Few holidaymakers are aware that the lush underwater meadows they swim above are formed not by algae, but by posidonia oceanica, an endemic plant unique to the Mediterranean and essential to the survival of many of its species.
In its meadows life thrives: a nursery for animals such as octopusses, sea stars, seahorses, shrimps, crabs, sea urchins, yet still at the mercy of careless visitors who might not know that turtles rest on posidonia and dolphins feed on animals that live there.
Sunlight
Covering nearly two million hectares across the sea (an area as large as Slovenia), these submerged green valleys host 20 per cent of the Mediterranean’s biodiversity.
They filter and clarify seawater as they continuously produce oxygen through photosynthesis, releasing it directly into the water and supporting marine life. They also shield coasts from erosion and, crucially, absorb carbon dioxide on a scale comparable to terrestrial forests.
On an annual basis, Amazon forests sequester more carbon than posidonia per hectare, but over centuries, seagrass sediment stores much more carbon than forests, making it an extremely effective long-term carbon sink.
Some meadows are thought to be among the oldest living organisms on Earth. One found between Ibiza and Formentera has been dated to around 100,000 years old, and some scientists consider it one of the oldest known living organisms.
Posidonia forms vast prairies that spread through clonal growth across the Mediterranean coast and up to 45 meters deep. It needs sunlight to live. That’s why it grows close to shores, where waters are more shallow.
Absorb
Yet, the very industry that sustains the economies of Mediterranean islands and shores — tourism— poses a grave threat to posidonia’s survival. This plant, like coral, is a major casualty of uncontrolled vessel anchoring.
In 2024, the Balearics welcomed 18.7 million visitors who spent €22.38 billion, breaking all previous records. Sardinia jumped from 3.4 to 4.5 million tourists in a single year, while Corsica reached 3.1 million in 2024. With airports overflowing, cruise ships docking daily and marinas filled to capacity, economic growth is colliding with ecological fragility.
Scientists agree on the plant’s exceptional role. An international study from 2014 claimed that posidonia potentially stores “11 per cent to 42 per cent of historic CO₂ emissions from Mediterranean countries since around 1850.
The plant has a great capacity to absorb carbon dioxide and store it in the matte, the dense layers of roots and sediment beneath posidonia meadows.
In fact, a new study co-authored by Australian, Spanish and Argentinian institutions and universities states that all marine seagrasses across the world- which occupy only 0.2 per cent of ocean space, absorb 10 per cent of the carbon dioxide that all oceans absorb annually and confirm the idea that posidonia oceanic is the seagrass with the greatest capacity to store carbon long-term.
Enforcement
The study is key to the development of carbon credits based on these deposits, an idea that projects such as SeaForest Life in Italy and Promethee-Med in France have already put on the table. It also confirms that seagrasses are a clean and cheap ally against climate change.
However, posidonia grows just one square centimetre a year, taking up to six centuries to form a single hectare. Anchors that rip up the seabed can undo millennia of carbon storage in minutes.
The 55,000 hectares of posidonia meadows around the Balearics absorb an estimated seven per cent of the archipelago’s annual emissions. Yet nautical tourism, which surged after the pandemic, has become the plant’s greatest threat, according to all the scientists interviewed for this story.
Posidonia thrives where waters are at their clearest — producing the very turquoise tones that draw millions of visitors. But many of the small rental boats crisscrossing these seas each summer are captained by tourists with no licences, no training and no awareness of what lies beneath their anchors.
Spain, Italy and France allow anyone to rent a vessel up to a certain engine power without certification. The result: anchors are dropped indiscriminately onto posidonia meadows. The Balearic Islands banned the practice in 2018, but enforcement remains patchy.
Protecting
In Italy, the damage continues largely unchecked, while France passed a law in 2020 prohibiting yachts longer than 24 meters from mooring near the coast to protect seagrass. These vessels must now anchor at least 300 meters offshore, a measure that has sparked protests from yacht associations.
Today, the anchoring of recreational boats seems to be the main threat to the plant across the Mediterranean as a whole. Interestingly, when the European Union issued Directive 92/43/EEC in 1992, granting posidonia the status of a Protected Habitat, the greatest dangers were trawling and pollution discharges.
In 2006, the European Union adopted a regulation banning fishing methods likely to damage marine habitats. The law specifically prohibited bottom trawling, pelagic trawling, purse seining, and dredging within all marine protected areas (MPAs) that host vulnerable habitats such as seagrass meadows and coral reefs.
This legislation has had a positive effect on Posidonia oceanica; however, several studies show that trawling continues to occur, even illegally—particularly in France, but also in Italy, Spain, and other European countries.
Greece is the only nation to have enacted a national law that will completely prohibit this practice within its MPAs and national parks by 2030—an important step towards achieving the global “30×30” target of protecting at least 30 per cent of the world’s land, freshwater, and ocean areas by 2030, as established under the Kunming-Montreal Global Biodiversity Framework.
