Sargassum: the seaweed that became a coastal problem

What sargassum is

Sargassum is a genus of brown seaweed. Most of its species grow attached to rocks, but two of them, Sargassum natans and Sargassum fluitans, spend their whole life floating at the surface, kept up by small gas-filled bladders and reproducing by fragmentation. They form loose mats that shelter fish, turtles and invertebrates in the open ocean. For centuries these mats were known mainly from the Sargasso Sea, the calm region of the North Atlantic named after them.

The Great Atlantic Sargassum Belt

In 2011, satellite images showed something new: a recurring band of sargassum stretching across the tropical Atlantic, from the coast of West Africa to the Gulf of Mexico, well south of the Sargasso Sea. Researchers named it the Great Atlantic Sargassum Belt. At its 2018 extent it ran for about 8,850 kilometres. The amount of seaweed varies strongly from one year to the next, but the trend since 2011 points upward: satellite estimates reached 38 million tonnes of wet seaweed at the July 2025 peak and about 34 million tonnes in June 2026.

Why the belt appeared and keeps returning is still being studied. Nutrients carried to the ocean by large rivers, by upwelling off West Africa and by dust from the Sahara feed the seaweed; currents and winds carry it westward; water temperature affects how fast it grows. Recent research indicates that the drivers themselves are changing, from physical forcing in the early years to ecological control now, which makes the belt more likely to persist. Monthly bulletins from the University of South Florida track it from space and give coastal authorities a few weeks of warning.

What happens when it strands

At sea, sargassum is an ecosystem. On a beach, it is a problem. Piles a metre high can build up within days. As the seaweed rots, it releases hydrogen sulphide and ammonia: the smell of rotten eggs reaches homes and hotels, health authorities issue guidance for residents and clean-up workers, and metal fittings corrode. Beneath the mats, seagrass beds and coral are smothered, turtle nesting is disrupted, and small-scale fisheries lose access. Tourism, the main income of many of the affected islands, suffers first.

Clean-up is slow and expensive. The seaweed is heavy with water and sand, has to be collected within a day or two before it decomposes, and then has to go somewhere. In the French Caribbean departments the state has run dedicated sargassum plans since 2018 to fund collection, barriers and research; similar programmes exist in Mexico, the Dominican Republic and the smaller islands.

How coasts manage it today

• Forecasting: satellite bulletins and drift models announce arrivals.
• Barriers: floating booms deflect the mats away from beaches and harbours, with mixed results in rough weather.
• Collection at sea: boats gather the seaweed before it reaches the shore, which keeps it cleaner and fresher.
• Collection on the beach: machinery and hand crews, which also remove sand and disturb nesting sites.
• Storage and disposal: spreading, burying or landfilling, which moves the problem inland.
• Attempts at reuse: compost and fertiliser, biogas, building blocks, paper and bioplastics have all been tried at small scale. The composition limits them: salt, sand, a large water fraction and heavy metals such as arsenic that the seaweed takes up from seawater.

What Salacia sees in it

Salacia treats sargassum as a material rather than a nuisance. Its process is built for a wet, salty, variable marine feedstock and keeps the salts and heavy metals out of the fuel path from the first conversion step. The scale is deliberately modest against the problem: one platform at full capacity would use 0.4 million tonnes of wet sargassum a year, a fraction of the 2 to 10 million tonnes that models estimate strand on Atlantic and Caribbean coasts annually. How the conversion works is explained on the Technology page; how the seaweed gets from the shore to a plant is the subject of the supply-chain page.