Every summer, divers from the Marseille Sports Loisirs Culture (MSLC) club organise their traditional dive at Callelongue, to clean up the harbour of the rubbish that has accumulated over the year. This year, however, they were surprised to find that the bottom of the calanque was covered with an impressive green algae.
Jean-Claude Eugène, who was part of the team, had already spotted it in 2019: "it was beginning to cover the bottom of the calanque, but not to this extent", he says with alarm. According to his calculations, the algae has grown to a height of nearly 80 centimetres in some places, covering the waste dumped in the harbour.
For Thierry Thibaut, marine ecologist at the Institut méditerranéen d'océanologie, there is no doubt about it: it is the species Rugulopteryx okamurae, an invasive Asian algae that has arrived from Japan. It now remains to be seen how it will impact the ecosystem it has begun to invade.
A still little-known influence on the environment
This Asian variety was first observed in the Mediterranean in the early 2000s. It was imported oysters from Japan that brought it to the Etang de Thau on the Sétoise coast," explains this seaweed specialist.
Little by little, Rugulopteryx okamurae has gradually spread along the Bouches-du-Rhône coastline. Just last week, it was spotted for the first time in Carry, Sausset-les-Pins and Carro, in the waters of the Côte Bleue Marine Park.
The proliferation of this algae has accelerated over the last two years, causing concern among divers and regular visitors to the Callelongue cove. "We're afraid it's going to destabilise the local flora and fauna," says Jean-Claude Eugène, urging the authorities to take action against its spread.
With the arrival of the heat, local residents also fear that this algae will cause strong sewage odours as it decomposes, as was the case in August 2019. At that time, they were already worried about the appalling stench escaping from the cove, caused by rotting algae.
In fact, the experts at the Parc national des Calanques have been quietly investigating this algae for the past three years. "This invasive species could have an impact on algal communities, and in turn on invertebrates and certain fish", says Thierry Thibaut, who is actively involved in the investigations.
According to initial observations, Rugulopteryx okamurae also clogs cracks in the seabed, with the risk of disturbing local species that live there.
However, the marine ecologist from Aix-Marseille University points out that the development of this variety is more visually impressive in the port of Callelongue than in the rest of the national park, due to its narrow, enclosed configuration.
17 species of invasive algae in Bouches-du-Rhône
When contacted, the Calanques National Park did not respond on Wednesday to our request for information on the proliferation of this algae within its boundaries.
What worries specialists most is the ease with which this variety can spread along the coast. "It produces spores that can travel kilometres on currents and then settle on favourable surfaces," explains Éric Charbonnel, scientific manager of the Côte Bleue Marine Park.
What's more, rugulopteryx okamurae is only a tiny part of the problem: there are no fewer than 17 invasive species now dispersed in the waters of Bouches-du-Rhône.
The caulerpa cylindracea, for example, colonised the entire Mediterranean basin a few years ago, influencing its natural habitats, before its population stabilised a few years later.
The red filamentous alga Womersleyella setacea has also covered the Mediterranean seabed at an exponential rate in recent years. It forms dense mats that increase sedimentation of the seabed and limit access to the substrate, which serves as food for local species.
Thierry Thibaut, who is one of the few specialists on the subject in France, regrets that the proliferation of invasive algae species in the Mediterranean is not the subject of more in-depth research, for lack of interest or funding, in his view.
"It's a global problem linked to maritime transport and a lack of strict regulations," adds Éric Charbonnel, from the Côte Bleue Marine Park.
"To maintain local biodiversity, we need to intervene upstream with better legislation", he believes.