Spatiotemporal Quantification of <i>Rugulopteryx okamurae</i> in Beach Wrack Using UAV Multispectral Remote Sensing

ABSTRACT Excessive beach accumulations of stranded macrophytes, also known as wrack, are a growing challenge for coastal ecosystems and management, particularly when dominated by invasive species. The Asian brown alga Rugulopteryx okamurae has rapidly proliferated along the southern coast of Spain, causing significant ecological and socioeconomic impacts. This study applied unmanned aerial vehicle (UAV) and multispectral imaging integrated with GIS‐based analyses to quantify temporal and spatial variability of R. okamurae wrack on four Atlantic beaches of Cádiz (Marisucia, Hierbabuena, Punta Paloma and Lances North) during peak accumulation events in the summer of 2024. Orthorectified mosaics and normalised difference vegetation index (NDVI) rasters were used to delineate algal coverage and assess temporal change. Results revealed strong spatial heterogeneity, with Punta Paloma and Marisucia showing substantial biomass increases (+225% and +88%, respectively), while Hierbabuena and Lances North remained stable or declined. These contrasts reflected differences in beach morphology, exposure, wind regimes and management activities. The UAV–NDVI workflow provided a reliable, high‐resolution framework for mapping stranded macroalgae, capturing both coverage and density dynamics across complex coastal environments. Findings highlight the need for site‐specific management that integrates environmental drivers and human activity to mitigate R. okamurae accumulation and its cascading ecological and socioeconomic impacts. UAV‐based remote sensing represents a scalable tool for early detection and adaptive management of invasive wrack, supporting conservation strategies that balance ecological integrity with beach use and climate resilience.