Proliferation of <i>Undaria pinnatifida</i> along the Atlantic coast of the Iberian Peninsula

Abstract The spread of non-indigenous species is a critical concern for marine ecosystems, particularly in regions with high biodiversity and economic reliance on coastal resources. This study investigates the distribution, abundance, and expansion of the invasive kelp Undaria pinnatifida along the Atlantic coast of the Iberian Peninsula. A comprehensive survey was conducted in 2024, covering 59 locations across Spain and Portugal. This dataset was complemented by biodiversity records from citizen science platforms. Historical records were also reviewed to assess changes in its distribution over time. Results indicate that U. pinnatifida has notably increased its presence in Portugal since its first record, now occurring at 16 sites (compared to only 2 in 2007), while remaining well-established in Galicia. However, the species co-occurs with native kelps and, at this stage, does not dominate the communities where it has established itself. Furthermore, it remains absent from southern Iberia and the Bay of Biscay. Hence, environmental conditions and biotic interactions might be shaping their distribution, which warrants further investigation. Given the ongoing environmental changes and human-mediated dispersal, continuous monitoring is essential to track future range shifts. This study also showcases how integrating citizen science data can enhance the spatial and temporal resolution of non-indigenous species monitoring.