Hakai Magazine

Coastal science and societies

Danger Toxic Gas sign on beach in Brittany, France
Several beaches in Brittany, France, were closed last summer due to a toxic green tide that may have killed three people. Photo by Loic Venance/AFP via Getty Images

France’s Deadly Seaweed

Fueled by agricultural runoff, rotting seaweed on Brittany’s beaches is becoming an environmental and public health emergency.

Authored by

by Lorraine Boissoneault

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André Ollivro has spent years stomping around the beaches of Brittany, France, putting on a show for the cameras. Sometimes he wears a gas mask and carries a hydrogen sulfide monitor. On other occasions, a pitchfork and wheelbarrow serve as props. He’s often in a red cap and always fiery about the scourge of green algae, which, for decades, have encrusted the bays of Brittany. For nearly 20 years now, the retired engineer and head of the environmental group whose name translates to Stop the Green Tides has played a sort of Chicken Little to the French region’s disinterested masses.

“The beaches are becoming landfills,” Ollivro says. “Landfills of rotting green algae that kill the living.”

Included among those he says have succumbed to the green tide: dozens of wild hogs, several domestic dogs, a horse, and as many as six people, including three in the summer of 2019, although the precise cause of the human deaths has not been conclusively confirmed. (A blood test that can detect hydrogen sulfide intoxication was not carried out in the autopsies.)

The presumed killer is commonly called sea lettuce, an apt name for algae that resemble emerald-green plastic wrap. The two main species that grow along the Breton coast—Ulva armoricana and Ulva rotundata—live attached to the rocks of the intertidal zone. When the fronds break free of their mooring, they continue to grow while floating through the water. The algae are native to the region, and are ubiquitous, but in the 1970s things started to change.

Sea lettuce on beach

Sea lettuce, the common name for a genus of algae called Ulva, blankets the Baie de Douarnenez in Brittany, France. Photo by ImageBroker/Alamy Stock Photo

Since then, each spring and summer, unusually large masses of the seaweeds have festooned swaths of the shoreline. The carpet sometimes thickens to a meter, completely obscuring the sand. Once the Ulva starts decaying, it stinks of rotten eggs and emits hydrogen sulfide, which is thought to be responsible for the mammalian deaths. When people or animals walk on the seaweed sludge, perhaps assuming it’s safe because of the dried brown crust, they break through to the decaying organic matter below and release the noxious gas.

Ollivro blames one industry for the algal menace: intensive agriculture. Nitrogen from fertilizer and animal waste runs into the region’s rivers and out to sea, creating an all-you-can-eat buffet for the algae, which grow in profusion with the extra nutrients. Between 2007 and 2018, green tides hit a total of 114 different sites along the coast, with the algal accumulations from that time covering an area equal to 601 soccer stadiums.

The green tides are a bane to the local tourism economy, a hazard to the environment, and now a public health problem. But even as some beaches are repeatedly hit each year, the green tides only affect about five percent of the coastline, making it an easy problem to ignore or downplay. In a region whose lifeblood is agriculture—Brittany provides the rest of France with 56 percent of its pork products, 44 percent of its eggs, and 33 percent of its poultry—an inability to muster the political will to overhaul agricultural practices has stalled any meaningful action.

The agitation has mostly come from individuals like Ollivro, coordinating with or leading environmental groups to bring lawsuits against the government for its inaction in solving the crisis and for some of the deaths. Ollivro, for instance, helped in a lawsuit brought forward by the family of truck driver Thierry Morfoisse who died suddenly during his work removing mounds of rotting algae from the beach. His death occurred immediately after he deposited the last load, presumably because he was overcome by the deadly fumes.

André Ollivro

André Ollivro has been raising awareness about the dangers of algae blooms for years. Photo by Idealink Photography/Alamy Stock Photo

After decades of apathy, the increasing pressure to do something about the issue is nearing a breaking point. Activists and journalists whose work has drawn public attention have been denounced by protestors, had their phones tapped, and even received death threats. Climate change is now turbocharging algal growth, and, as the number of deaths of people presumably intoxicated by the algae creeps up, the political winds seem to finally be changing. What’s unclear is if it’s already too late.


