Your beach at risk? 37.5m tonnes of sargassum now spans 8,800 km, fuelled by the Amazon and sewage

An immense brown tide creeps across the Atlantic, defying seasons and satellites while coastal towns brace for another uneasy summer.

What looked like harmless seaweed is now a transoceanic conveyor belt of biomass, expanding year by year and reshaping daily life from West Africa to the Gulf of Mexico.

The great Atlantic sargassum belt today

Scientists now confirm a phenomenon that once seemed improbable: a continuous, ocean‑wide ribbon of seaweed tracking across the Atlantic. Traditionally, sargassum floated in patches within the Sargasso Sea. Over the past decade and a half, it has broken free of that historic confinement and organised into a sprawling belt.

In May 2025, satellites mapped roughly 37.5 million tonnes of sargassum extending more than 8,800 kilometres. That biomass, visible from orbit, approaches twice the width of the continental United States when stretched in a line. Ships pick it up on radars. Fishers meet it at harbour entrances. Beach crews confront it with excavators at dawn.

37.5 million tonnes. 8,800 kilometres. A living belt that now dictates when you swim, fish, or open the beach bar.

What is driving the surge

Long‑term monitoring reveals a decisive shift after 2011. Researchers compiling forty years of satellite records, seawater sampling, and chemical analysis report recurrent, near‑annual blooms since the early 2010s. The belt does not arise from a single cause. It feeds on a perfect mix of nutrients, currents, and seasons.

Farming’s footprint

Fertilisers and soil wash off fields into rivers, then into the Atlantic. Those nutrients turbocharge sargassum growth far from the farms that paid for them. Between 1980 and 2020, nitrogen measured in sargassum increased by about 55%, reflecting a diet richer in run‑off.

Sewage and wastewater

Ageing pipes and overflows leak nutrients during storms and peak tourist seasons. Untreated or partially treated effluent delivers nitrogen and phosphorus that algae rapidly convert into new fronds and gas‑filled bladders that keep mats afloat.

Dust and atmospheric fallout

Particles drifting from distant continents, including mineral‑rich dust, settle into the sea. These deposits top up the nutrient mix just enough to sustain growth between river pulses.

The Amazon’s seasonal pulse

The Amazon River acts like a fertiliser pump. During wet months it injects nutrient‑laden freshwater into the ocean, seeding rapid expansion. During drier months, the belt slackens, but persistent mats remain. Ocean currents, including the North Equatorial and the Gulf Stream, then ferry these rafts across basins and into coastal traps.

Richer waters, warmer seas, and reliable currents now align to turn a natural drifter into a transatlantic production line.

When abundance becomes a hazard

A modest raft of sargassum shelters life. It provides nursery cover for juvenile fish, sea turtles, and invertebrates; more than a hundred species use it as a floating habitat. Yet excess flips the script.

• Rotting piles release hydrogen sulphide, the “rotten egg” gas that can irritate eyes and lungs.
• Thick surface mats block light, stressing corals and seagrass and slowing reef recovery.
• Decomposition consumes oxygen, creating low‑oxygen zones where fish and crabs struggle to survive.
• Breakdowns also emit methane, adding to greenhouse gases from coastal waters.
• Weeds jam engines and water intakes; in one notable case, a Florida power station temporarily shut intake systems after a mass influx.

Short bursts of hydrogen sulphide above 10 ppm can trigger headaches and nausea; coastal workers need masks and careful shifts.

Counting the costs on your doorstep

For communities from Barbados to Cancún and along the Gulf of Mexico, summer now means bulldozers, booms, and bills. Resorts pile sargassum behind dunes. Councils contract skimmers. Fishermen lose days when harbour mouths clog. Tourists change plans when the smell lingers and the water turns tea‑brown.

Municipal budgets bend under regular clean‑ups that run into millions of pounds each year across affected coastlines. Disposal adds complexity: wet sargassum is heavy, salty, and can leach when stockpiled. Transport costs surge. Landfills resist. Composters worry about sand and salt content. Meanwhile, missed bookings and lost diving trips ripple through local economies.

Key figures at a glance

Can we predict and manage it?

Forecasting has improved. Satellite sensors now map density and drift, and ocean models estimate landfall windows weeks ahead. Early warnings help hotels stagger staff, ports schedule dredging, and health teams prepare protective gear.

Practical tactics that work best in combination

• Near‑shore barriers: Floating booms guide mats to collection points before they strand and rot.
• On‑water harvesting: Skimmers and barges remove biomass where feasible, reducing coastal impacts.
• Smart beach cleaning: Light equipment avoids tearing up sand and seagrass; crews work at first light to limit odours.
• Waste handling: Dewatering on site cuts transport weight; controlled composting or anaerobic digestion can convert loads into products.
• Source control: Upgrading wastewater plants, fixing sewer leaks, and precision fertiliser use reduce the nutrient tap.

Cut nutrients at source, manage arrivals at sea, and handle landings safely: three steps that shrink the seasonal shock.

Promise and pitfalls of turning a nuisance into a resource

Sargassum holds carbohydrates, trace minerals, and organic compounds. Entrepreneurs trial biogas, fertiliser blends, packaging fibres, and even building materials. Some products look promising, especially where de‑salting and sand removal are practical. Yet quality control matters: salt can burn crops, and heavy metals or microbes can hitch a ride. Standards and testing keep risks in check.

What this means for you this season

If you live or holiday near affected coasts, plan around peaks. Check local advisories before swimming. Keep pets off fresh strandings. Beach workers should wear masks and gloves, rotate tasks, and use gas monitors where odours concentrate. Hoteliers can install small barriers, coordinate with councils, and provide guests with timely updates rather than surprise closures.

Wider context you should know

This belt did not begin with people, but people have accelerated it. Ocean gyres and variable winds naturally corral floating life. Added nutrients now feed that system harder and longer. Warmer surface waters help sargassum grow faster and float farther. The result is a new seasonal rhythm for the Atlantic, with consequences from fisheries to power plants.

Two terms help frame the change. Eutrophication describes nutrient enrichment that overstimulates growth. Anoxia refers to oxygen‑starved conditions created when that growth dies and decays. Together they explain why a golden raft that shelters hatchling turtles at sea can turn into a suffocating blanket in a shallow bay. Smarter farming, modern sewers, and precise coastal management can trim the pulse without destroying the habitat that many species still need offshore.