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Posidonia oceanica in the Bay of Alcudia: the centuries-old underwater forest that keeps Mallorca alive
Mallorca has a forest no tourist sees — and it's one of the oldest on the planet
Before the pharaohs of Egypt, before the first stone of Stonehenge was placed, before any human ever set foot on the Balearic Islands — beneath what we now call the Bay of Alcudia, a forest was already growing. And it is still growing, silently, right under the keels of the boats that cross the north coast of Mallorca every summer. It is called posidonia oceanica, and almost nobody who visits the island knows exactly what it is. People swim over it without seeing it, anchor their boats on top of it without suspecting, photograph the turquoise waters that exist only because of it without having any idea who makes them possible. Posidonia is probably the most important living being in the Mediterranean — and at the same time, the most invisible. The first thing to clarify — because almost everyone gets this wrong — is that posidonia is not seaweed. It is a higher plant, with flowers, fruits and roots, that evolved millions of years ago from a terrestrial ancestor and returned to the sea. It is closer to meadow grasses than to marine algae. That is why, technically, what grows on the seabed off Mallorca is not 'seaweed': it is a meadow, an underwater prairie. A forest, if you want to give it a poetic name. And then comes the fact that blows the mind of anyone hearing it for the first time: in the protected zone between Formentera and the east coast of Ibiza, scientists from the Mediterranean Institute for Advanced Studies (IMEDEA) confirmed in 2006 the existence of a single posidonia plant 100,000 years old. Not one hundred. One hundred thousand. It is probably the longest-living organism on Earth — older than any sequoia, any baobab, any fungus ever recorded. And beneath Mallorca, inside the Bay of Alcudia, there are individuals that have been growing since before writing was invented. - Posidonia is a higher plant, not seaweed - It only grows in the Mediterranean (it is endemic) - Its meadows can live for more than 100,000 years - It only grows at depths between 0 and 40 metres, in clean waters - Its meadows along the east coast of Ibiza–Formentera are a UNESCO World Heritage Site since 1999 In other words: every time you put your head underwater in a cove on the north coast of Mallorca and see a dark green seabed waving with the current, you are looking at an organism that has probably been there longer than all of human civilisation. And that is only the beginning.
The Bay of Alcudia meadow: one of the Mediterranean's hidden jewels
The Bay of Alcudia has a geography that makes it especially well suited for posidonia: relatively shallow waters, a soft sandy bottom, moderate currents and an orientation that protects it from the most violent eastern storms. The result is that the posidonia meadow extending from Alcanada to the far end of Playa de Muro and beyond is one of the most continuous and best preserved on the entire north of the island.
The meadows in the bay start practically at the shoreline — barely a couple of metres deep — and descend to 30–35 metres, forming a living carpet of several square kilometres. If you fly a drone over the bay on a calm day, you can see it clearly from above: the dark patches alternating with the sandy areas are not rocks — they are the meadow. That difference in colour is exactly the difference between bare sand and underwater forest.
Along with Cabrera and the Ibiza–Formentera meadow, the Bay of Alcudia is one of the most intensively studied seagrass habitats in the Balearic archipelago. Teams from the CSIC, IMEDEA and the Universitat de les Illes Balears have worked here for decades, measuring growth, health, biodiversity and response to climate change. It is one of the few places in the Mediterranean where consistent long-term data exists.
It is also one of the reasons Alcudia's waters have that very particular blue — a milky, crystalline turquoise that is not exactly the blue of Sardinia or the Cyclades. It is a blue that only appears when posidonia is healthy and the water is clean. When a meadow dies, the sand rises, the waters cloud over and that colour disappears. You can see this in many overdeveloped Mediterranean areas where tourism has been more aggressive. In Alcudia, not yet.
If you want to understand what makes this geography unique, we recommend reading our guide to the history of the Bay of Alcudia and our piece on the marine species that live in the bay.
Five reasons why posidonia is the foundation of the Mallorca you know
If posidonia disappeared from Mallorca tomorrow, the island would change physically within months. No exaggeration. Beaches would shrink, water would lose its transparency, the fish you see while snorkelling would vanish, and the local climate would become more extreme. This is what the underwater forest beneath your feet does every day — without anyone applauding.
