Imagine a beach in the French Caribbean: turquoise waters, coconut palms, light-coloured sand. Yet on some mornings, the sand disappears under a thick brown carpet, and the smell of rotten eggs drifts to the houses.
These algae are sargassum. Since 2011, their massive wash-ups have disrupted the lives of many residents in Guadeloupe, Martinique, and throughout the Caribbean. Yet out at sea, they support a teeming life, and some already see them as a raw material. A scourge or a resource? The answer is more nuanced than it seems.
What is sargassum?
Sargassum is a type of brown algae. The species that reach the West Indies, Sargassum natans and Sargassum fluitans, have a rare characteristic: they spend their entire lives floating, never attaching themselves to the seabed. Scientists call them holopelagic.
In the open sea, their rafts form true oases of life in the middle of the blue. Hundreds of species find food and shelter there: fish such as mahi-mahi, seabirds, and even young sea turtles.
Why are sargassum wash-ups dangerous?
The problem begins when these rafts wash up by the thousands of tonnes. On the shore, the algae pile up, block access to beaches and ports, and suffocate some coastal environments.
Then comes decomposition. Deprived of oxygen, the sargassum releases hydrogen sulfide (H2S), the gas responsible for the rotten egg smell, as well as ammonia.
These emissions are more than just a nuisance. In Martinique, doctors followed patients who came for treatment during the massive wash-ups of 2018: neurological, digestive, and respiratory problems were the most common complaints. More recent studies also suggest possible effects from prolonged exposure, even at moderate concentrations. Sensors now measure the gas near the affected areas.
Fishing, tourism, boating: the local economy also suffers. For some coastal communities, the sea, usually a source of life and income, becomes a daily burden.
Where does the sargassum in the West Indies come from?
Sargassum is nothing new in the Atlantic. The Sargasso Sea, off the coast of Bermuda, even owes its name to it.
What has changed is the scale. Since 2011, satellites have observed a massive band of algae crossing the tropical Atlantic, from West Africa to the Gulf of Mexico: the Great Atlantic Sargassum Belt. In June 2018, it stretched 8,850 kilometers and exceeded 20 million tonnes. Winds and currents push part of this biomass toward the Caribbean.
Why such a proliferation? Researchers point to several drivers: nutrients carried by the Amazon, deep-water upwelling off the coast of West Africa, and ocean circulation. The sargassum in this belt also contains more nitrogen and phosphorus than that in the Sargasso Sea. The exact contribution of each source, including human activities, is still being debated.
One thing is certain, however: the phenomenon extends far beyond the affected islands. It has to be managed across the entire Atlantic.
Can sargassum be put to use?
Faced with these mountains of algae, the idea quickly becomes obvious: rather than storing them, why not use them?
There is no shortage of ideas. Sargassum contains alginates, natural thickeners widely used in industry. Research teams are also exploring its conversion into fertiliser, bioplastics, recycled paper, energy, or building materials such as adobe bricks and road coatings.
On paper, the scenario is appealing: turning an environmental burden into a local activity, with new jobs to show for it.
One major obstacle holds back this momentum: arsenic. A study conducted in Guadeloupe measured high levels in these algae, all year round. Their arsenic content varies with how deficient they are in phosphorus, an element chemically similar to arsenic.
It is therefore impossible to view sargassum as a natural fertiliser or a risk-free raw material. Every use requires testing, sometimes a decontamination step, and appropriate processes. There are also logistical challenges: quickly collecting highly variable volumes from one year to the next, transporting them, and processing them at a reasonable cost. Markets exist, but none are yet operating on a large scale.
Sargassum: learning to live with a changing ocean
Sargassum tells us something about our relationship with the ocean. A seaweed essential to life at sea becomes a major problem when it accumulates in the wrong place. Conversely, what we treat as waste can open up unexpected prospects.
The priority remains to protect residents and coastal ecosystems: better predict arrivals, quickly collect washed-up algae, and develop safe processing industries. If some of these industries create jobs and useful resources tomorrow, all the better, provided this can be demonstrated without moving the pollution elsewhere.
Do you live in the West Indies or are you going there soon? Before going to the beach, check local bulletins on wash-ups and air quality. Understanding the ocean before judging it: that may be the best lesson these algae leave us.