Improving satellite monitoring of coastal inundations of pelagic Sargassum algae with wind and citizen science data

Massive blooms of pelagic Sargassum algae have caused serious problems to coastal communities and ecosystems throughout the tropical Atlantic, Caribbean Sea, and Gulf of Mexico since 2011. Efforts to monitor and predict these occurrences are challenging owing to the vast area impacted and the complexities associated with the proliferation and movement of Sargassum. Sargassum Inundation Reports (SIRs) were first produced in 2019 to estimate the potential risk to coastlines throughout the Intra-American Sea at weekly intervals at 10 km resolution. SIRs use satellite-based data products to estimate beaching risk from the amount of offshore Sargassum (quantified by a Floating Algal density index). Here we examine whether including wind metrics improves the correspondence between the offshore Floating Algal density index and observations of Sargassum along the coastline. For coastal observations, we quantified the percent coverage of Sargassum in photos obtained from the citizen science project "Sargassum Watch" that collects time-stamped, georeferenced photos at beaches throughout the region. Region-wide analyses indicate that including shoreward wind velocity with SIR risk indices greatly improves the correspondence with coastal observations of Sargassum beaching compared to SIR risk indices alone. Site-specific analyses of photos from southeast Florida, USA, and data from a continuous video monitoring study at Puerto Morelos, Mexico, suggest potential uncertainties in the suite of factors controlling Sargassum beaching. Nonetheless, the inclusion of wind velocity in the SIR algorithm appears to be a promising avenue for improving regional risk indices.