Abstract

Since 2011, massive amounts of Sargassum algae are washing ashore on the coasts of the West Indies, Brazil and West Africa. These algae are supposedly coming from a new region, called North Equatorial Recirculation Region, spanning the tropical and equatorial Atlantic south of 20 °N. Tracked through satellite (Alternative Floating Algae Index, aFAI from MODIS), the extent, location and spreading of Sargassum aggregation in the NERR show large changes at seasonal and interannual time scales. Although hypothesized, the role of passive transport of algae in the observed evolving distribution of these aggregations was not explicitly tested. Here we used oceanic currents from a data-assimilative model to simulate the Lagrangian passive transport of algae from month to month over 2017. Simulations show reasonable agreement with satellite monthly distribution that validate the hypothesis of passive transport as being the main driver of the distribution changes. The seasonal cycle is driven by the North equatorial current, the North Brazil current and the Equatorial counter current. It starts with accumulation in the cen-tral Atlantic, drifting westward, then northwestward from North Brazil to Caribbean in the Spring, then splitting : part drift-ing further northwest, part returning east in the summer to accumulate off West Africa in the fall. The windage impact on Sargassum transport was also tested. Potential source and sink regions are discussed.

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