11. It is the real reason behind those turquoise waters
When you see those crystal waters where you can pick out the seabed at 8 metres depth, it is not magic, geological luck or 'just good Mediterranean fortune'. It is posidonia. The meadow filters and traps sediments carried by currents, preventing sand from being lifted with every wave and clouding the water column. Without posidonia, the bay's water would be an opaque blue similar to many Atlantic beaches, and you would never see the seabed with such clarity. Mallorca's turquoise is, literally, a biological consequence of having a healthy underwater forest below.
22. It produces the oxygen we breathe (more than you think)
A well-preserved posidonia meadow produces between 4 and 20 litres of oxygen per square metre per day. To put it in perspective: one hectare of meadow can generate as much oxygen as one hectare of Amazon rainforest. And the Bay of Alcudia meadow covers several square kilometres. Every time you breathe by the sea in Mallorca, part of that air comes literally from the seabed beneath you. Posidonia is not just an ecosystem — it is a lung.
33. It captures more CO₂ than a tropical forest
This surprises even climate scientists. Posidonia is one of the best carbon sinks on the planet: it captures between 11 and 42 tonnes of CO₂ per hectare per year, and stores it in its roots and matte for thousands of years. Per unit of surface, it traps more carbon than the Amazon. Mediterranean meadows together store as much carbon as many entire boreal forests. Conserving posidonia is, in practical terms, one of the most efficient climate actions that exist — and Mallorca holds one of the largest reserves in the Mediterranean beneath its waters.
44. It protects the very beaches you love
Have you ever noticed those brown, dry, tangled remains piled up on some Mallorcan beaches — especially after a storm? That is dead posidonia leaves, naturally shed and washed ashore. What almost nobody knows is that this accumulation has a technical name, *banquette*, and plays a critical role: it breaks the force of the waves and prevents sand from being washed away during winter storms.
On beaches where posidonia banquettes are removed 'so the beach looks nicer', erosion accelerates dramatically. Some studies estimate that 80% of Mediterranean beach sand comes from shell fragments of organisms that lived in posidonia meadows. If you want a beach, you need a meadow. There is no way around it.
55. It is home to hundreds of species — the same ones you see while snorkelling
Posidonia meadows are one of the most biodiverse ecosystems in the Mediterranean. They host more than 1,000 animal and plant species, from seahorses and noble pen shells (the largest bivalve in the Mediterranean) to cuttlefish, octopus, sea bream, salema, white sea bream, common pandora, stingrays, starfish, sea urchins and all kinds of small invertebrates that form the base of the food chain. Every time you see a school of salema flashing in Alcudia's waters or a sea bream darting between the leaves, you are watching the meadow at work. Without it, no snorkel is worth its salt. In our guide to the best snorkel spots in Alcudia you will see that almost every place with abundant marine life has a living meadow right below.
How to see it with your own eyes: snorkel, kayak and boat to discover the underwater forest
The good news is that, unlike many of the planet's unique ecosystems, seeing posidonia in Mallorca is easy. You don't need a submersible, a diving qualification or professional gear. A mask, a snorkel, a calm day and knowing where to look are enough.
How to recognise it when you see it:
Best ways to see it in the Bay of Alcudia:
1. Snorkel from the shore at spots like Coll Baix, Alcanada or the small coves north of Mal Pas. You see the meadow practically from the first stroke.
2. By kayak — a clear-bottom kayak is great if you want to see it without getting wet, and there are rentals at several beaches. But respect anchoring rules: never drop an anchor on a meadow.
3. From a boat with stops, probably the most comfortable option to cover several spots in a few hours. On our morning tour we make specific stops over living meadows so you can snorkel directly above them, with a guide and gear included.
4. From the air — a drone over the bay on a windless day shows you the meadow in panoramic view. You won't see it 'up close', but you'll understand the shape of the entire forest.
If you're going snorkelling, we recommend reviewing our guide to the best hidden coves accessible by sea beforehand: many of the spots we recommend have a posidonia meadow right below, and they're the most spectacular ones for discovering the underwater forest.
