Monitoring pelagic Sargassum in the Atlantic Ocean from space: Principles and practices.

While pelagic Sargassum as a critical habitat to marine animals is known to be abundant in the Sargasso Sea, Gulf of Mexico, and the Caribbean Sea, attempts to detect this brown macroalgae from space did not start until 2006 when the proof of concept was demonstrated with medium-resolution satellite sensors. The annually recurrent Great Atlantic Sargassum Belt (GASB) since 2011 motivated efforts to develop new algorithms and approaches to detect and quantify Sargassum using data collected by various satellite sensors. This is mainly because of this macroalgae's harmful impacts on the coastal environments. Here, we review the principles and practices of using satellite remote sensing to map, quantify, and monitor pelagic Sargassum in the Atlantic Ocean. We first present the concept of how Sargassum can be detected, discriminated (against look-alikes), and quantified, where four types of data products are defined to meet the various needs. Then, we present the various published approaches in realizing such a concept using data collected by different satellite sensors. Following this concept and using recently developed algorithms and medium-resolution satellite data, we show the spatial distribution patterns and temporal changes of the GASB as well as a near real-time system to monitor Sargassum in the GASB. Finally, we discuss the gaps in the current technology and propose pathways forward to fill these gaps.