Relentless
Nevertheless, over the past decade, tourism has expanded to an unprecedented scale, exerting significant pressure on Posidonia meadows, which has never received the endorsement of a national law against anchoring as clear as the Greek law against trawling.
Across Spain’s Balearic Islands, legislation is clear, but the reality is murky. At a local level, Decree 25/2018 made the Balearics the first Mediterranean region to ban anchoring on seagrass outright.
It also prohibited trawling, dumping, the construction of submarine cable projects and port expansions in areas where seagrass is present. Offenders face fines of up to €450,000. However, laws don’t necessarily translate into enforcement.
Dressed in a dark green T-shirt — the color of the plant he is devoted to protecting — Marcial Bardolet, head of the governmental agency Posidonia Surveillance Service in the Balearic Islands, leans over the edge of a boat on a sweltering June day.
He lowers a bathyscope, an orange plastic tube that lets him check whether a luxury yacht anchored in Cala Major, Mallorca, is touching the seagrass. “It’s ok, but very close to it,” said Bardolet to the patrol boat’s driver, who has spent hours under the relentless summer sun, shielding himself with a white hat as he moves from one vessel to the next.
Safe
Patrol boats operate from May to October, yet most carry only a driver who can request that yachts move, but does not issue fines. Only occasionally do they travel with environmental agents, such as Bardolet.
Even though the law was passed in 2018, it only gained sanctioning power in 2021. In 2024, patrols asked 6,764 vessels to relocate; just 43 were fined either by environmental agents, Guardia Civil or Seprona (a state-based environmental protection unit).
“The first years were chaotic because we had to dive to check the seabed, since it wasn’t visible, and only then could we fine boats anchored on posidonia,” said Bardolet. “With revenue from the sustainable tourism tax — an ecotax on overnight stays in the Balearic Islands — we created detailed maps and a mobile app for boaters.”
In addition, a call center now allows people to request information or report boats anchoring illegally. Apps such as Donia and Posidonia Maps help boaters find safe anchoring spots, but they remain little known among tourists.
The apps are popular in Spain and France but not in Italy, where an official mapping of the plant meadow – the most ambitious attempt worldwide to map posidonia ever – and a future app is still underway.
Awareness-raising
In 2025, for the first time, Posidonia Surveillance Service employed detection technologies that show real-time geolocation in the Balearics. It used the mandatory automatic identification system for ships over 24 meters in length. But for it to really be effective, more funds will be needed to control luxury yachts.
However, the toughest challenge is controlling small rental boats. “There are 3,600 registered recreational boats, but more than twice as many illegal ones,” said Pedro Francisco Gil, president of the Balearic Charter Companies Association.
Despite increased monitoring with drones and other tools, enforcement is still lacking. “There are inspections, but there is a lack of police presence,” he points out.
Gil said that rental companies that do their job properly tell clients about restrictions, but he acknowledges that many still break the law, mainly out of ignorance.
In Italy, some share this view. Sara Pincioni, owner of the San Rafael Marina in Sardinia, which can accommodate about 30 yachts, said: “There is a lack of information and awareness-raising from the authorities. We do our part in our small corner, but it is not enough.”
Underwater
In Corsica, anchoring restrictions for yachts over 24 metres long were introduced in 2020.Those violating the rules are supposed to face fines of up to €100,000, banishment of the vessel from French waters and even jail time. To keep enjoying secluded coves or privileged beach access, wealthy tourists try to find loopholes, such as building vessels just under the 24-metre threshold.
“We can’t do anything [about that],” admits Julien Courtel, a police officer with the Ministry of Ecological Transition, as he inspected a yacht one afternoon last July. After verifying the registration papers, the vessel—compliant because it is shorter than 24 meters by just a few centimeters, but featuring multiple cabins, a gym, kitchens, and a large deck—remains close to shore, to the delight of its clients.
“In principle, anchoring over seagrass meadows is prohibited for all vessels, regardless of their size. But to be able to impose a fine, you need to get a photo of the anchor being lifted with uprooted plants,” explained Michel Mallaroni, director of the Port of Bonifacio.
Over the past fifteen years, the number of superyachts in the world has nearly doubled, reaching 6,000 — half of which spend most of the year in the Mediterranean.
When they anchor over underwater meadows, each one can destroy, in a single night, an area equivalent to a football field, according to Quentin Fontaine, an oceanographer at the Calvi Marine Research Station.
Restricted
Between 2010 and 2018, the number of anchorings by vessels between 24 and 60 meters in French Mediterranean waters increased by 449 per cent. However, according to a study published this year, since the French law was approved, large-yacht illegal anchorings on posidonia in Corsica have dropped dramatically — from 13,630 before 2020 to 1,955 in the past year.
