• Sargassum is a toxic alga that proliferates between March and October, primarily in the Caribbean, the Gulf of Mexico and Florida.
• After several years of anomalous proliferation, sargassum quantities have reached record levels, creating a range of environmental and economic problems.
• Some people are finding ways to limit the damage — or to turn the phenomenon into an opportunity.

Close your eyes and think of Miami or the Caribbean islands, and you naturally picture paradise beaches. The reality captured in photographs these past few weeks looks rather different: brown patches floating on the ocean, washing up along the coastlines and harbours of seaside resorts. This is Sargassum (sargasso), a toxic alga that proliferates between March and October. In April, when the blooming season had only just begun, the University of South Florida calculated that it had already reached 31 million tonnes — 40 per cent more than the previous record set in June 2022. According to CNN's reporting in mid-May, it forms a strip across the ocean stretching 8,850 kilometres.

Why the sargassum bloom is out of control

Sargassum is a brown alga found primarily in the Atlantic and the Caribbean. When conditions are favourable — meaning high temperatures, abundant nutrients and an absence of strong currents — it reproduces rapidly through vegetative growth, forming enormous floating mats. Since 2011, experts consulted by CNN explain, the Caribbean, the Gulf of Mexico and southern Florida have been inundated by ever-increasing quantities of sargassum.

In itself, this alga is an integral part of the marine habitat. An anomalous proliferation, however, brings harmful consequences — and not merely aesthetic ones. These brown patches on the water's surface block light from reaching corals and marine vegetation, reduce dissolved oxygen levels and disrupt the food chain. They also harbour larvae and other marine organisms that irritate swimmers' skin. Once they wash ashore, they rot, releasing foul-smelling gases — primarily hydrogen sulphide, which is notoriously toxic.

While sargassum blooms are a natural phenomenon, this uncontrolled pace is driven on one hand by rising water temperatures, and on the other by an excess of nutrients — particularly nitrogen, arriving either from the atmosphere (carried by desert sand or released through the combustion of fossil fuels) or from the land via agricultural fertilisers. The Amazon, in particular, has just endured two consecutive years of extreme drought: the first rains washed away the now-dry organic matter, feeding the sargassum.

Matthew-P Rogers @Rogers_M_P

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In the Caribbean, we know about Sargassum but it was still shocking seeing a normally pristine bay filled to the brim with toxic, smelly seaweed. I spent a month with 10 other Masters students from around the world at NIOZ course in curacao to try and understand what's going on.

♥ 12💬 2 15 mag 2025 · Apri su X →

Innovations to limit the damage caused by sargassum

It comes as no surprise that mounds of sargassum on Caribbean island beaches are a serious blow to tourism — as well as to fishing, since the seaweed becomes tangled in nets and can even tear them under its weight. But there are also entrepreneurs and local organisations rolling up their sleeves and trying to turn the phenomenon into an economic opportunity. An article published by The Conversation reports that Legena Henry, a university lecturer from Barbados, has developed a technique for producing a seaweed-based biofuel that can even power cars. Algas Organics, meanwhile, is a Saint Lucia-based company that sells sargassum-based plant tonics.

At the same time, islands such as Jamaica have adopted early warning systems. Normally used to prepare for hurricanes, in this case they flag ocean currents that could carry large quantities of seaweed towards the coastlines, giving fishermen time to prepare. The startup Sos Carbon, which emerged from the Department of Mechanical Engineering at the Massachusetts Institute of Technology, has designed a boat that collects sargassum before it is washed ashore.