UAV multispectral validation of Sentinel-2 super-resolution toward a scalable observatory of the invasive seaweed Rugulopteryx okamurae

The brown seaweed Rugulopteryx okamurae is an aggressive invader of the Strait of Gibraltar and the Gulf of Cádiz (SW Spain), forming massive intertidal strandings whose management requires frequent, large-area observation. Coupling UAVs with satellite super-resolution could enable operational, low-cost monitoring, yet the reliability of super-resolved imagery over dynamic coastal habitats has not been thoroughly evaluated. Using UAV multispectral orthomosaics (DJI Mavic 3 Multispectral, ~5 cm) as a high-resolution reference dataset, we assessed two Sentinel-2 super-resolution products, S2DR3 (1 m) and SEN2SR (2.5 m), over an urban beach affected by R. okamurae-dominated strandings in the city of Cádiz. Three UAV flights spanning about two hours sampled different water levels and illumination conditions during the same tidal cycle and were compared pixel-by-pixel across spectral bands and the Normalized Difference Vegetation Index (NDVI). Over exposed seaweed, the 1 m product (S2DR3) matched drone reflectance and NDVI far better than the 10 m and 2.5 m products (NDVI RMSE 0.09 vs 0.20 and 0.28), capturing the seaweed signature at metre scale and enabling finer delineation of strandings. The UAV series further showed that acquisition conditions strongly influence the signal: the NDVI of the same patch varied by about 0.14 across tide and illumination states, highlighting tide-synchronised, low-tide imaging as a key requirement for reliable tracking. Moreover, drone validation revealed a radiometric anomaly in one band of one product, which would have remained undetected without the UAV reference data. These results show that UAVs are critical for validating satellite super-resolution, and that super-resolved Sentinel-2 acquired at low tide can provide the foundation for a scalable, metre-scale observatory of R. okamurae strandings along the Andalusian coast.