Monitoring the Colonisation Dynamics of Rugulopteryx okamurae on Algal Communities in the Northwestern Mediterranean Sea

ABSTRACT

Biological invasions are a major driver of biodiversity loss, causing profound and often irreversible ecological changes. Among these invaders, the brown alga Rugulopteryx okamurae provides a striking example. Since its arrival in 2016 in Calanques National Park (Marseille, southern France), it has drastically transformed underwater landscapes by reshaping native algal communities. To better understand the mechanisms underlying its colonisation, we conducted an experimental study involving the complete removal of existing algae within experimental quadrats. Using the photoquadrat method, we monitored the spatio‐temporal dynamics of algal succession. In parallel, we performed an independent field survey to quantify seasonal changes in biomass over time. Our results show that turf‐forming algae, characterised by rapid growth, are the first to recolonise cleared surfaces. This initial colonisation phase is rapidly followed by the massive establishment of R. okamurae , which reached 100% cover within 1 year. This expansion was accompanied by a strong restructuring of benthic communities and a marked decline of native algae, whose relative abundances dropped below 2% over time, highlighting the species' ability to thrive after disturbance. In addition, we observed that the emergence of the invasive alga Asparagopsis armata in April significantly reduced R. okamurae cover, suggesting biotic interference and potential allelopathic competition. Overall, R. okamurae stands out as a highly impactful invader whose rapid dominance and interactions with other invasive species emphasise the urgent need to improve our understanding and management of marine invasions to mitigate biodiversity loss.