Marine biologists team up with tiger sharks to help discover the world’s largest seagrass ecosystem

Posted on: 01 November 2022

Dr Carlos Duarte (King Abdullah University of Science and Technology, Saudi Arabia), was one of the study authors. He said:

“We thought that The Bahamas likely had an extensive seagrass ecosystem, but the true spatial estimate had never been properly quantified, because surveying this vast area remains challenging. Seagrass meadows cannot be readily mapped from space, as they yield signals similar to those of seaweed and are often covered by carbonate deposition, which returned a glare – backscatter – similar to that of bare sand.”

In order to map the total area covered by seagrass in The Bahamas, the global team led by US NGO Beneath The Waves performed their own remote sensing analysis, while integrating previous satellite imagery estimates to generate a composite estimate of the Bahamas seagrass ecosystem. These records were paired with extensive ground-truthing validation, involving over 2,400 diver seagrass surveys throughout the area.

Part of the seagrass measow ecosystem in The Bahamas. Image: Beneath The Waves.

Working with tiger sharks allowed the researchers to map even more seafloor, with the team attaching video cameras to the sharks as they swam throughout the Bahamas Banks.

“Research led by Beneath The Waves had shown that tiger sharks spend about 72% of their time patrolling seagrass beds, which can be observed by the 360^o cameras we deployed, for the first time in marine animals, on the sharks,” Dr Duarte adds.

“This provided an opportunity to expand ground-truthing across the vast, challenging depths of the Bahamas Banks, as tiger sharks cover about 70 km in one day, can work with us 24/7 – with no complaint yet on record – and are not constrained – as human divers are – to shallow depths.”  The additional validation provided by tiger sharks helped raise the estimates of the area seagrass habitat across The Bahama Banks from a minimum of 66,000 km², up to 92,000 km².

This first-time partnership between tiger sharks and scientists led to this major discovery and provides a model for collaborating with large marine animals to explore the ocean. The research team also collected sediment cores from the extensive seagrass ecosystem, to evaluate how much carbon is stored in the sediment, ultimately revealing that The Bahamas likely holds up to 25% of the global stock of seagrass-based blue carbon. 

Seagrass ecosystems play an increasingly recognised role in supporting ocean health. Seagrasses promote biological productivity, ocean biodiversity, fishery resources, and carbon sequestration, while protecting shorelines from storms. Seagrasses trap and permanently store massive amounts of organic carbon – termed blue carbon – in their sediment, and this ecosystem service renders seagrass a nature-based solution to mitigating the effects of climate change. Tropical seagrass meadows are kept healthy by tiger sharks, which, as the top predators in these ecosystems, prevent overgrazing by sea turtles, dugongs, manatees and other herbivores.

“This discovery should give us hope for the future of our oceans,” says Dr Austin Gallagher from Beneath The Waves and lead author of the research article.

“It also demonstrates how everything is connected. Because tiger sharks had been protected in The Bahamas for many years, we were able to study and monitor the ancient processes these animals had been engaged in for millennia. 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. If protected, these seagrasses can play a crucial role in slowing the climate emergency, as the world moves to deploy a diverse range of strategies to capture carbon from the atmosphere.”

A tiger shark patrols a seagrass meadow in The Bahamas. Image: Beneath The Waves.

The Prime Minister of The Bahamas, Philip Brave Davis, has emerged as a key leader for the region’s climate change advocacy. He said:

“The innovative work done by Beneath The Waves, in partnership with Bahamians, to map our country’s vast seagrass meadows, represents extraordinary scientific progress, and will make a very significant contribution to our national development and security. The Bahamas may be in the top of rankings on climate vulnerability, but we now have evidence that we top the list of the world’s blue carbon hotspots, too – which means our seagrasses can play a critical role in generating the resources we need to transition to renewable energy and to adapt and become more resilient to the changing climate.”

Nicholas Payne, Assistant Professor in Trinity’s School of Natural Sciences, and Lucy Harding, PhD Candidate under his supervision, were part of the research team and helped deploy the video cameras on the tiger sharks.

Dr Payne said:

“This is an exciting and important discovery for a range of reasons. Here in Ireland, we have a huge coastal area that likely supports significant seagrass ecosystems. However, as was the Bahamas, we don’t have a great understanding of their distribution and extent. It’s important that we now put more research effort into mapping these crucial habitats, especially so we can design the best ways to conserve them.”

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