The protected coastline of the Calanques threatened by an invasive Asian green alga in the Mediterranean
Every summer, the divers of the Marseille Sports Loisirs Culture (MSLC) club organize their traditional dive in Callelongue, in order to clean the harbor from the waste accumulated during the year. Except that this year, they were surprised to see the bottom of the cove covered with an impressive green algae.
Jean-Claude Eugene, who was part of the team, had already spotted it in 2019: "it began to cover the bottom of the cove, but not to this extent," he said. According to his calculations, these algae have gained nearly 80 centimeters in height in some places, covering the waste dumped in the port.
For Thierry Thibaut, marine ecologist at the Mediterranean Institute of Oceanology, there is no doubt: it is the species Rugulopteryx okamurae, an invasive Asian algae arrived from Japan. It remains to be seen, from now on, how it will impact this ecosystem that it has started to invade.
An influence on the environment still unknown
This Asian variety was observed for the first time in the Mediterranean in the early 2000s. It is the importation of oysters from Japan that brought it to the Thau lagoon on the coast of Sète, says this specialist in algae.
Little by little, Rugulopteryx okamurae has gradually spread to the coast of Bouches-du-Rhône. Just last week, it was observed for the first time in Carry, Sausset-les-Pins and Carro, in the waters of the Blue Coast Marine Park.
The proliferation of this algae has accelerated in the last two years, worrying divers and regulars of the Callelongue cove. "We are afraid that it will destabilize the fauna and the local flora," said Jean-Claude Eugène, urging the authorities to take measures against its expansion.
With the arrival of heat, residents also fear that this algae causes strong odors of sewage by decomposing, as was the case in August 2019. At that time, they had already worried about appalling smells escaping from the cove, caused by rotting algae.
In fact, the experts of the Calanques National Park have been discreetly investigating this algae for three years. "This invasive species could indeed have consequences on the algal communities, and by ricochet on the invertebrates or on certain fish", estimates Thierry Thibaut, who takes an active part in the investigations.
According to the first observations, Rugulopteryx okamurae also comes to block the faults of the sea-beds, with a risk to disturb the local species which live there.
However, the marine ecologist from Aix-Marseille University specifies that the development of this variety is more visually impressive in the port of Callelongue than in the rest of the national park, because of its narrow and closed configuration.
17 species of invasive algae in the Bouches-du-Rhône
Contacted, the National Park of the Calanques had not answered on Wednesday to our request of information on the proliferation of this alga in their perimeter.
What worries the specialists above all, is the facility that this variety has to spread along the coasts. "It produces spores that can travel kilometers with the currents and then settle on favorable supports," explains the scientific manager of the Blue Coast Marine Park, Eric Charbonnel.
Moreover, rugulopteryx okamurae is only a small part of the problem: there are, in fact, no less than 17 invasive species that are currently dispersed in the waters of the Bouches-du-Rhône.
The caulerpa cylindracea, for example, had colonized the entire Mediterranean basin a few years ago, influencing its natural habitats, before its population stabilized a few years later.
The red filamentous alga Womersleyella setacea has also carpeted the Mediterranean sea floor at an exponential rate in recent years. It forms dense carpets which increase the sedimentation of the sea-beds and limit the access to the substratum, which serves as food for the local species.
Thierry Thibaut, who is one of the rare specialists of the subject in France, regrets that the proliferation of the invasive species of algae in the Mediterranean is not the object of more thorough researches, for lack of interest or of financing according to him.
"It is globalization that has created these exchanges of species, it is a global problem related to maritime transport and a lack of strict regulations," adds Eric Charbonnel, of the Blue Coast Marine Park.
"To maintain local biodiversity, it would be necessary to intervene upstream with better legislation," he believes.