Sargassum info & research

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What sargassum is

Sargassum is a brown seaweed that floats its whole life, held up by small gas-filled bladders. Floating mats have been known in the Sargasso Sea since Columbus. They give shelter to fish, shrimp, crabs, birds and turtles.¹

Since 2011, satellites have seen a new band of weed called the Great Atlantic Sargassum Belt. It often stretches from West Africa to the Caribbean and the Gulf of Mexico. In June 2018 it was about 8,850 km long and held more than 20 million tonnes of weed. Researchers called it the world's largest seaweed bloom.¹ Its shape mostly follows the ocean currents. Year-to-year changes seem linked to nutrients: upwelling off West Africa in winter and Amazon River water in spring and summer.¹

Seasons

The belt builds up in the warmer months and is mainly a summer event.¹ USF reports seasonal peaks of up to 39 million tonnes. Its satellite system puts out a monthly bulletin with the current state and an outlook for the next months.⁵ In some months from 2018 to 2023, more than 2 million tonnes reached Caribbean coasts, and about 10 million tonnes in 2018 alone, by one model estimate.⁵

On our maps, weed seen in mid-Atlantic usually takes weeks to months to reach the islands, not days. That is why the History view on the map can show up to 6 months.

Health: the rotting-weed smell

Weed out at sea is not the main problem. About 48 hours after it lands and starts to rot, it gives off hydrogen sulfide (rotten-egg smell) and ammonia.^2,3

• Symptoms reported include eye and airway irritation, nausea, headache, dizziness and confusion. People with asthma or other breathing problems are more sensitive.³
• In 2018, doctors in Martinique and Guadeloupe reported about 11,400 cases of acute hydrogen sulfide exposure linked to rotting weed.^2,3
• In a study of 154 patients there, the most common complaints were neurological, digestive and breathing problems.⁴
• The US EPA notes a possible higher risk of early preeclampsia for pregnant women exposed to these gases.⁶
• Its advice: stay away from piles of rotting weed if you feel symptoms, and limit time near them if you have asthma.⁶ Public-health advice in France favours collecting weed in shallow water before it reaches the shore.² Crews clearing large amounts face a workplace hazard.²

Uses and research

Arsenic comes first. Sargassum soaks up arsenic from seawater, more so where the water is low in phosphorus.⁷ Samples stranded in Mexico held 12.7–62.9 mg/kg of the more toxic inorganic arsenic (dry weight).⁸ In Barbados, 17% of 23 samples were above the EU limit for fertilizer. 22 of 23 were above a common limit for farm soil. All were above the limits for animal feed.⁹

• Fertilizer and gardens. In a WWF-Mexico and STINAPA Bonaire test, vegetables grown in 50/50 dried sargassum and potting soil had much more arsenic and cadmium than vegetables grown in plain soil (bok choy 37× the arsenic). The researchers advise against using raw weed on food crops until more is known.¹⁰ That is why raw weed should not go straight into a food garden.
• Compost. A Florida study found sargassum compost could grow radishes. Most metals were within guidelines, but arsenic (6.64–26.5 mg/kg) was above one residential soil limit. The authors see it as suited to fill and some farming uses.¹¹
• Rinsing and treatment. In a lab proof of concept, hot water (90 °C, 15 min) cut total arsenic from 62.2 to 7.2 mg/kg. Adding a citric-acid step brought it to 0.8 mg/kg.¹² These are small trials, not a proven farm process.
• Biostimulants, asphalt and concrete. Companies in Mexico (for example Carbonwave) make biostimulants for soil and crops. Public agencies there are testing ground sargassum in asphalt and concrete.¹³ Lab tests of cement with 5–7.5% sargassum looked at its strength and heat insulation.¹⁶
• Building blocks (Sargablock). Blocks made from about 40% sargassum and 60% other organic material, pressed and dried in the sun. Houses have been built with them in Quintana Roo, Mexico.^14,13
• Biogas. Raw weed breaks down slowly in digesters. Pressing out the liquid gave about twice the methane of raw weed (159.7 L per kg of volatile solids).¹⁵
• Bioplastics. A 2023 Green Chemistry study modelled a calcium-alginate bioplastic made from sargassum. It looked economically feasible. Its baseline climate footprint was higher than PLA or PET, and much lower when run on low-carbon energy.¹⁷

How our forecast works, and its limits Beta

• We map weed seen by satellites (NOAA AOML/USF AFAI near the Caribbean; Copernicus Marine for the whole Atlantic). Then we drift it with ocean-current and wind forecasts for the next 72 hours, and stop it at coastlines.
• Clouds hide weed, and thin mats can be missed. An empty patch of sea does not always mean clean water.
• In our own back-test for 2025–26 shore-days ("weed within 15 km in the next 72 h"), the shore forecast was right about 65% of the time. A coin flip would be 50%. The live map shows this score.
• Past 3 days we have no reliable forecast. The History view shows where weed travelled, not where it will go.
• The forecast is a guide, not a promise. Check the beach and local advice before you go. Beach reports from people on the shore help us test and improve it.