“Posidonia is no longer declining since anchoring by large yachts was banned,” Fontaine said. And contrary to the opinions of yacht owners, Fontaine said yachting traffic has remained steady since the law came into effect. “There have been declines in some places, but we’re far from a mass exodus, as they might claim,” he explained.
And yet, there are barely any resources to enforce the rules. In France, there are only ten patrol boats to monitor the 1,000 kilometers of Corsican coastline.
Only two vessels were fined in 2024, and just a handful of shipowners and captains have had to pay up to €100,000 since the law was passed. Even when a coast guard patrol spots a vessel anchored over the plant, in most cases they are merely asked to move elsewhere. Still, the law appears to serve as a deterrent.
Yachting groups argue that the 300-metre coastal ban is excessive, since many safe sandy areas without posidonia also fall within the restricted zone.
Enforce
“The boundary lines are poorly drawn; sometimes we’re forced not to anchor in what turns out to be just sand,” explained Marilyn Sarti, representative of Mar’Isula Yacht Services Corsica, a yacht owner association leading some of the protests against the 2020 law.
Meanwhile, smaller boats — also prohibited from anchoring on seagrass under national law since 1988 — continue to moor wherever they want with impunity because very few checks are carried out.
Unlike France, Italy has no national law protecting posidonia. Regulations vary by region. In Sardinia’s Maddalena Archipelago National Park, which is home to some of the most pristine seagrass beds in Italy, anchoring is banned in just the eight per cent of the reserve that falls under Zone MA, which means there is total protection of everything that lies underneath: no entrance, no sailing, no mooring.
“Without a national law, our hands are tied,” said park director Giulio Plastina, who lacks the staff or authority to enforce even local rules.
A boat ride in July with his team proved his point: several ships are anchored on the plant but Plastina cannot kick them out. He could only advise them to move.
Anchoring
He also has no powers or staff to carry out checks or impose fines, which are, in any case, purely administrative, of derisory amounts and do not mention posidonia.
In 2024, the Coast Guard of the island of La Maddalena fined 29 vessels €50 for crossing into protected areas such as Zone MA, and they weren’t necessarily anchored on posidonia.
However, for local residents like Lucrezia Scotto and Gianna Fancello, who grew up in La Maddalena, the main village of the park, posidonia is a treasure they have learned to protect.
Aware of its importance for the health of the waters, they don’t hesitate to call out anyone who breaks the rules—as they did when they saw that illegal mooring in June.
A WWF report released in June estimated that 50,000 hectares of posidonia were destroyed by anchoring across the Mediterranean in 2024, with Italy the hardest hit, followed by Spain.
Trawling
According to the study, 60 per cent of the impacts are caused by yachts over 24 meters in length. The French law makes a clear difference here: France is one of the countries losing the least Posidonia due to anchoring by large yachts, as the study mentioned by Quentin Fontaine confirms as well.
The Italian Institute for Environmental Protection and Research (ISPRA) is currently mapping the 7900 km of Italian coast to have a real understanding of their posidonia meadows.
Using satellite data, aerial pictures and underwater robots ISPRA wants to provide “a complete, up-to-date, and scientifically accurate picture of the posidonia habitat, to guide protection policies and restoration plans in the coming years,” explained Francesco Rende, a marine biologist in charge of the mapping.
ISPRA has not yet published data, but Rende confirms that in the images taken in areas such as Sardinia or Sicily, you can clearly see the ‘wounds’ caused by anchors in heavily trafficked zones. ‘It’s like looking at a Gruyère cheese, with all its holes.”
The mapping will also make possible to see the highways left behind by illegal bottom trawling. “All this information will help to better protect Posidonia, perhaps change regulations, fully understand all the impacts, and preserve an extremely important habitat for the future.”
Buoys
In fact, Rende also talks about climate change and how rising water temperatures are causing posidonia to bloom continuously, whereas until a few years ago it did so only rarely. ‘It’s its way of reacting to stress.
It blooms in its own attempt to adapt to the new temperatures and perhaps create a variety more resistant to warmer water. “But its capacity as a carbon dioxide reservoir is essential for the future of coming generations, and that is why it must be preserved,” Rende said.
One solution used to protect seagrass meadows around the Mediterranean – and coral reefs around the world — is the installation of mooring buoys, which boats can tie up to and which prevent damage to marine ecosystems.
However, buoys are only a partial solution: the entire coastline cannot be filled with moorings, and their installation is expensive—ranging from €1,000 to €7,000 per buoy, depending on the country.
In Corsica, residents criticised the use of public funds to expand moorings for mega-yachts and successfully limited part of the expansion near the coast of Ajaccio.
Fragile
For the billionaires who anchor along the Corsican coast, these buoys are essential, allowing them to access protected areas without anchoring directly on seagrass—and they pay a hefty price of €4,000 a day.
