Taking advantage of its massive flowering to replant its seeds at sea and restore this endemic plant that provides us with numerous services. That is the project led by the GIS Posidonia since 2022. A project of gentle restoration, inexpensive, which could inspire elsewhere in the Mediterranean. But which primarily aims to remind us of how precious marine ecosystems are. And how vital it is to preserve them.
You don't need to dive very deep to see its long green leaves dancing. An endemic species of the Mediterranean Sea, posidonia takes root up to 30 meters deep, sometimes just a few steps away from our coasts.
Drawing its name from the Greek god of the sea Poseidon - no less! - it hosts a whole world of fish, mollusks, and other bacteria... And serves as a significant carbon sink, absorbing between 10 to 20% of emissions depending on its location.
Nothing to envy to terrestrial forests, with which it actually shares many similarities. "The seagrass meadow is the most terrestrial of marine ecosystems," says Charles-François Boudouresque, a marine biologist who has devoted a large part of his work to seagrass and has long presided over the GIS Posidonie.
"One heck of a woman"
"The Posidonia is not an algae but a flowering plant. It descends from terrestrial species that returned to the marine environment between 60 and 100 million years ago. And since then, its appearance has changed a little because it has reached a perfect adaptation to its environment, like the shark," the researcher tells, amazed.
A perfection that allowed it to withstand everything: the extinction of the dinosaurs, the drying up of the Mediterranean, and its sudden refilling, the ice ages...
And all the better! Because the seagrass provides us with many services. In addition to absorbing carbon and hosting numerous species with high heritage value, it acts as "a sand-making machine" thanks to the organisms that live in it and end up crushed; either because other species eat them, or due to erosion. "The seagrass also helps us protect the beaches by reducing the force of the waves and making the seabed rougher." A role played especially by its dead leaves when they cover the beaches.
Repairing the damage of the past
If the Posidonia has resisted everything, threats still remain. Among them, the artificialization of the coastlines and the anchoring of ships have greatly reduced its surface until laws came to limit the impact of such nuisances, prohibiting, for example, anchoring above the seagrass bed for ships over 24 meters long.
"However, this protection is not present throughout the Mediterranean," regrets Charles-François Boudouresque. Trawling, which regularly receives exemptions, also causes considerable damage. Finally, due to climate change and the globalization of trade, invasive species sometimes cause damage to seagrass meadows.
Hence the need, in certain circumstances, to restore it. This is the whole purpose of the Reposeed project, established in 2023 by the GIS Posidonie.
In Marseille and Bonifacio, a gentle dining solution
This adventure takes root in 2022. A year of massive flowering of the posidonia, a phenomenon that occurs only once every ten years. Instead of producing the same amount of fruits every year, the strategy of these plants is to play the card of unpredictability so that predators cannot adjust their numbers. Otherwise, they would risk devouring all the fruits, preventing the tree or the posidonia from reproducing sufficiently.
In the case of Posidonia, massive blooms produce seeds that can give rise to new seagrass meadows. However, the quantity produced is such that many flowers and seeds wash up on the beaches and can no longer be sown. Therefore, the Posidonia GIS decides to collect these seeds and plant them in the sea.
Taking the lead is Patrick Astruch, a research engineer within the GIS Posidonia. "To sow these seeds, we had to find places that are preserved from any human pressure, with few herbivores such as sea breams, sea urchins, or crustaceans." Criteria that lead the GIS to select two sites.
The first one is located in the harbor of Marseille, off the coast of Prado Beach where all human activity is prohibited. The second one is further south, in Bonifacio, in the bay of Sant’Amanza. Within an area where anchoring is forbidden for large boats.
On each of these sites, 9000 seeds are planted between April and May using two methods: either kept with coconut fiber or directly in the substrate. And at two densities: 100 or 200 seeds per square meter.
Encouraging results to follow over time
Difficult to make it any simpler. And Charles-François Boudouresque is delighted. "I really admire the idea that Patrick Astruch had. It's brilliant because it's completely natural. Many restoration operations artificialize the environment and often generate more harm than good. That's not the case here. If the project fails, it will have no consequences on the environment," unlike methods based on the use of cuttings, seagrass plots, or those requiring significant ground infrastructure.
Experimentation whose initial results seem encouraging. "We do not have monitoring at the moment in Corsica, but another university team has conducted an experiment in parallel on the same site as us with a different substrate. And their initial results are very positive. There is no reason why it should not work for us," hopes Patrick Astruch.
Following this, if all goes well, there will be monitoring over several years, at least five. Because a seagrass meadow does not grow in a day. "For Posidonia to regain the place it once held in the bay of Marseille, it will take a good hundred years," says Charles-François Boudouresque.
It remains to find the funding that will allow long-term monitoring. In a context where public research tends to lack human, material, and financial resources. And where, adds Patrick Astruch, "an entire generation of ecology experts is retiring".
By the end of June, several seeds of Posidonia had sprouted. It will be necessary to wait until September to have more complete results. @GIS Posidonia
Restore, but above all preserve
If the experimentation is successful, it could be adopted elsewhere and help restore damaged ecosystems, with, as much as possible, the participation of citizens. But this prospect faces a barrier: the limited frequency of massive blooms.
Actually, Reposeed's main goal is to raise awareness among decision-makers and the general public about the need to preserve marine ecosystems. These ecosystems are very difficult to restore due to the complexity of interactions in this environment. "Replanting cannot be a systematic solution," insists Patrick Astruch. "It is a helping hand that can be provided under specific conditions. The priority is to conserve ecosystems. And for that, the best thing to do is to do nothing. Let nature take its course." Because only by reducing pressures on posidonia will we help it face the new threats that await it.