The connection between agriculture and green tides is what drew investigative journalist Inès Léraud to Brittany in 2015. At the time, she thought her investigation might last three months. But understanding the role of agriculture and its political power was a convoluted process and three months turned into three years.

“It took me at least two years to create trusting relationships with farmers and for them to tell me about their working conditions, their relationships with food industry groups,” Léraud says. “In the beginning, I ran into a lot of shut doors. I had to spend a lot of time in cafés, drinking coffee with people, just talking.”

The end result of her investigation is a 160-page graphic nonfiction exposé called Algues vertes: l’histoire interdite (Green algae: the forbidden history), published in the summer of 2019. Léraud’s story dives back to the Second World War, when France began recovering with help from the United States’ Marshall Plan and revolutionized its agricultural sector. In Brittany, this meant the introduction of tractors, hybrid corn, pesticides, and fertilizers, all to maximize the food production process.

By the early 1970s, Léraud discovered, the earliest green tides caught the attention of municipal councilors who asked scientists to determine why the algae blooms were occurring. The phenomenon was still relatively small scale, but nobody wanted smelly beaches. A number of researchers—some at government-funded institutes, others working through universities—quickly singled out two culprits: phosphates from detergents and nitrogen from farming.

man shows toxic seaweed on French beach

Decomposing green algae smell like rotten eggs and can emit toxic hydrogen sulfide. Photo by Loic Venance/AFP via Getty Images

The suggestion that nitrogen might be a leading factor in the green tide phenomenon was vehemently countered by the agriculture industry, the federal minister of agriculture, and a group whose name translates to the Scientific and Technical Institute for the Environment and Health (ISTE). The ISTE, led by a scientist whose work cast doubt on the role of nitrogen and intensive agriculture as causes for the green tides, began publishing books and conducting lectures to sway public opinion. The group is funded by numerous food corporations, including SunBird, Père Dodu, and others. Although ISTE’s work has been heavily criticized by other scientists, Léraud says it raised enough doubt that politicians dragged their feet in implementing new regulations of the agricultural industry.

As Léraud dug deeper into the connections between the food industry and political lobbies—“assembling a 100,000-piece puzzle,” as she describes it—her work began drawing ire. In 2016, the news organization where Léraud is a reporter aired a segment on Alain Ménesguen, a scientist who had been studying green algae for decades and repeatedly ran into trouble for publishing his research. After the segment ran, Ménesguen told Léraud he’d been chastised by his department for speaking out about the impact of intensive agriculture and its link to green tides. He told her he wouldn’t appear in the media anymore.

And then, at a journalism festival, Léraud met a former French spy who shared some of his intelligence know-how. From him, Léraud learned that one of the huge food conglomerates had tapped her phone and computer.

“It definitely makes you a little paranoid,” Léraud says. “But it can be stimulating because we know that we’re upsetting people in the right places.”

Léraud felt safe as a journalist. She could use her platform to publicize any threats and felt her public profile would work as a shield to ensure her safety. It wasn’t the same case for Ollivro.

Two incidents in 2009—the death of Morfoisse and the death of a horse and the sickness of its rider—spurred Ollivro’s efforts to enact change. Clearly the situation was more dangerous than anyone had believed. “Because I have a big mouth and talked a lot about it, the regional tourism committee asked to meet me,” Ollivro says. Members of the group told him that his message about the dangers of the algae was making the region look bad and would have a negative impact on tourism. Despite Morfoisse’s death being the second human death linked to green tide, the problem still wasn’t seen as a public health crisis.

seaweed being removed from French beach

Seaweed has to be regularly removed from the beach in Saint-Michel-en-Grève, France, where a horse collapsed and died in 2009. Photo by Cyril Folliot/AFP via Getty Images

But worse than the confrontation with the antagonistic committee was receiving written death threats, finding a pile of manure outside his driveway, and then discovering a dead fox with its head burned off by acid near his garden wall. Ollivro worked with a lawyer to file charges against persons unknown (he had no clue as to the identity of the harassers), but no action ever came from the district attorney’s office.