Off the island of La Maddalena in Sardinia, on the other hand, tying up to the buoys is free with a park entry ticket, which costs between €2 and €5 per meter of boat length.
Giulio Plastina has already installed several dozen buoys with support from the European SeaForest Life project, whose funds come from climate change mitigation efforts of the Italian government.
Plastina’s goal is to create a comprehensive map of the park, where all areas containing seagrass will have mooring buoys. “You will have to register to anchor, and once all the buoys are occupied, that’s it.
Right now, boats are all over. We need to control the number of boats, otherwise it is very difficult to protect an ecosystem as fragile as this,” he explained. In 2024 alone, 13,500 permits were issued for private vessels to enter the Sardinian park.
Ambitious
Anchors are not the only threat to posidonia. Illegal discharges of sewage along the coast and of brine from desalination plants also cloud waters and deprive the seagrass of light, particularly in the Balearic Islands. “Very few vessels comply with the rule to discharge waste more than three miles offshore,” said Raquel Vaquer of the Marilles Foundation, in Palma de Mallorca. “Most empty their tanks between coves, or keep their systems permanently open.”
At Ibiza’s Talabarca Bay, satellite images show posidonia meadows shrinking near a desalination outfall, according to green criminology researcher Esteban Morelle-Hungary. “Many plants,” he noted, “were built decades ago for far smaller populations”.
Poorly treated water affects posidonia not only because it is polluting but because this seagrass needs a lot of light and the discharges reduce water clarity. “They also affect many other sensitive organisms and habitats that are essential for the conservation of marine ecosystems,” Vaquer explained.
In a secluded village deep in Mallorca’s Sierra de Tramuntana, reached by winding roads that cut through pine-clad mountains, stands the Mediterranean Institute of Advanced Studies.
Resident scientist Jorge Terrados has dedicated much of his career to posidonia — and to the ambitious task of replanting it. In 2018, he coordinated Spain’s largest restoration effort: seeding two hectares along the coast of Pollença, in Mallorca.
Conservation
“Restoration means bringing back the ecosystem as a whole,” Terrados explained. “With replanting, we’ve started the process, but it takes time — decades, even centuries — to see whether it survives and recovers.”
Faced with the accelerating loss of seagrass throughout the Mediterranean, researchers have been experimenting with restoration for years. Pilot projects have been launched in the Balearics, Italy and France.
The results are promising, but the obstacles are daunting: the plants grow slowly, and costs reach around €1,000 per square meter, according to Sardinia’s Maddalena Archipelago National Park. Italian studies suggest that seven or eight out of ten transplanted cuttings take root. In Pollença, survival rates in Terrados’ project reached as high as 90 percent.
Italy is betting heavily on replanting, while Spanish and French administrations lean toward conservation. For Terrados, the priority is clear. “The strategy must be to protect the meadows we still have,” he said. “Not a single square meter should be lost. And if we do lose ground, then we can use replanting to speed up the recovery.”
Laws
Marine biologist Roberto Barbieri is even more blunt: “Neither buoys nor restoration projects provide the full solution. They are costly and, to some extent, marginal interventions,” he said.
“If we truly want to protect posidonia and preserve this underwater forest that absorbs carbon and helps fight climate change, we must legislate for the sea as we legislate for terrestrial forests.
“There must be stricter controls, higher fines, and greater efforts in education. European countries need common rules, because posidonia doesn’t recognise borders.
“Calling it a protected habitat is not enough — it must be genuinely safeguarded by national laws. Action is needed at multiple levels, and above all, anchoring on it must be banned.”
These Authors
Barbara Celis is a Spanish freelance reporter, filmmaker and communications expert currently living in Rome. She is a journalist with over twenty years’ experience working internationally as a freelance correspondent in New York, London, Taipei, Madrid and Rome. Throughout her career, she has regularly contributed to leading media outlets such as El País, CTXT, El Confidencial, Rolling Stone and LA Weekly, covering politics, culture and human rights. Her work combines narrative journalism with in-depth investigation.
Ana López García is a freelance journalist and photographer specialising in human rights, environmental issues, and international relations. Before becoming a journalist, she worked primarily with the Red Cross in the humanitarian sector and lived for several years in West Africa and Lebanon. In the field of media development, she has collaborated with the Journalism Trust Initiative of Reporters Without Borders France, promoting transparency and the fight against disinformation.
Alban Leduc is a French freelance journalist specialising in biodiversity. After studying digital journalism and new media formats at the Lille School of Journalism (ESJ Lille), he worked as a reporter for emerging media outlets focusing on ecology, such as Vert le Média and Reporterre. He then co-founded a newsletter and La Corneille Media, a news site dedicated entirely to the biodiversity crisis.
This investigative report has made with the support of the JournalismFund Europe.
