A box filled with seagrass seedlings is bouncing around at the back of the boat. It’s a warm day in June and marine biologist, Manu San Felix, is preparing for a dive a few kilometers off the coast of Formentera, Spain.
“This is where we plant the seeds,” said San Felix, as he put on his scuba gear. “You should jump in, have a look around.”
Beneath the surface, a green seagrass meadow—believed by some scientists to be the oldest living organism on the planet—is swaying across the seafloor. The green expanse stretches 15 kilometers wide and consists of clones dating back 200,000 years. Swallow fish swim slowly above it; jellyfish dance amongst it; San Felix, a native to this environment, dives towards it.
Posidonia oceanica or Neptune grass, is a seagrass species endemic to the Mediterranean and a critical tool for fighting climate change. P. oceanica—which is a plant, not to be confused with seaweed, an algae—covers over 25,000 square kilometers of the seafloor. The species protects the coastline and provides a home for molluscs, crustaceans, echinoderms, fishes, and sea turtles. The average square kilometer of seagrass stores 83,000 metric tons carbon—more than double the storage capacity of a typical forest.
“[One of] the most effective ecosystems on the planet for absorbing carbon is the Posidonia meadow,” said San Felix.
Around the world, however, seagrass is quickly disappearing. Although Posidonia is protected at both the international and European level, the plant is nevertheless struggling to survive amidst rising tourism, pollution, and sea temperatures. Scientists estimate that between 1842 and 2009, up to half of Mediterranean P. oceanica meadows might have been lost.
“If we are the ones responsible for Posidonia disappearing, that is a mistake with no return for humanity,” said San Felix. “We have to protect it.”
A Magic Meadow
San Felix remembers the first day he swam with the Posidonia like it was yesterday.
“When I dove into the water, I was blown away,” he said, recalling his first dive in the Formentera meadows in 1992. “Something happened at that moment. I fell in love with this amazing plant that creates an entire ecosystem.”
San Felix is not the only one to have become mesmerized by the Posidonia meadows of the Balearic Islands. The ancient Phoenecians sent their wounded warriors to swim in the meadows, believing the water could heal all injuries, according to Miguel Soler-Roig, a conceptual artist whose 2018 series Atlantis & Posidonia, explores the mythology of the plant. The ancient Greeks cherished Posidonia so much that they named it after their god of the sea, Poseidon, who was said to have the power to unleash storms as well as protect sailors and their ships, Soler-Roig said.
Much like the god it’s named after, Posidonia has always safeguarded the sea. The meadows provide shelter and breeding grounds for thousands of animal and plant species that live in the Mediterranean. Because of the large amount of oxygen it produces, Posidonia gives water life. The meadows are also critical to preventing coastal erosion: the plant’s leaves trap sediment before the waves carry it away from shore. Posidonia is one reason why many parts of the Mediterranean enjoy white sandy beaches and turquoise clear waters.
“The lungs of the planet are not on land. They are in the sea.” —Antonio Jesús Sanz Igual, Former Councillor of Environment and Inspection Services of Formentera
Historically, seagrass was a useful resource for people on land. Posidonia, which develops a strong, paper-like texture when dried, was used as pillow inserts, as wrapping packages for fish, and as insulating material for thatched roofs in Tunisia’s coastal communities.
It provides ecological services today, too, protecting the region from climate change. The Mediterranean is warming 20% faster than the global average, leading to more frequent and severe storms. By reducing the speed and strength of waves, Posidonia helps avoid destructive flooding during storm surges. It also is one of the most effective carbon sinks on the planet. Experts estimate that Posidonia has sequestered between 10 to 40% of Mediterranean countries’ greenhouse gasses since the Industrial Revolution.
“The lungs of the planet are not on land,” said Antonio Jesús Sanz Igual, the former Councillor of Environment and Inspection Services of Formentera. “They are in the sea.”
A Threatened Species
The Spanish Posidonia meadow has survived for 200,000 years. The plant has seen the rise and fall of civilisations. It has withstood the ice age. But 50 years of pollution and rising sea temperatures has put the species’ future in jeopardy.
“When I first dove in the meadows, they were pristine, intact, there were no prints of human activity,” said San Felix. “This has changed dramatically.”
