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Field and laboratory experiments are designed to measure Sargassum biomass per area (density), surface reflectance, nutrient contents, and pigment concentrations. An alternative floating algae index‐biomass density model is established to link the spectral reflectance to Sargassum biomass density, with a relative uncertainty of ~12%. Monthly mean integrated Sargassum biomass in the Caribbean Sea and central West Atlantic reached at least 4.4 million tons in July 2015. The average %C, %N, and %P per dry weight are 27.16, 1.06, and 0.10, respectively. The mean chlorophyll‐a (Chl‐a) concentration is ~0.05% of the dry weight. With these parameters, the amounts of nutrients and pigments can be estimated directly from remotely sensed Sargassum biomass. During bloom seasons, Sargassum carbon can account for ~18% of the total particulate organic carbon in the upper water column. This study provides the first quantitative assessment of the overall Sargassum biomass, nutrients, and pigment abundance from remote sensing observations, thus helping to quantify their ecological roles and facilitate management decisions.

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