Vincent Petit was walking his horse along a beach at Saint-Michel-en-Grève in northern France, when the pair suddenly sank into and found themselves stuck fast.

"It was impossible to get out of there and within 10 seconds, my horse had passed out," Petit recalls.

He says he just had time to call for help before fainting himself.

Luckily, that day eight years ago, a man was working to clear the beach of green algae just a few meters away. He rescued Petit. The horse died from poisonous hydrogen sulphide gas released by the rotting algae.

Since then, two people and a few wild boar are believed to have died from the gas, although the government has never officially recognized this.

"It's incredible," Petit told DW. "You wouldn't expect to be exposed to such danger on a beach in France."

A global problem

Yet it's a danger that's becoming increasingly common around the world. Algae of various kinds thrive on nitrates used in fertilizers that run off agricultural land and end up in rivers, lakes and coastal waters.

A NASA image shows a floating green mat of algae off the coast of New York

The worst outbreaks have been seen in parts of the world where agriculture is particularly intensive - including North America, Europe and parts of Asia.

Florida's Lake Okeechobee has become seriously infected with algae, as have coastal waters off the state of Georgia. Parts of the Gulf of Mexico have been labelled "dead zones" because of huge expanses of the blooms which suck up all oxygen.

Last year, algae was blamed for the deaths of 23 million salmon in Chile, and this week tourists were warned to keep out of the water at beaches on the Canary Islands because of dangerous "sea sawdust" algae.

In Brittany, famous for its picturesque rocky coves, green algae is present at eight beaches. All are in small bays where the water doesn't flow away easily.

Green algae are not toxic per se. But when they occur in large amounts and cannot be cleaned away fast enough, they start to rot. Bacteria decompose them and release hydrogen sulphide in the process, a toxic gas smelling of rotten eggs.

Agriculture versus the environment

Farming is one of Brittany's biggest industries, and the region's struggle to tackle the algae is indicative of the competing interests of the agricultural sector and surrounding environment.

Cattle farmer Laurent Guernion says nitrate-based fertilizer is essential to keep his farm competitive

Laurent Guernion keeps about a hundred cows near the coastal town of Saint-Brieuc, and grows corn to feed them. He says the chemicals are crucial for farmers to stay competitive.

"If we don't use fertilizer, we won't be able to produce enough food for our animals," he told DW.

"We would have to buy fodder and that would bring down our revenue even further. We are already struggling - the milk price has gone down from 300 to 250 euros per ton over the past two years."

Guernion says he calculates exactly how much fertilizer his crops need, so none ends up in natural waters. But the corn stops growing in August - and no longer soaks up the nitrates in the soil, which are then washed into the rivers and the sea.

Cutting down on fertilizers

Still, Guernion is trying to reduce the amount of fertilizer he uses and the proportion of corn he feeds his cows. He is among the 80 percent of Brittany farmers who have signed up to the region's anti-algae plan.

The government has spent 40 million euros ($47 million) over the past five years trying to combat the problem, and compensating farmers for reduced productivity.

That sum will now be topped up with another 55 million euros, says Thierry Burlot, the regional government's vice-president in charge of environmental matters.

"We will reduce the proportion of nitrates in the water even further," Burlot says. "We will continue to reshape the land so that the water can flow off more easily. Brittany is on the move - we are adapting our agricultural model."

This beach near Hillion in Brittany has been closed to tourists because it's coated with green algae

The first anti-algae plan has had some success. Brittany's water quality has improved, with its average nitrate content falling from 50 to 30 milligrams per liter.

But scientists say to eradicate the algae, that figure needs to drop below 10 milligrams.

This spring saw more algal blooms than for the past 15 years, after relatively high temperatures followed a mild winter with few storms to clear the water of nitrates.

And scientists fear weather like this could become increasingly common as a result of global warming.

Rethinking farming

Yves-Marie Le Lay, vice-president of the "Stop the Green Tides" association was born in Brittany and wants his children and grandchildren to grow up on clean beaches.

He believes this will only be possible with a radical switch to less intensive farming.

Yves-Marie Le Ley of Stop the Green Tide says only a radical change to agriculture can save the beaches he grew up with

"We need to thoroughly overhaul Brittany's agricultural model," he told DW, while measuring gas levels at one beach - something he's been doing for the last seven years to flag up the problem.

Le Lay insists this wouldn't be a death knell for the local farming sector.

"Farmers should focus on quality and not quantity," he says. "They would then need less fertilizer. And they wouldn't even lose money, as organic products fetch considerably higher prices.”

But farmers like Guernion say such a strategy is just too risky.

"The prices for organic products are subject to fluctuations on the world markets - just like for conventional products," he says. "We would have no guarantee we could make a living if we produced less, even with higher quality."

So far, the government's anti-algae initiatives have all been voluntary. Which means unless farmers are enthusiastic about going organic, the algae are probably here to stay.