How do you find the world's biggest seagrass meadow?
First start with satellite images — a method known as remote sensing. With those you can pick out the darker patches of seagrass from the blue water.
But it's difficult to distinguish things like phytoplankton and algae from seagrass in those images, and some of the sparser, deeper, or harder-to-spot meadows could be missed.
So your estimate made using satellite images would need to be checked on the ground, or "ground truthed".
To do that, send down divers and tow snorkellers behind research vessels to take photos of the seafloor, which you can then analyse for the presence of seagrass.
But people are slow, have to come up for air, and can't reach the depths of up to 90 metres that seagrasses have been found in.
So the next thing to do is catch a bunch of tiger sharks. About seven should do it.
Then attach cameras to their dorsal fins on swivels designed to corrode in saltwater after about 24 hours.
Attach radio and satellite beacons to the cameras, and you're away.
At least that's how a team of researchers who've published their results in Nature Communications decided to do it.
Using these methods, they say they've found what could be the world's biggest seagrass meadow in the Bahamas, covering a "conservative estimate" of 66,000 square kilometres.
Previous research had confirmed at least 2,250 sq km of seagrass stretched across the Bahamas Banks, though it was known that there was significantly more there.
"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."
Blue carbon is carbon stored in coastal and marine ecosystems.
Turtles also lead to seagrass discovery
So why did they use tiger sharks? Because they spend a lot of time around seagrass meadows where they feed on things like turtles and dugongs.
Studying tiger sharks in the Bahamas for over a decade led Dr Gallagher to come up with the unique research approach.
"Putting cameras on animals for science is not necessarily a new thing, but using this approach to map seafloors is a relatively new concept," Dr Gallagher said.
"It is something we are scaling to other areas. I think it is honestly the only way to properly survey the seafloor throughout expansive and remote shallow ocean regions."
Though using sharks to assess seagrass is a novel approach, it's not the first time animals have helped scientists discover meadows, according to Associate Professor Michael Rasheed, head of the Seagrass Ecology Lab at James Cook University.
"There are some really neat stories of [satellite] tagged green turtles turning up in places where people think, 'why would they be out there?'" Dr Rasheed said.
"And when people have gone and had a look they've found these magnificent seagrass meadows in the middle of the Indian Ocean."
Dr Rasheed said he wasn't sure whether the Bahamas Banks meadow qualified as the world's biggest, because it would depend on your parameters, but he said it was certainly a globally significant area of seagrass.
"We have, just in the Torres Strait alone, close to 13,000 sq km, and you can add that to the northern Queensland and the Great Barrier Reef seagrass, which tallies about 35,000 sq km, because they all join up.
"But [the Bahamas] is certainly a large seagrass system."
Seagrasses make great carbon traps
So why do we need to know how much seagrass we have?
Firstly, carbon storage.
Carbon stored in coastal and marine ecosystems is what is referred to as "blue carbon", and seagrasses are responsible for a good proportion of all the carbon buried in marine sediments each year.
Unlike terrestrial plants which release their stored carbon when they die and decompose, seagrasses can store it away long-term, Dr Rasheed said.
"They are really good at taking carbon and locking it away. They're really efficient at that because most of the carbon gets stored in the sediment rather than in the living tissue of the plant itself.
"A lot of the carbon ends up down in the soil where it can be trapped for millennia if it's not disturbed."
But seagrasses are being disturbed, largely because of direct destruction by people, and through extreme weather and climate events.
When that happens, the stored carbon can be released to the atmosphere as carbon dioxide, which in turn adds to warming.
"We are losing up to 7 per cent of our seagrasses globally, per year," Dr Gallagher said.
Extent of seagrass meadows a mystery
Seagrasses also provide food for turtles and dugongs, and habitats for many fish species, Dr Rasheed said.
"They're a huge fish-nursery ground for juvenile commercial and recreational species.
"Seagrasses [also] help prevent coastal erosion and in places where you've got good seagrasses growing, they're filters if you like, removing sediment and nutrient flowing from the land out to the Great Barrier Reef."
But we're also not really sure of the full extent of the world's seagrasses.
Estimates around the globe range from a verified expanse of just over 160,000 sq km (now over 215,000 thanks to this study), to anywhere up to 1.6 million sq km, based on remote sensing estimates.
If we don't know what we've got, it's very hard to work out how best to protect it.
According to today's research, the size of the Bahamas meadow means it's estimated to hold between 19-26 per cent of the blue carbon buried in seagrass globally.
Given the climate and biodiversity crises, there needs to be more protection for these habitats, Dr Gallagher says.
"The documented extent of seagrass meadows remains poorly understood.
"So 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."
