They may look like brown dead leaves on the beaches of Marseille. However, the Posidonia oceanica seagrass forms « benches » that cushion the waves eroding the coastal sediments.
Marseille (Bouches-du-Rhône), report
Squeezed between the Corniche Kennedy and the protective dike, Plage du Prophète in Marseille’s 7th arrondissement defies marine erosion. It has the unique ability to naturally accumulate sand, thanks to an astonishing underwater plant.
This plant is Posidonia oceanica, whose dead leaves form a thick brown layer along the waterfront, the « benches. » These cushions absorb wave energy and reduce their ability to carry sand offshore. In front of the benches, the water is also filled with dead leaves. « This adds viscosity, which further reduces the water’s erosion power, » explains Patrick Astruch, a marine biologist at Aix-Marseille University. Depending on exposure, wind, and tides, you can find them on all Marseille beaches.
Reporterre visits the beach at the beginning of spring, accompanied by this scientist, a member of the Posidonia Interest Group (GIS) – which studies and acts to protect the « Mediterranean endemic » plant – and Hervé Menchon, Marseille’s Deputy Mayor for the Sea.
A Champion of Biodiversity and Climate
Unlike many Mediterranean towns that remove dead posidonia, deeming them unsightly and inconvenient, for the second year in a row, the City of Marseille has decided to leave them on the beaches. The goal is to restore the plant’s natural cycle, which plays multiple ecological roles vital to ecosystems.
Dead, the grass provides nutrients to marine flora like glasswort, sea fennel, rock samphire, and beach grass, and to small sand-dwelling fauna like sea fleas and other arthropods and mollusks.
Alive, it forms extensive underwater meadows where the GIS Posidonia has observed a quarter of the plant and animal species in the Mediterranean. These seagrass beds serve as food sources, shelter, breeding grounds, and nurseries for many marine species, including sea bream, saupe, white seabream, scorpionfish, cuttlefish, and large clams.
Per unit area, this flowering plant stores, via its deep-rooted rhizomes, « 3 to 5 times more carbon than tropical forests and up to seven times more than a deciduous forest in France, » calculated the Calanques National Park.
The long, slender green leaves of Posidonia oxygenate the water through photosynthesis and also dampen wave force with their swaying in the current. In autumn, the plant sheds some of its leaves, akin to trees losing leaves as sunlight wanes.
« The True Mediterranean Landscape »
« What you see here, this dark mass, is the true Mediterranean landscape, » says Hervé Menchon, as he invites us to walk on about fifty centimeters of dead posidonia leaves.
The official wants to depart from a « coastal economic model that has sanitized beaches by removing dead grass in spring, altering the Mediterranean landscape’s imagery to conform to the stereotype of white sand beaches typical of the Maldives. »
The Prophète’s posidonia bed extends to the rocks supporting the Corniche. In some places, a meter thick layer faces the lapping waves. « You feel how soft it is under your feet. Even in a forest, it’s hard to find this sensation, » enthuses researcher Patrick Astruch. « My 8-year-old loves it. He jumps on it like it’s a trampoline, » says Hervé Menchon.
As we approach the small Yachting Club Prophète, two members raise concerns. In swimsuits, with sun-weathered skin, Henri and Stéphanie Tamburini are annoyed by an excess of posidonia.
« It’s not the posidonia that bothers us, it’s the smell. We’re careful around posidonia with our boat, not anchoring to avoid uprooting them. But look at this beach; it looks like a swamp, » says Henri, pointing to the stagnant water in the bench.
A faint odor of decomposition emanates. The couple insists it will worsen as temperatures rise, potentially causing health issues. « It’s not the posidonia that smells; it’s other organic matter that gets trapped and decomposes, » responds Hervé Menchon.
« I’m Ashamed of Marseille »
« This has never been like this before. We can’t even launch our boats, » says Henri to Reporterre. The club’s concrete boat ramp is now cluttered with dead posidonia leaves, stretching more than ten meters into the sea. Small inflatable boats are lined up under tarps. « See that buoy? We used to tie our boats there. It used to be in 80 cm to a meter of water, » explains Stéphanie, pointing to the white mooring laid on the sand and brown posidonia.
She would prefer a compromise for an accessible beach. In other words, removing part of the bench. « It should be moved to the other side of the beach, where swimming is not supervised and where we don’t launch boats, » suggests Henri.
On the beach, other opinions are even more polarized. « Look at what our mayor has done. It’s not beautiful. I’m ashamed of Marseille, » fumes Raymonde Castellan, heading into the water under a wide-brimmed hat. The sexagenarian, a daily visitor, wishes the entire bench to be removed.
Other beachgoers, like Jeanne Lavialle, rejoice at its presence. She frequents the beach with her partner and two children, aged 5 and 3. They play with sticks left by the last storm on the sand and bench. « It’s great to leave the ecosystem in place. Cleaning it up would destroy it. It shelters many little creatures, » says the Marseillaise, a professional botanist.
A « Protected Plant, Alive or Dead »
« Posidonia is protected, alive or dead, » says Patrick Astruch of the GIS Posidonia. To move or manipulate the benches for development or scientific purposes, one must apply for a government permit.
Permits are usually granted, provided the dead leaves are left in their natural environment, on land or at sea. Thus, to prepare for the summer season, many Côte d’Azur towns practice « clapping. » Boats with opening bottoms load dead leaves and drop them offshore, sinking into marine pits.
This method provides nutrients to deep-sea species, touted as eco-friendly in local press.
Patrick Astruch doubts its effectiveness. « We no longer dump dead posidonia in landfills, which is good. Naturally, it should return to the sea, even into the deep ocean. But not all at once with these enormous volumes. Deep-sea ecosystems have slow dynamics. They risk being smothered by this excessive mass of organic matter, » explains the marine biologist.
The City of Marseille does not endorse complete non-intervention on beaches where posidonia accumulates. « Every year, before the bathing season,