The threats: how Mallorca's meadow is slowly dying
Here comes the hard part. According to the most recent scientific data, Mediterranean posidonia is in clear regression for several decades. And it is not something happening 'somewhere else'. It is happening here, in the Bay of Alcudia. Every year, part of the meadow dies. And posidonia has a cruel particularity: as it grows less than 1 cm per year, what is destroyed today takes centuries to recover, if it ever does. These are the five main threats:
1. Boat anchoring — the worst visible threat.
An anchor falling on a meadow tears out, in a single gesture, what the plant has taken decades to build. When the anchor chain 'walks' with wind or current, the damage multiplies: a single badly anchored vessel can destroy more than 100 m² of meadow in one morning. In the Bay of Alcudia, on a peak August day, dozens of boats can be simultaneously anchored on posidonia. Do the maths.
2. Urban and agricultural pollution.
Poorly treated wastewater discharges, fertilisers reaching the sea via runoff, oils and fuels from boats. Posidonia is extremely sensitive to water quality: any nutrient excess causes opportunistic algae to grow over it, blocking light and suffocating it.
3. The warming Mediterranean.
Posidonia is adapted to average summer water temperatures of around 22–25°C. Marine heatwaves — increasingly frequent in the Mediterranean, with peaks above 30°C — biologically stress it and reduce its photosynthetic capacity. The summers of 2003, 2022 and 2024 left documented damage in meadows across the basin.
4. Invasive species.
The arrival of tropical algae — such as *Caulerpa cylindracea*, *Lophocladia lallemandii* and *Halimeda incrassata* — is covering meadow areas across many Mediterranean islands. They compete directly for light and nutrients and are very well adapted to ever-warmer water. Some areas of Cabrera and Mallorca are starting to have serious problems with this.
5. Historical bottom trawling and dredging.
Fortunately, bottom trawling has been banned over much of the Balearic coast for years, but the historical damage it caused is enormous. And one-off projects of beach regeneration or port works can also affect specific areas. Sand regeneration 'to make the beach larger' usually kills the adjacent meadow.
The combination of these five factors means that, according to Save Posidonia Project and IMEDEA data, some areas of the Mediterranean have lost between 30% and 50% of their meadow in the last 50 years. Mallorca has held up relatively better — but it is not immune.
What is being done: regulation, eco-friendly mooring buoys and protection projects
The good news is that in the last ten years there has been a more serious institutional and scientific response than at any earlier moment. It is not perfect, it is not enough, but it has changed the game. These are the most important measures directly affecting the Bay of Alcudia and the rest of the Balearics:
1. Decree 25/2018 of the Balearic Government: ban on anchoring on posidonia.
Since July 2018 it has been forbidden by law to anchor on posidonia oceanica meadows in Balearic waters. The regulation applies to all vessels, regardless of size or nationality. Fines can reach several thousand euros, and Customs Surveillance and Maritime Rescue carry out active checks in summer. The full regulation is available on the official BOIB of the Balearic Government.
In practice, this means that when you see a modern boat in the Bay of Alcudia using a nautical chart with posidonia mapping (the *Posidonia Maps* app from the Balearic Government, or charts like Navionics with the posidonia layer), it is complying with the law. If you anchor by eye, you are almost certainly committing an offence.
2. Eco-friendly mooring buoys.
In several points of the archipelago, fixed mooring buoys that don't damage the meadow have been installed. The chain doesn't touch the bottom and the buoy is anchored with a system that doesn't affect posidonia. In Mallorca there are buoy networks in areas like Cabrera, Sa Dragonera and parts of the north. The idea is that vessels moor to these buoys instead of anchoring on the meadow.
3. Marine reserves and protected zones.
The Cabrera Maritime-Terrestrial National Park (south of Mallorca) and several Marine Reserves of the archipelago have stricter rules to protect specific meadows. While the Bay of Alcudia is not a marine reserve, it does include zones with partial protection.
4. UNESCO declaration of the Ibiza–Formentera meadow.
In 1999, UNESCO declared the posidonia meadow stretching between Ibiza and Formentera a World Heritage Site. It is one of the few 'plants' recognised by UNESCO worldwide, and reinforces international acknowledgment of the ecosystem's importance throughout the archipelago.
