Tiger sharks help researchers to discover world's biggest seagrass meadow beneath the Bahamas banks

Researchers reported a 41 percent increase — an area up to 35,521 square miles — from the projected estimate of seagrass habitat across the Bahamas banks.
Deena Theresa
Tiger sharks are often found around seagrass meadows.
Tiger sharks are often found around seagrass meadows.

Cristina Mittermeier and SeaLegacy, 2021 

Seagrass ecosystems are fascinating; they form lush, underwater meadows host to an incredibly diverse range of sea animals like fish, turtles, crabs, and other marine mammals, they're one of the most productive ecosystems in the world, and they're extremely efficient global sinks of carbon dioxide on Earth.

They're also little explored, underrepresented, and therefore harbor secrets of the ocean - to illustrate, earlier this year, scientists found colossal reserves of sugar in seagrass ecosystems. Who would have ever guessed?

In another episode of revealing ocean mysteries, a new study revealed that tiger sharks helped researchers locate the biggest seagrass meadow in the world, located in the waters of the Bahamas.

The most significant blue carbon sink on the planet

Previous research had confirmed that at least 869 square miles (2,250 square kilometers) of seagrass stretched across the Bahamas Banks, but it was always known that there was more out there.

Thanks to the tiger sharks, the researchers reported a 41 percent increase from the projected estimate - an area up to 35,521 square miles (92,000 square kilometers) of seagrass habitat across the Bahamas banks.

Tiger sharks help researchers to discover world's biggest seagrass meadow beneath the Bahamas banks
Seagrass meadows are feeding and nursery grounds for an incredibly diverse community of marine life.

The researchers from Carlton University in Canada, King Abdullah University of Science and Technology in Saudi Arabia, Trinity College in Ireland, and Beneath the Waves, a Virginia-based conservation nonprofit, published their results in the journal Nature Communications.

"This discovery should give us hope for the future of our oceans. It demonstrates how everything is connected," said study lead author Austin Gallagher, chief executive officer of global blue carbon not-for-profit Beneath the Waves.

"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."

Tiger sharks help researchers to discover world's biggest seagrass meadow beneath the Bahamas banks
Seagrasses store carbon in their dense root systems, called rhizomes.

When sharks helped researchers map the ocean floor

The researchers attached bio-logging cameras on the dorsal fins of the sharks, and then used satellite tags to track the individual tiger sharks as they swam in the Bahamas. Though cameras have been used on animals before, this study showed the first-ever use of 360-degree cameras on a marine animal, to map seafloors, the authors told Popular Science.

"In confirming the various benthic habitat types in The Bahamas, and their corresponding use by sharks, we found that tiger sharks spent around 70 percent of their time swimming over seagrass meadows," Wells Howe, a program manager on Beneath the Waves‘ Blue Carbon project, said in an interview with PopSci.

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The sharks proved to be of immense help; they allowed researchers to peer into areas inaccessible by humans. They swam more than 2,465 miles in both regions of the Bahamas banks.

"The tiger sharks extend well below the depth limit of seagrass and further into the interior of the vast Great Bahama Bank, areas not logistically possible for human access," Howe, who is also a co-author of the new study, continued. "When reviewing the camera footage, the biggest notable behavior was just how much of their time is spent patrolling seagrass meadows."

Tiger sharks help researchers to discover world's biggest seagrass meadow beneath the Bahamas banks
The scientists attached bio-logger tags equipped with cameras on tiger sharks.

Seagrass conservation must be taken seriously

Over the previous decades, seagrass cover has been reducing rapidly. This, in turn, has lowered the sequestering capacity of their ecosystems, releasing massive amounts of carbon into the atmosphere. Prior studies suggest the removal capacity of seagrass can be 30 times faster than the rainforest.

Therefore, it is of paramount importance to ensure the conservation of these seagrass systems. "When referred to global seagrass carbon stock estimates, our results indicate that seagrass in The Bahamas may contain 19.2 to 26.3 percent of all the carbon sequestered in seagrass meadows on Earth," Howe told PopSci.

In an interview with Forbes, Gallagher said: "What this discovery shows us is that ocean exploration and research are essential for a healthy future. The untapped potential of the ocean is limitless. Further, this discovery underscores the role that science can play in developing resilient communities."

Study Abstract:

Seagrass conservation is critical for mitigating climate change due to the large stocks of carbon they sequester in the seafloor. However, effective conservation and its potential to provide nature-based solutions to climate change is hindered by major uncertainties regarding seagrass extent and distribution. Here, we describe the characterization of the world’s largest seagrass ecosystem, located in The Bahamas. We integrate existing spatial estimates with an updated empirical remote sensing product and perform extensive ground-truthing of seafloor with 2,542 diver surveys across remote sensing tiles. We also leverage seafloor assessments and movement data obtained from instrument-equipped tiger sharks, which have strong fidelity to seagrass ecosystems, to augment and further validate predictions. We report a consensus area of at least 66,000 km2 and up to 92,000 km2 of seagrass habitat across The Bahamas Banks. Sediment core analysis of stored organic carbon further confirmed the global relevance of the blue carbon stock in this ecosystem. Data from tiger sharks proved important in supporting mapping and ground-truthing remote sensing estimates. This work provides evidence of major knowledge gaps in the ocean ecosystem, the benefits in partnering with marine animals to address these gaps, and underscores support for rapid protection of oceanic carbon sinks.

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