From the Field: Blue Carbon on the Great Barrier Reef

By Emily Darling

Fifty feet above the Great Barrier Reef, a plastic toy kite in the shape of a stingray whips in the wind attached with a tiny camera shooting photographs of the reef below. From the research boat, Dr. Elizabeth Madin waves her arms in big gestures showing the direction of the patch reef, only barely visible from the water above the wind and waves. Fighting to pull the kite towards the reef, Brian Sullivan from Google Earth Outreach swims through the choppy water to position the camera in the right place.

Flying a camera above the reef allows Dr. Madin, a researcher with World Wildlife Fund and Macquarie University in Australia, and her team (scientists, Google imagers, and drone specialists) to search for ‘grazing halos’. Grazing halos are areas of coral patch reefs where herbivorous fishes act like lawn mowers to graze and eat algae surrounding the reefs. On healthy reefs with predators like sharks or groupers, the lawnmower fishes only forage small distances away from the safety of the reef, leaving distinct rings or ‘halos’ of algae around the patch reef.

“Fish show highly-developed responses to danger – in other words, they do their best to avoid it while balancing their need to find food, such as seagrass and algae. The result of so many small fish grazing close to their coral reef shelter are these quite spectacular patterns around reefs, known as grazing halos that can be seen all the way from space.” – Dr. Elizabeth Madin
Images from Google Earth show a patch reef (yellow box) with a halo of green algae surrounding the reef, allowing for remote observations of predators and prey on coral reefs. At Heron Island, Dr. Elizabeth Madin and Dr. Trisha Atwood research grazing halos to investigate how healthy coral reefs store blue carbon.

Images from Google Earth show a patch reef (yellow box) with a halo of green algae surrounding the reef, allowing for remote observations of predators and prey on coral reefs. At Heron Island, Dr. Elizabeth Madin and Dr. Trisha Atwood research grazing halos to investigate how healthy coral reefs store blue carbon.

Dr. Madin’s research has lent support for the idea that satellite images from Google Earth may be a quick, inexpensive tool to monitor coral reefs from space as the size of grazing halos can indicate the health of reef predators and prey. At Heron Island, the team is trying out a new technique to obtain real-time aerial photographs, a surprisingly challenging enterprise in the ocean where only a few drops of saltwater can destroy even the most sophisticated electronics. In this hostile environment for electronics, a plastic toy kite might be just what the scientists need.

Grazing halos might also be able to tell us how healthy coral reefs help the ocean store more carbon. Dr. Trisha Atwood, a postdoctoral fellow at the Global Change Institute at the University of Queensland, is collecting shallow sediment samples across Heron Island’s grazing halos to see how predators influence carbon storage in the sand flats surrounding coral reefs. She hopes that this research will tell her how global declines in marine predator populations may be altering carbon storage in coastal ecosystems.

Blue carbon is a promising new approach to mitigating global climate change by sequestering carbon emissions in coastal ecosystems like mangroves, salt marshes and seagrass beds. In the past decades, coastal habitats have experienced shocking declines from development, water pollution and habitat conversion to aquaculture (i.e. shrimp farms), destroying the ability of these ecosystems to sequester carbon. By restoring coastal habitats, we also restore their ability to sink carbon as a cost-effective strategy for emerging carbon markets. Shallow, coastal carbon-sinking habitats can sequester carbon 50 to 100 times faster than trees in forests, suggesting that restoring and protecting intact coastal habitats is a key action for climate mitigation and adaptation.

“Because of their remarkable capacity to sequester carbon as well as their many other ecosystem services, coastal ecosystems should be prominent in our strategies to combat climate change” –Trisha Atwood

Coral reefs and their algal halos play a potentially large role in blue carbon storage, which Dr. Atwood is here to investigate. By understanding the links between predators, grazing halos and blue carbon storage, the team at Heron Island can provide the first evidence of how coral reef food webs are linked to blue carbon.

Understanding the source-sink dynamics of blue carbon on coral reefs is critical to funding climate mitigation efforts of coastal ecosystems. Countries that effectively manage their coastal carbon stocks can also apply for international funding for their efforts through initiatives like REDD+ (Reducing Emissions from Deforestation in Developing countries), actions that not only reduce the amount of atmospheric carbon emitted but can also sink carbon back into trees, mangroves, seagrasses, salt marshes and algae on reefs. Demonstrating how blue carbon works on coral reefs on the expedition to Heron Island can provide the science behind new approaches to carbon management and carbon marketing.

Back on the boat, Dr. Madin winds up the kite by hand and pulls out her tablet with satellite images of the next reef halo. Dr. Atwood revs up the outboard engine and the team pulls off to another site in the lagoon. With a crew of ecologists, biogeochemists and technology collaborations with Google and ConservationDrones.org, it’s only a matter of time before the team works out a blue solution for climate adaptation on the world’s coral reefs.


Learn more about grazing halos at Dr. Elizabeth Madin's website and the effects of predators on blue carbon storage at Dr. Trisha Atwood’s website. The research was done in collaboration by Google Earth Outreach and ConservationDrones.org and funded by Global Change Institute at the University of Queensland, World Wildlife Fund, and the Australian Research Council.