Part of the problem is overtourism. In 2017 alone, the Balearic islands welcomed 15 million international tourists, a 12% increase since 2015, according to the Balearic Islands Tourism Board. With more tourists comes more waste discharged into the sea.
It also comes with more boats—up tenfold in recent years, according to San Felix. Anchors rip up roots, destroying the meadows and the ecosystems they sustain. Because seagrass grows extremely slowly—taking up to 100 years for one square meter of Posidonia to grow—the meadows have been unable to replenish themselves.
“The loss of Posidonia is directly related to the invasion of human beings on the shores since the 1960s,” San Felix said.
Another problem is climate change. Posidonia has an upper lethal temperature limit of 28.9 degrees Celsius. Although the summer temperature of Mediterranean waters has historically averaged around 24 and 26 degrees Celsius, last summer, they reached 30 degrees in Corsica. This summer, a marine heat wave swept across the Mediterranean, bringing water temperatures off the coast of southern Spain up to 9 degrees Celsius higher than they normally are this time of year.
“We are bringing Posidonia to the brink of extinction if we continue raising temperatures,” said San Felix.
Until recently, most Spaniards or tourists visiting the Balearic Islands did not know about Posidonia.
Although the seagrass was revered in ancient cultures, it was largely forgotten in the 20th century. Many hotels actually removed seagrass when it washed up onto its shores in order to keep beaches sandy for tourists.
Today, however, it is hard to visit Formentera without hearing about the plant. Around the island, ‘Save Posidonia’ logos are stuck on cash registers, ferry terminals, and even etched onto the concrete. Local gyms and small boats are named after the seagrass. Several hotels and restaurants display informational brochures about Posidonia in their receptions.
“People are only just starting to discover Posidonia,” said Soler-Roig. “There’s been a huge change in the past few years.”
This is largely thanks to the Formentera council. In 2017, the council launched the Save Posidonia Project, which seeks to increase research and advocacy for Posidonia as well as promote sustainable tourism.
Alongside advocacy, the council has put in place several regulations to protect the meadows. In 2018, they approved a ‘Posidonia law’ which requires human activity to be compatible with conserving seagrass. The council has placed a cap on the number of cars on the island in the summer, for instance, in order to reduce pollution runoff into the sea.
They’ve also tried to reduce damage caused by anchoring: In 2013, the Government of the Balearic Islands introduced an anchoring patrol and assistance service in Formentera’s natural park to help with both on the ground education of sailors as well as enforcement for those who break the rules. The council has placed buoys to signal meadows to boaters.
Without San Felix, many of these changes would not have been possible. He has spent years mapping the meadows, providing critical information to the council to create legislation. He made these maps publicly available on “Posidonia Maps”, a free app that alerts boat owners if there are meadows below their vessel.
San Felix has not stopped there. Although he said legislative changes and scientific advancements are important, he believes that the key to protecting the plant lies in better communication and education.
“Most people won’t see the underwater world,” said San Felix. “That’s why for marine conservation, communication is so important.”
“Posidonia is giving us a lot of things. In return, it asks only that we take care of it.” —Manu San Felix, Marine Biologist
Over the years, San Felix, who is also a National Geographic Explorer, has partnered with brands to make educational videos about Posidonia and started a diving school with his wife to educate the next generation of Spaniards about the importance of seagrass.
“If you explain to a child how to protect the sea, you are teaching them how to construct a better society for the future,” said San Felix. “It takes them a day to understand this. They get it instantly.”
When San Felix emerges from his dive, his hands red from jellyfish stings, he is smiling: the seeds they sowed months ago have taken.
San Felix, however, is cautious with his optimism: even if pollution is reduced and parts of the meadows are restored, rising temperatures could reverse progress.
“We are starting to witness the Posidonia die because of high temperatures,” he said. “I just try to focus on what I can control, which is to keep Posidonia in the best condition possible.”
For San Felix, doing so is not so much a choice as it is a duty.
“Posidonia is giving us a lot of things,” he said. “In return, it asks only that we take care of it.”
This story was produced with support from Internews' Earth Journalism Network. It was first published in Atmos on August 2, 2023. It has been lightly edited for length and clarity.
Banner image: Seagrass / Credit: Théophile Bremond.