Spatial-temporal dynamics of decaying stages of pelagic Sargassum spp. along shorelines in Puerto Rico using Google Earth Engine.

Coastal social-ecological systems in the Caribbean are affected by pelagic Sargassum spp. influxes and decomposition, but most satellite monitoring efforts focus on offshore waters. We developed a method to detect and spatial-temporally assess sargassum accumulations and their decaying stages along the shoreline and nearshore waters. A multi-predictor Random Forest model combining Sentinel-2 MultiSpectral Instrument reflectance bands and several vegetation, seaweed, water, and water quality indices was developed within the online Google Earth Engine platform. The model achieved 97 % overall accuracy and identified both fresh and decomposing sargassum, as well as the Sargassum-brown-tide generated from decomposing sargassum. We identified three hotspots of sargassum accumulation in La Parguera, Puerto Rico and found that sargassum was present every month in at least one of its forms during the entire time series (September 2015-January 2022). This research provides information to understand sargassum impacts and areas where mitigation efforts need to focus.