Impact of the invasive brown alga Rugulopteryx okamurae on the benthic communities in the Northwestern Mediterranean Sea

Over the centuries, the Mediterranean Sea has been significantly impacted by biological invasions, hosting more than 1000 non-indigenous species. In the early 2000s, the brown alga Rugulopteryx okamurae, native to the northwestern Pacific Ocean, was introduced to the Thau Lagoon (Occitania, France) due to extensive shellfish farming activities. Since 2016, this species has proliferated, completely covering the rocky habitats along the Marseille coasts (Provence, France) and becoming the dominant alga. The impact of R. okamurae on flora and fauna was investigated by comparing rocky benthic assemblages of invaded and non-invaded sites. Results showed a change in the communities, with a significant decrease in species diversity and a homogenization of algal and invertebrate populations in invaded sites. Rugulopteryx okamurae is a habitat transformer species inducing a significant habitat shift. Rugulopteryx okamurae also seems to affect the different algal strata of the benthic community, thus impacting the entire canopy. However, a weak effect of R. okamurae was observed on the functional structure of native algal communities, comparing invaded and non-invaded areas. This lack of structural variation between these zones could be explained by the degradation of habitats within the Calanques National Park, due to decades of intense overgrazing by Sarpa salpa and sea urchins.

But an impact on the ecosystem functioning should not be excluded. It is noteworthy that R. okamurae, a species presumed to prefer colder waters, has shown resistance to successive marine heat waves, allowing it to continue spreading and dominating without impairment.