Posidonia meadows: Lungs of the seas at risk

Issued on: 24/08/2023 - 11:33

They produce twice as much oxygen in the Mediterranean as the Amazon rainforest. They are often confused with algae. But seagrasses, known as "posidonia" in the Mediterranean, are plants that are crucial to our planet. Yet they are under threat. Scientists believe the Mediterranean has lost 14 percent of its seagrass meadows over the past 100 years. Tourism, boats dragging their anchors and pollution have driven the rapid decline of this ancient, carbon-absorbing plant.

It is a glistening summer day and a swarm of boats are berthing between the Lerins Islands, off the coast of Cannes. For Samuel Jeglot, a marine biologist, it is the perfect example of mass tourism taken up a notch too far. A “pizza boat” stops by to give out menus – the kitchen is stationed on a catamaran a few meters away. A “cocktail boat” whips up drinks on board.

There can be hundreds of vessels at a time, says Jeglot, who co-founded NaturDive, an ocean conservation non-profit. The problem is that right underneath there is a dense posidonia meadow, an endemic species of seagrass.

“Boats drop their anchors here,” explains Jeglot. “And everytime they’re pulled back up, seagrasses get crushed, torn and destroyed.”

It is often confused with seaweed, but seagrass is in fact a plant. It is one of the most widespread ecosystems on Earth, found in 159 countries. The Mediterranean, however, is home to the oldest species of its kind, Posidonia Oceanica or Neptune Grass. Seagrass meadows have long been overshadowed by other, more colorful forms of marine life. But their role is just as vital, if not more.

They have been dubbed “the lungs of the Mediterranean”, as their long leaves are known to produce large amounts of oxygen, while at the same time absorbing carbon dioxide from the atmosphere. A hectare of posidonia can store 15 times more carbon than a hectare of Amazon rainforest.

Their role as carbon sinks is also under threat, warns Heike Molenaar, who has studied posidonia for more than 30 years.

“An anchor that drops into a posidonia meadow will crush the roots and rip out the plant, eventually releasing all this carbon into the environment, as the plant will decompose,” she says.

Posidonia also acts as a bulwark against erosion. During winter, posidonia will shed some of its leaves, some of which can wash up on the beach, forming thick, brown layers of posidonia “beds”. They are an entire ecosystem of their own, providing shelter to crustaceans and insects. When storms hit, they are the perfect natural seawall.

At the height of summer, you can still see confetti-like strips of posidonia scattered along the beach. Not everyone is happy about their presence.

“People still see posidonia as waste,” says Clelia Moussay, a municipal officer from Le Lavandou, a seaside town prized by tourists for its turquoise waters and sandy beaches.

Unlike other cities, which have traditionally removed the posidonia beds ahead of summer, Le Lavandou has made the bold choice of leaving them untouched. But it is not always easy to stay on track, says Moussay. The municipality has received several complaints from beachgoers, requesting the leaves be removed.

Officially, posidonia has a protected status in France and much of the Mediterranean. Little is done to actually enforce it. But initiatives are slowly emerging to reverse the rapid decline of meadows.

In 2020, Cannes imposed restrictions on vessels over 24 meters long, which can no longer drop their anchors on posidonia meadows. This automatically bans yachts, those with the biggest anchors, from stationing over the protected habitat. So far, it has proven successful despite a rocky start, says Samuel Jeglot.

“Now, we can deal with the smaller boats,” he adds. “Individually they cause less harm, put together their impact is huge.”

Just east of Marseille, a rescue mission is underway. GIS Posidonie, an association of scientists, have launched REPOSEED, an experimental project testing out different restoration methods.

Two sites were selected, off the coast of Marseille and Corsica, where more than 9,000 posidonia seeds have been planted. Think of it as reforestation, but underwater.

The seeds have sprouted and leaves have grown, according to Bruno Belloni, a marine biologist overseeing the project. He insists it is “a little extra help” but plantation efforts cannot be the ultimate solution.

“Nature-based solutions are probably the real solution, instead of direct intervention,” he concludes.

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