5. Long-term research and monitoring.
Teams like those at IMEDEA-CSIC and the University of the Balearic Islands have been monitoring Balearic meadows for over 30 years using methods like the Posidonia Networks. This gives concrete numbers on which areas are doing better or worse, helping to focus measures. Mallorca is one of the places in the world with the best data on the state of its meadow.
6. LIFE projects and citizen campaigns.
Projects like LIFE Posidonia and campaigns like Save Posidonia Project work both on active restoration (transplantation) and awareness. Some of the best local nautical companies — including ours — collaborate with these projects by sharing data, avoiding harmful routes and training crew.
What you can do — seven simple, real actions
The good part of all this is that protecting posidonia doesn't require being a scientist, a biologist or a full-time activist. A few conscious decisions during your holiday are enough for your visit to Mallorca to add up rather than subtract. These are the seven most impactful actions you can take:
1. If you rent a boat, ALWAYS anchor on sand.
Sand looks light from above, white, bright. Posidonia looks dark green or brown. If in doubt, don't anchor. And if you plan to rent a vessel, before you set off read our guide on boat rental without licence in Mallorca — we explain how to read the seabed and where the eco-friendly buoys are.
2. If you go on a boat with a company, ask how it manages anchoring.
A responsible company uses eco-friendly buoys or charts with posidonia mapping. If they tell you 'we anchor wherever it suits', that's a bad sign. At Coral Boats, for example, every stop is planned in zones with a buoy or over confirmed sand. It's not optional — it's the baseline.
3. Don't take souvenirs from the seabed.
No live shells, no noble pen shells (they are protected and on the verge of extinction!), no posidonia balls (the famous 'aegagropilae' that appear on some beaches), no leaves of the plant. Photograph yes, take home no.
4. Support local restaurants and businesses working with sustainable produce.
Traditional Mallorcan fishermen are natural allies of posidonia (not bottom trawling). Restaurants offering certified local fish help maintain a balance that doesn't damage the ecosystem.
5. Respect posidonia banquettes on the beach.
No, it's not 'sea rubbish'. No, the beach isn't 'dirty'. Those dry remains protect the sand from storms. On Mallorca beaches that follow European recommendations, banquettes are left in place at least during winter. If you visit in May and see remains, be a bit patient: the beach will be perfect by the time the season starts.
6. Spread the word.
This sounds cliché but it works: the more people know what posidonia is, the fewer harm it through ignorance. Talk about it in your photos, your social media, your comments back home. It's one of the best-kept treasures of the Mediterranean and deserves to be known.
7. If you're passionate, donate to or collaborate with a protection project.
Projects like Save Posidonia Project, Posidonia LIFE or the Marilles Foundation work directly on protecting Balearic meadows. A €10 donation can help maintain eco-friendly buoys, fund transplants or pay for educational campaigns. It's probably one of the best uses of small money in terms of real environmental impact.
And if what you want is to experience seeing the meadow with your own eyes, accompanied by a team that knows and respects it, book a boat trip with us: our stops are chosen not only for their beauty but also so you can get close to the meadow without harming it. And when you go home, you'll take more than photos: you'll take with you the story of the forest that keeps Mallorca alive.
Posidonia oceanica is probably the most beautiful answer Mallorca offers to the question 'what makes this island special?'. It's not the beaches — those exist because there's posidonia. It's not the blue of the water — that blue exists because there's posidonia. It's not the abundance of fish, the unspoilt coves, the clean summer air: all of that, thread by thread, is connected to the underwater forest that has been growing for thousands of years beneath our feet.
Posidonia is not a postcard. It is a living, vulnerable, millennial heritage that belongs both to the sea and to those of us who live by it. And, in the next few years, what we humans do at this very moment will determine whether future generations still see turquoise waters in Alcudia or have to settle for photographs.
So the next time you come to Mallorca, when you put your head underwater and see a green seabed waving with the current — pause for a moment. You're looking at an organism that has probably been there longer than all of humanity. And you have the chance, simply by making the right decisions during your holiday, to make sure it stays there for many centuries more. It's one of the most beautiful privileges this sea offers. Come and see it on a boat, with a local guide, snorkel included and the awareness of visiting one of the oldest forests on the planet. It's the closest you can get to time travel.
