Multiple airborne sensors to monitor rafts and beached Sargassum in the Mexican Caribbean: Documenting different UAVs applications for management actions.

The assemblage of multiple sensors on Unmanned Aerial Systems (UAS) to collect high resolution geospatial data represents one of the most significant advances in remote sensing, including oceanographic applications. Coastal inundation of pelagic Sargassum has been thoroughly documented as a natural hazard that jeopardizes the ecological integrity of coastal ecosystems, unbalancing several livelihoods and local economies. Sargassum patches (rafts) are drifted offshore by surface ocean currents, with distinct drivers at different geographic and time scales. UAVs have revolutionized the immediate local remote sensing of Sargassum as they can identify rafts that are expected to reach the coast in terms of hours, becoming a strategic tool for rapidly management actions, bridging the on-site actions with high and medium resolution satellite detections. To obtain primary data on the extent, frequency, and magnitude of floating and beached Sargassum in the Mexican Caribbean, a rapid assessment protocol based on aerial photogrammetric techniques was implemented in the Yucatan Peninsula. We documented the arrival of sargassum rafts in the nearshore environment used to perform statistical comparisons with other remote sensing products. High resolution orthomosaics, DSMs, and 3D reality models were created to document the extent and quantity of beached Sargassum and the contiguous “brown tide” areas. Floating sargassum rafts were also identified in real time using long-range telemetry UAVs between 2 and 20 km offshore, that were consistent with field-based observations. Ocean circulation model outputs are also presented, which demonstrate that including UAV-mounted multi-sensors data acquisition is fundamental towards a comprehensive description and monitoring of the Sargassum coastal dynamics. These results strongly suggest that UAV-derived cartographic products represent an efficient tool for Sargassum-management actions, downscaling satellite detections and linking them with local observations, a strategy that needs to keep addressing as the future research agenda in Operational oceanography.