The zodiac of the maritime patrol of the city of Marseille sails straight towards the submerged artificial reefs of Prado Bay, a sanctuary zone between the Corniche and Frioul. On board, on a sea of oil this Tuesday, divers from “Operation Posidonia”, overseen by the scientific interest group (GIS) Posidonia and supported to the tune of 15,000 euros by the city. They are about to jump into the water with, in a basket, a precious treasure picked up a few days earlier on the Var beaches: posidonia seeds, these underwater herbs with a thousand ecological virtues (if they are not destroyed…) and present only in the Mediterranean.
“Today, they are going to sow 4,000 seeds under mats of coconut fiber, 25 meters deep, continues sea assistant Hervé Menchon (EELV). We have a very ambitious roadmap on the Posidonia issue in Marseille. Until now, we were faced with the observation that it is possible to protect but difficult to develop these meadows. The perspective changes with the exceptional flowering of Posidonia this autumn, undoubtedly linked to the sea heat wave. A phenomenon which is observed, according to the elected official, every 4 to 10 years. So, when the scientists told him about the project to collect the numerous seeds to come, to plant them in the bay of Marseilles, he did not hesitate for a moment. It remains to wait for the flowering to bear fruit. Once detached from the plant, these float and wash up on the beach. It is then possible to recover the seeds.
“A first outside the laboratory”
“And then suddenly, one evening last week, I received a call telling me that Operation Posidonia had started,” smiles Hervé Menchon. “We communicated a lot on social networks to have eyes and ears everywhere”, continues Thomas Schohn, study engineer at GIS Posidonia. The team goes to Londe-les-Maures, in the Var, where it has been informed of a large stranding of Posidonia fruits, normally prohibited for collection: the association has had special authorization from the State. “In a few hours we were able to recover the equivalent of 10,000 seeds”, indicates the scientist. The whole is brought back and sheltered in the cool, without its natural element, under a pontoon in the Frioul archipelago. A first dive takes place on Friday. Each time, the seeds are sorted beforehand, to keep only those that can germinate.
If we stick to the natural process, the seed flows to the place where the Posidonia flower opens. So it’s a bit random. “The idea, with this operation, is to target an area where we know that the dynamics of the Posidonia meadow is positive, to go and place these seeds in a slightly denser way and increase their chance of survival, explains Thomas Schohn. For us, this is the first time that this experiment has been done by directly taking seeds that are not germinated in the laboratory. Such a process relies more on the quantity of seeds collected than on their quality obtained in the laboratory. If it works, it would be “very easily replicable on a larger scale, even by recreational divers or voluntary associations. »
” A helping hand “
However, the approach only makes sense in areas that are already protected from human activities, in other words prohibited from fishing, mooring, etc. “We are not going to save the herbarium like that,” recalls Thomas Schohn. It’s a little boost that we can bring when we first put an end to human pressures. In the bay of Marseilles, the meadow has regained 1 meter in ten years, it’s very slow. Sowing some 25 m² more, on a scale of four or five years, can give interesting results. »
On board the zodiac, Hervé Menchon recalls the importance of Posidonia seagrass beds for the “decarbonization” strategy of the city of Marseille, and more. “The herbarium makes it possible to capture large quantities of CO2 in depth and to fight against global warming, he lists. Posidonia also produces oxygen, and helps to fight against the acidification of the sea and for the good quality of bathing water. Finally, these are places where fish find refuge to create their nursery. They contribute to marine biodiversity. A first assessment of the experiment will take place within a year, before a longer-term follow-up. The next dive is scheduled for September, to see if the Posidonia seeds have spent the summer well.