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Shark ‘eyes’ reveal biggest blue CO2 trap

Tiger shark swimming over manatee grass on Little Bahama Bank (Austin Gallagher)
Tiger shark swimming over manatee grass on Little Bahama Bank (Austin Gallagher)
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Fifteen Bahamas tiger sharks kitted out with camera and satellite tags have played a key role in the discovery of what is now claimed to be the world’s largest seagrass ecosystem.

Marine biologist Dr Austin Gallagher of US-based “blue carbon” charity Beneath The Waves had hardly expected such a dramatic outcome when his team fitted cameras and satellite tags to the sharks’ dorsal fins – including what is claimed to have been the first use of 360° bio-logging cameras on a marine animal.

The carbon-storing seagrass beds now revealed on the Bahama Banks are estimated to cover between 66,000 and 92,000sq km. About the size of Portugal or Hungary at the upper estimate, this would make the area one of Earth’s most significant climate assets, boosting previous estimates of seagrass coverage by up to 41% and representing a massive 19-26% of the blue carbon buried in seagrass around the world. 

“What this discovery shows us is that ocean exploration and research are essential for a healthy future,” says Dr Gallagher, who worked on the study with an international team of scientists and is lead author of their report, just published in Nature Communications. “The untapped potential of the ocean is limitless.” 

“We are proud and humbled to share this finding with the world and provide a glimmer of ocean optimism in the fight against climate change before the UN COP27 next week,” stated Beneath the Waves.

Sharks are attracted to seagrass meadows because they can find prey including turtles and dugongs there. Dr Gallagher described the use of shark-mounted cameras to map the seabed as a relatively new concept: “I think it is honestly the only way to properly survey the seafloor throughout expansive and remote shallow ocean regions,” he says.

Sharks prey on turtles in seagrass (Cristina Mittermeier / SeaLegacy)
Turtle in seagrass (Cristina Mittermeier / SeaLegacy)

It was not the sharks alone that established the extent of the Bahama Banks meadows, however – their data was integrated with that from 2,542 scuba diver surveys. 

“This discovery should give us hope for the future of our oceans,” said Dr Gallagher. “It demonstrates how everything is connected. The sharks led us to the seagrass ecosystem in the Bahamas, which we now know is likely the most significant blue carbon sink on the planet.”

Capture and store

Besides their proven ability to capture and store carbon, seagrass meadows provide nursery facilities for fish and can prevent or reduce coastal erosion.

Until the Bahamas discovery only around 160,000sq km of seagrass areas had been verified around the world, although it has been estimated that the figure could extend to 1.6 million sq km. The world’s seagrass meadows have been reckoned to absorb 10% of the ocean’s carbon annually.

The biggest-known such ecosystem previously measured – by a combination of scuba divers and towed cameras in 2009 – lies between the Australian mainland and the Great Barrier Reef and covers some 40,000sq km. 

The Bahama Banks, with and without seagrass, covers up to 135,000sq km. The densest meadows there consist of the largest seagrass species turtle grass (Thalassia testudinum), along with shoal grass (Halodule wrightii) and manatee grass (Syringodium filiforme). Sparser meadows tend to combine Caribbean seagrass (Halophila decipiens) and shoal grass.

The depths routinely reached by tiger sharks were significant for the study because the very clear waters on the Bahama Banks allow seagrass to grow there well beyond non-technical scuba-diving range, as deep as 90m.

“There is a clear need to do the mapping and science to swiftly document these areas, and then protect them, given the myriad benefits they provide to humans and our own survival,” commented Dr Gallagher.

Also on Divernet: Tiger Tiger, Seagrass Lifts ‘Blue Carbon’ Hopes In Cornwall, Blue Meadows Seagrass Project Underway, Sunscreen Chemicals Build Up In Med Seagrass

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