Revealing Halimeda bioherms in the northern Great Barrier Reef: origin, function, and fate

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1. Please give a brief summary of your work.

The calcareous green alga Halimeda is a major contributor to inter-reef carbonate sediments and is found along the entire northern Great Barrier Reef (GBR). Previous studies of extensive Halimeda sediment deposits, or bioherms, show they represent important inter-reef habitats and potential carbon sinks, covering an area at least equal to the modern coral reef system in the north. Our study utilised new high-resolution seafloor bathymetry data to reveal a completely different picture of their spatial extent and surface morphology. These new findings suggest Halimeda bioherms are far more complex than previously thought – challenging existing paradigms describing their origin, development and significance.

The Halimeda bioherms in the northern Great Barrier Reef are remote, in deep water (compared with shallow coral reefs) and little is known about the ecological status and trends of this ecosystem, which is clearly very different to that of the more familiar shallow coral reefs.

2. Describe your approach and broader findings.

The build-up of Halimeda sediment has developed extensive ~20 m thick reef-like calcium carbonate deposits, or bioherms, in the northern GBR over millennial time scales. To re-assess their spatial extent and surface morphology, we undertook a high-resolution mapping exercise covering the northern GBR from Torres Straight to Port Douglas, over 7 degrees of latitude. This approach incorporated all previous legacy data, and integrated these with new multibeam echo-sounder and airborne Lidar seafloor bathymetry data sets collected by the Royal Australian Navy and provided by the Australian Hydrographic Service. After processing the raw data, a new 25 m pixel resolution Digital Elevation Model (DEM) was compiled of the GBR seafloor, then converted to an ESRI raster grid for further spatial analysis.

The resulting imagery revealed the true nature of this part of the GBR seafloor for the first time. The Halimeda bioherm morphology is far more complex than previously thought, challenging existing paradigms describing their origin, development, and significance. Rather than a series of sub-parallel linear ridges and troughs as proposed by early workers, we found that these bioherms are in fact characterised by complex reticulate ring-shaped and hollow-centred mounds, commonly over 250 m in diameter, and coalescing together to form bioherm complexes in the order of hundreds of square kilometres in size. To date the mapped spatial extent of the Halimeda bioherms covers an area over 6000 km2 on the outer continental shelf. To put this figure in context, that area is equivalent to over 7 times the size of New York City, or around 240,000 MCG’s! This area is three times larger than previous thought, and likely exceeds the volume of calcium carbonate stored in the adjacent modern shelf-edge barrier reefs. 

We identified three morphological sub-types which we classified as reticulate, annulate, and undulate, and found a general east-west cross shelf gradient in the variation of these morphologies. We are currently developing new hypotheses around the physical and hydrological processes which would influence these sediments to form circular ring shapes at this scale.

Previous biodiversity management and zoning of the inter-reef areas of the GBR have been based on a very basic understanding the Halimeda bioherm distribution and their true nature. Therefore, their distribution and significance has been vastly underestimated - they have been commonly misclassified as either coral reefs or other inter-reef habitats, resulting in conflicting management evaluations and implications for the bioherm benthic habitat. The data from this project will be of undeniable use to the management of the GBR Marine Park on both short- and long-term time scales.

These new estimates of bioherm spatial distribution and morphology have implications for understanding the role these geological features play as structurally complex and productive inter-reef habitats, and as calcium carbonate sinks which record a complete geochemical history of the Holocene post-glacial marine transgression in the northern GBR over at least the past 10,000 years.

3. What is the wider contribution, or impact, to your scientific field(s)?

This study significantly advances our understanding of the vast spatial extent of the northern GBR Halimeda bioherms, their morphology, and structural complexity, and provides a crucial new geomorphic context for future studies. Their vast size and volume perhaps elevate their importance to rival that of the northern GBR coral reefs as calcium carbonate sinks and structures of geological significance. This new bioherm map will facilitate managers in clarifying the northern GBR Halimeda bioherms bioregion classification and contribute to the appropriate management and conservation of these important benthic habitats.

Since publication in the international journal Coral Reefs, this research has captured much public attention in the mainstream and online media. The story has featured on over 70 individual news outlets including ABC news, SBS, Science Daily, The Daily Mail, The Telegraph, Fox News, Mashable, numerous radio stations in the United States, and even featuring on the popular science websites Science Alert and IFL Science. As a measure of popular attention and impact, this research has resulted in an Altmetric score of 646, placing it in the top 0.01% of all research outputs and the #1 output for the journal Coral Reefs since Altmetric scores have been tracked.  The paper currently has two citations in the published peer-reviewed literature.

4. Are there any potential ideas you would like to explore to take this research further?

The Halimeda bioherms on the GBR are some of the most extensive, actively accumulating Halimeda deposits in the world. These deposits extend back to the pre-Holocene, preserving evidence of global climate change, glaciations and sea level changes, often prior to coral colonisation as the earlier Holocene conditions were more conducive to Halimeda than coral. Halimeda bioherms are therefore particularly important for understanding the history of reef development in the GBR.

We are continuing to work on the Halimeda project, bringing together a multi-discipline team of colleagues with expertise in carbonate sedimentology, oceanography, marine ecology, geophysics, isotope geochemistry, and numerical modelling. Our goal is to undertake a large-scale multi-year assessment of the origin, formations, processes, and fate of the Halimeda bioherms in the context of a changing environment, specifically addressing the following objectives:

1.     Explore the relationship of bioherms to the undersea landscape (adjacent reefs, channels, passages and submarine canyons).

2.     Develop new models of their origin and development, including generating Holocene paleo-climate data, including novel archives of paleo-floods, water quality, and nutrient upwelling cycles.

3.     Calculate their total volume/area as a geological carbonate sink and contribution to the global marine carbon budget; and

4.     Assess the importance of the bioherms as modern benthic habitats.

This research will increase our fundamental understanding of the processes that control bioherm development, and have direct implications for environmental managers tasked with predicting how these poorly studied inter-reef environments might respond to future climate change.

We would like to acknowledge the Royal Australian Navy and Australian Hydrographic Service for provision of raw data, Geoscience Australia for providing access to archived cores and sediment material, and colleagues at James Cook University and University of Sydney Geocoastal Research Group.

5. Please share a link for researchers to access a relevant publication, data-set, or thesis.

McNeil, M. A., Webster, J. M., Beaman, R. J., & Graham, T. L. (2016). New constraints on the spatial distribution and morphology of the halimeda bioherms of the great barrier reef, australia. Coral Reefs, 35(4), 1343-1355. doi:10.1007/s00338-016-1492-2


richard dawson
almost 3 years ago

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