Spread and Population Dynamics of <i>Rugulopteryx okamurae</i> Across the Azores Archipelago (NE Atlantic)

ABSTRACT Rugulopteryx okamurae is a recently introduced brown macroalga that has expanded rapidly across the Azores (NE Atlantic). This study provides an update of the archipelago‐wide spatial spread and the first assessment of its local population dynamics. Multi‐year surveys (2019–2024) revealed a swift progression from the initial detection in São Miguel in 2019 to records in eight of the archipelago's nine islands by 2024. Early species occurrences were consistently located near main islands' harbours, indicating maritime traffic as the primary vector of introduction and inter‐island dispersal. Once established, populations persist and frequently reach high cover along extensive rocky coastlines. Monthly demographic sampling demonstrated year‐round persistence with pronounced seasonality: biomass and thallus length peaked in spring and declined sharply in summer due to widespread thallus detachment, while density fluctuated moderately, increasing in spring and again in late autumn. Asexual reproductive structures were most abundant in late winter–spring, with no record of gametangial sori. Size‐frequency analyses revealed up to seven overlapping cohorts, indicating continuous recruitment apparently maintained through both asexual reproduction and vegetative propagation. Generalised linear models detected significant negative influence of temperature and a significant positive influence of photoperiod on the species' thallus length, biomass and reproduction, suggesting that increasing daylength in winter and spring may trigger its growth, which is then halted as seawater warms up. Overall, these findings indicate that R. okamurae is now a widespread and persistent invader in the Azores, with its expansion driven primarily by maritime vectors and influenced by regional‐scale variation in environmental conditions associated with seasons.