Temporal Shifts in Subtidal Rocky Benthic Communities Associated With the Invasion of Rugulopteryx okamurae in a Port Environment

ABSTRACT

The invasive brown alga Rugulopteryx okamurae , native to the northwestern Pacific, has rapidly expanded along the European Atlantic coast, reaching the Basque Coast in 2023, and was likely already present in 2022. Using annual data from a monitoring survey conducted between 2010 and 2025, this study assesses the spatio‐temporal variability of subtidal rocky benthic communities at two locations within a port environment on the western Basque Coast affected by the presence of R. okamurae . Following its establishment, marked shifts in benthic community composition, structure and functioning were detected. Taxa richness, density and diversity declined sharply from 2022 onwards, whereas assemblages from both study locations became increasingly similar, indicating strong community homogenization. Multivariate analyses revealed a rapid reorganization of communities coinciding with the arrival of R. okamurae . The diverse native macroalgae and associated invertebrates present prior to the invasion were replaced by a single persistent community dominated by R. okamurae and the bryozoan Tubulipora spp. Functional and structural analyses showed a pronounced loss of calcareous and structurally complex macroalgae, together with the collapse of basal and crustose layers and a transition towards canopy‐dominated assemblages largely associated with the dominance of the invasive species. Overall, our results suggest that R. okamurae acts as a strong habitat‐modifying species in this port environment, rapidly transforming subtidal rocky communities and highlighting the urgent need for early detection, long‐term monitoring and management strategies in newly invaded areas.