Ollivro’s work and that of other activists undoubtedly pushed the issue, but government action still fell short of resolving the problem. In February 2010, politicians announced the €120-million Green Algae Plan, which largely focused on collecting and removing algae from the beaches rather than setting new regulations on farming practices. Two of the main goals—to convert 10 percent of farmland to organic practices and reduce the nitrate level in the region’s rivers and bays from 50 mg/L to 10 mg/L—failed spectacularly, partially due to being voluntary.

In 2017, the government announced an updated plan to lower nitrate levels at eight watersheds around the region. Among the measures to be attempted: planting grass along with crops like corn to prevent fertilizer runoff, raising cattle in pastures rather than in feedlots, and raising pigs on straw. Once again, these measures were voluntary for farmers. The government didn’t pass any new regulations to overhaul the agricultural system.

Activists protest the lack of government action on toxic green tides in Brittany, France

Activists protest the lack of government action on toxic green tides in Brittany. Photo by Loic Venance/AFP via Getty Images

Even in the unlikely case that the government began enforcing farm practices to limit nitrate runoff, it’s hard to say if a decrease in nitrogen would even work. Based on models and lab tests, scientists long ago proposed that nitrates in the watershed must be kept below 10 mg/L to avoid algal blooms. But there are other factors at play besides excess nutrients flowing into the water.

Even if farmers move away from mass production and transition to organic practices, nitrogen may have accumulated in surface waters and take years to go away. With climate change, there’s an increased risk of flooding (which will send more nitrogen into the water) and higher temperatures (which allow the algae to thrive).

“There are a lot of studies, but these studies cannot completely reduce to zero the uncertainty around the phenomenon,” says Magalie Bourblanc, a research fellow at the French Agricultural Research Centre for Development. “There are a lot of interactions in the system that we don’t understand. Can it be solved? Nobody knows yet.”

Bourblanc, herself a Breton, returned to the region last summer to do further research on the health component of the green tides—and to go swimming in the Baie de Saint-Brieuc before parts of it were closed by green algae. When speaking with elected officials about the 2017 plan, she encountered a lot of frustration, especially with what’s viewed as the media sensationalizing the story and attacking farmers. The officials also felt that a definitive way forward was uncertain and more complicated than the narrative presented by journalists.

For Léraud and Ollivro, farmers are as much victims as everyone else. The real culprits, they say, are politicians and industrialists who created a system where farmers can only succeed financially by operating huge farms so that mass quantities of food can be sold outside of France. Both Léraud and Ollivro think the farmers should be offered financial support to move away from current practices.

André Ollivro checks how much hydrogen sulfide is being emitted by decomposing algae

André Ollivro checks how much hydrogen sulfide is being emitted by decomposing algae. Photo by Idealink Photography/Alamy Stock Photo

And at long last, it finally seems as if some politicians are listening. In February 2018, the regional Administrative Court fined the state government €556,509 to be paid to the city of Saint-Brieuc, one of the places most severely affected by the algal invasion, because of the state’s failure to remedy the problem. The 2009 death of Morfoisse, the algae collector, has been recognized as a workplace accident and his family recompensed. Then, in July 2019, Yannick Jadot, a member of the European Parliament, and Delphine Batho, a deputy in the French National Assembly, visited the beaches of Brittany to learn about the green tides.

“We have a catastrophically explosive cocktail between a certain model of agriculture and the fact that the climate is changing,” Batho said to the assembled crowd, including Ollivro. “I’m stunned that no one from the world of politics has come here, even after the beach closures and weeks of talking about it at the national level.”

Combine all that with the popularity of Léraud’s book—it’s already been reprinted eight times and been nominated for several awards—and the fact that the algae collection center in Saint-Brieuc had to shut down last summer because they couldn’t safely compost anymore algae, and it seems the issue has become impossible to ignore. Even worse, three people are thought to have died from hydrogen sulfide poisoning caused by the green tides during the summer of 2019: an 18-year-old boy collecting oysters, a 69-year-old man, and a woman, age 42.

Léraud believes the region is at a critical moment, a tipping point. Brittany has seen repeated record-breaking heatwaves each summer, along with the rest of France. People know climate change is wreaking havoc on the weather. They’re starting to understand that will also mean higher growth rates for green algae.

“In a way, we can stay hopeful,” Léraud says. “Since things are getting worse, that might increase public awareness. And not only awareness, but the obligation to act.”