Sargassum floating on the waters of Guadeloupe in the Caribbean.Photograph: Hemis/Alamy

Carib - The creeping threat of the Great Atlantic Sargassum Belt

Visible from space, an explosion of harmful seaweed now stretches like a sea monster across the ocean. Could robots save us from it – and store carbon in the process?

Seaweed has been having a moment. Eco-influencers and columnists rave about its benefits, in everything from beauty products to biofuels. Jamie Oliver has embraced it as a recipe ingredient; Victoria Beckham uses it to keep off the pounds. And they’re right: seaweed is packed with nutrition, it sucks up carbon and is an amazingly versatile addition to the green economy.

But one type of seaweed is not a benign force. Vast fields of sargassum, a brown seaweed, have bloomed in the Atlantic Ocean. Fed by human activity such as intensive soya farming in the Congo, the Amazon and the Mississippi, which dumps nitrogen and phosphorus into the ocean, the sargassum explosion is by far the biggest seaweed bloom on the planet. The Great Atlantic Sargassum Belt, as it’s known, is visible from space, stretching like a sea monster across the ocean, with its nose in the Gulf of Mexico and its tail in the mouth of the Congo.

“I think I’ve replaced my climate change anxiety with sargassum anxiety,” says Patricia Estridge, CEO of Seaweed Generation, a UK startup working to make seaweed commercially viable.

Sargassum’s appearance can be deceiving. It is beautiful, layered like golden mats on the surface of the open ocean. Distinguished by bubble-like formations in its stems that keep it floating on the surface, pelagicsargassum has sloshed about in the Atlantic since well before Christopher Columbus sailed across the wide Sargasso Sea: in 1492, he wrote that he feared his boat would be trapped in it. But even early witnesses recognised its value: it provides a safe harbour and breeding ground for fish, turtles and other marine life. Under the surface it teems with life, like an upside-down reef.

What is alarming, is the rate at which it is growing. Oceanographer Ajit Subramaniam, who has run scientific research expeditions in the South Atlantic for 25 years, first noticed it in 2018. “Here was something I’d never seen before,” he says. “One moment we were in the blue sea, then bam! It was all around the ship for tens, hundreds of metres.”

Unwilling to believe his eyes, he talked to other oceanographers, who confirmed that there was a huge sargassum bloom in the South Atlantic. Chuanmin Hu and his team from the University of South Florida’s (USF) optical oceanography laboratory had been monitoring it using satellite imagery since 2011 and had seen it explode in size. In June 2022, Hu estimated the size of the Great Atlantic Sargassum Belt to be 24.2m tonnes – about four times the weight of the Great Pyramid of Giza.

A sargassum landing event is a spectacular phenomenon. Vast rafts arrive without warning, often in calm sunny weather at the height of tourist season, smothering miles of coastline in golden seaweed, which piles up, sometimes metres high, turning brown and fetid as it rots.

The first places to really feel the impact of sargassum were the Windward Islands in the Caribbean. Shelly-Ann Cox, chief fisheries officer for the Barbados government, has been working on sargassum mitigation for more than a decade. She describes the environmental and economic fallout as catastrophic: “Every year we’re seeing more and more countries reporting of the influx, and the devastating impacts on tourism, fishery sectors and transport.”

Sargassum assaults harm coastal wildlife and fish, and interfere with vital infrastructure, including water and power supplies. In addition, the hydrogen sulphide released when it decays has been shown to cause a range of human health problems, from mild headaches or eye irritation to unconsciousness and worse, while a 2022 paper has linked it to an increased risk of serious pregnancy complications in women living on the coast.

Wildlife is affected when sargassum blocks light to the seabed in shallower waters, while newly hatched turtles are unable to crawl to the sea over mountains of rotting seaweed.

Sargassum blooms fluctuate: it is most abundant in summer when the sea is calm and blue, before storms break up and scatter the golden mats. But also clear from the data, is that it is growing inexorably, fed by the climate crisis. Increased sea surface temperatures, upwelling and changing currents have combined with nutrients caused by human activity such as sewage and soya farming in the basins of the great rivers of North and South America, and Africa. Sand blown from the Sahara also brings with it iron and other essential minerals.

A more basic problem, however, is how to dispose of this contaminated biomass. Some have suggested using it as a fertiliser, but the heavy metals it contains – particularly arsenic – make it dangerous to feed to plants. Composting, too, could allow the arsenic to leach into groundwater, then drinking water and the food chain, leading several Caribbean nations to ban sargassum composting. As for industrial uses, removing the heavy metals requires so much processing that it is not cost-effective.

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