolution was increased, the more we were amazed,” says McNeil. “It was very exciting to see their true extent unfold.”
The unusual limestone mounds are shaped like donuts, averaging 200-300 metres across, with central indents of up to 10 metres. According to the new map data, the total area these Halimeda bioherms cover is 6167 km2, which is slightly more than the entire northern Great Barrier Reef.
“That’s three times the previously estimated size, spanning from the Torres Strait to just north of Port Douglas,” says McNeil. “They clearly form a significant inter-reef habitat which covers an area greater than the adjacent coral reefs.”
The finding is a significant addition to research surrounding the Great Barrier Reef, which has been increasingly succumbing to coral bleaching due to warmer oceanic conditions. While these reef-like bioherm structures are a different kind of ecosystem from the coral reefs as we know them, the discovery opens a slew of questions that researchers hope to address, according to study co-author Dr Robin Beaman from JCU.
“For instance, what do the 10-20 metre thick sediments of the bioherms tell us about past climate and environmental change on the Great Barrier Reef over this 10,000 year time-scale? And, what is the finer-scale pattern of modern marine life found within and around the bioherms now that we understand their true shape?”
“We need to completely re-assess the significance of this vast inter-reef habitat in terms of the sheer volume of calcium carbonate, their role in carbon storage,” adds McNeil.