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The effect of inertia (resistance to a change in velocity of buoyant finite-sized objects) on the advection of pelagic Sargassum, a macroalgae, is a function of the size and density of natural Sargassum rafts. Here we present observations of Sargassum density and an approach for estimating an effective radius of Sargassum rafts from remote-sensing observations. This allows the existing theoretical framework for Lagrangian modeling of inertial effects on spherical particles to be applied to Sargassum. Accounting for inertia yields up to a 20% increase in Sargassum export from the Sargasso Sea southward and provides a return pathway to the tropics that may be important to maintaining a self-sustaining population. Resolving inertial effects also leads to increases in retention in the Gulf of Mexico and Caribbean Sea, where Sargassum inundation events are increasingly common. Including inertial effects in models of Sargassum advection could improve predictions of these events.

Plain Language Summary Sargassum seaweed has been washing up on beaches in the tropical Atlantic more frequently and in greater volume in recent years. Few measurements have been made of the physical properties of Sargassum that would allow more accurate modeling and prediction of these events. In this study, the size, weight, and shape of Sargassum rafts are calculated from both field measurements and satellite observations. These properties are then applied to a model of Sargassum transport in the Atlantic, where they cause more Sargassum to enter and then remain in the Caribbean and Gulf of Mexico. This suggests forecast models of Sargassum washup events should include these effects.