Great Atlantic Sargassum Belt

Pelagic Sargassum is free-floating brown macroalgae, rather than seaweed rooted to the seabed. Two holopelagic species, Sargassum natans and Sargassum fluitans, are prominent in the North Atlantic. The Great Atlantic Sargassum Belt is the recurrent accumulation of these floating algae across the tropical Atlantic and adjacent seas. It is not a solid, continuous raft: satellite imagery records variable patches and concentrations that can collectively extend over thousands of kilometres.

Satellite records show a major expansion of Sargassum across the central Atlantic in 2011, after relatively low abundance there in much of the preceding decade. Wang and colleagues reported a belt extending from West Africa across the Atlantic to the Caribbean and Gulf of Mexico, with a very large observed accumulation in June 2018. Currents and winds transport and concentrate the algae. Nutrients are supplied through multiple pathways, including river discharge, upwelling and vertical mixing; the relative contribution of these sources and conditions varies. The US Environmental Protection Agency describes the belt as seasonal and identifies nutrients, temperature, salinity, winds and circulation as interacting factors. No single driver explains every annual fluctuation.

At sea, floating Sargassum rafts provide food, shelter and breeding habitat for many organisms, including juvenile fish, turtles and seabirds. NOAA identifies Sargassum as essential fish habitat for several managed fisheries in relevant US waters. When large masses are driven onto coasts, however, accumulations can smother nearshore organisms and reefs, interfere with water infrastructure and disrupt tourism. Decomposition on beaches can release hydrogen sulfide, which may irritate the respiratory system. The ecological consequences therefore depend strongly on location and whether the algae remains offshore or strands in large quantities.

Researchers use satellite observations, field sampling and ocean models to track abundance and investigate the belt's changing sources and pathways. NOAA and partners publish tracking and forecasting products intended to help anticipate coastal arrivals. These tools address a difficult problem: ocean currents and weather can shift both the location of offshore mats and the likelihood of beaching, while the abundance of the belt changes markedly from year to year.