A chapel stands on a headland above a sandy bay, streaked with slimy green weed. Woodland and scrub descend to the beach's edge. A concrete bunker, built in 1944, awaits the allied invasion that never came (not to this part of France, at least). The Plage de Saint-Maurice could be one of 100 pretty inlets on the north Breton coast except that, on a sunny August day, there was not a soul walking on the sand, not a single sunbather, not a single child with a bucket or spade. This summer, the beach is the symbol of an ecological calamity which has been 40 years in the making.
Two bored but courteous gendarmes guarded a temporary fence, blocking access to the sand. They allowed a small group of people to pass the barrier. They included a local television crew, The Independent and an ecological campaigner, carrying a toxic-gas detector and a gas mask. We walked 100 yards on to the empty beach. Below the surface crust of the sand, our feet sank into an evil-smelling black mud, composed mostly of rotting weed.
The environmental campaigner, Yves-Marie Le Lay, 61, put on his gas mask and walked a few steps ahead. He stuck his detector into the mud. The machine emitted an angry screech, like a seagull searching for food. Mr Le Lay announced that the mud was, in places, approaching the toxic danger threshold for human health.
"We shouldn't be doing this. It should be the government doing this work," he said.
"OK, they have closed off the beach, but no one is removing all of this filth. Why? It is as if they are waiting for a child to die."
At one end of the Saint Maurice beach, in Saint Brieuc bay, is the mouth of the river Guessant. In the sand and mud of this narrow estuary, 38 wild animal corpses have been discovered since mid-July – a coypu, a badger and 36 wild boar.
This week it was officially confirmed by the French government that the animals had been killed by seaweed. To be precise, autopsies ordered by the government concluded that the animals had died from inhaling hydrogen sulphide, sometimes known as "sewer gas", a toxic gas emitted by the residue of rotting heaps of "ulva" or "sea lettuce".
The weed, always present here in small quantities, has been washing ashore along the Breton coast in great, stinking heaps since the 1970s. Only in the last few years has it been officially admitted that the proliferation of this primitive form of weed has been caused by nitrogen pouring into streams and rivers and then the sea, from the scores of intensive pig and cattle and maize farms in the heart of the Breton peninsula. In truth, the role of nitrogen pollution is not yet fully admitted. It is denied, against all the scientific evidence by the farming industry and by the powerful French agri-pharmaceutical lobby. They blame global warming or phosphate pollution. Even the official warning signs erected on Breton beaches are coy. The signs say that the algues vertes (green weeds) are a "potential danger" but are "naturally present". There is no mention of the nitrogen run-off from Breton farms, which has increased seven-fold since the 1970s. President Nicolas Sarkozy, addressing a farmers' conference in Brittany last month, mocked what he called the "ecological fundamentalists" who blame the seaweed invasion entirely on intensive methods of farming. A few days later the first dead wild boar was found. Local environmental and community groups have been campaigning for government action since the 1980s. The European Union has imposed large fines on Paris for breaching EU norms for clean river water. Relatively limited measures have been taken to cart the weed from the beaches, or to restrain the use of nitrogen fertilisers, or to discourage the dumping, or spreading, of animal waste.
Some progress has been made. Over 1,000 acres of beaches have been covered in weeds this summer, compared with a peak of over 2,000 acres in 2008. At the same time, evidence is piling up (like the weed) that the algues vertes are not just an unsightly and smelly nuisance but a deadly health hazard. Four years ago, two dogs were found dead on a north Breton beach. Three years ago a horse was found dead on a beach not far from the Plage de Saint-Maurice. An autopsy showed that the horse had died from inhaling the toxic gases emitted by the rotting weed. The same year, a man employed to shift the weed collapsed and died – officially from other causes. The legal battle over his death continues.
This summer the animal holocaust at the Plage de Saint-Maurice has raised the arguments and tensions in Brittany to a new level. For many days after the animal carcasses were found, farmers' groups claimed that the wild boar had been poisoned, either accidentally or deliberately by ecologists. Farming was not responsible for the weed, they said and, in any case, the weed was not dangerous.
The corpse of a poisoned fox was dumped last month on the doorstep of a well-known anti-weed campaigner, André Ollivro. "They say we are anti-farming and we want to destroy the farming industry," Mr Ollivro, president of the association for safeguarding the Penthièvre region, told The Independent.
"We are not anti-farming. But we are against the kind of intensive farming that pours nitrogen fertiliser into the ground or dumps vast quantities of animal effluent." The official autopsy results on the wild boar from the Saint-Maurice beach, delayed for many days, were finally published this week. The animals had died from cardiac arrest caused by inhaling hydrogen sulphide (H2S). Some of the animals had 2.45mg per kg of H2S in their lungs – almost double the level needed to kill a human.
How did this happen?
Scientists believe that, as the weed rots, concentrated pockets of toxic gas become trapped in the mud. The wild boar, which roots in the estuary mud for worms, must have disturbed the bubbles of deadly gas.
But the farmers' leaders ask why this has never happened before. If the weed has been present for 40 years, why have there been so few animal deaths until now?
Mr Ollivro points out that the herd of wild boar had only recently taken residence in the scrubland above the estuary.
"The truth is that, for years, there have been cases of people complaining of sickness and eye problems after visiting these beaches," he said. "It is only recently that the possible health hazards of HS2 from the rotting weed have been understood." The Ecology Minister Nathalie Kosciusko-Morizet – torn for nearly a month between the scientific evidence and the farm lobby – has finally been obliged to take sides.
She said earlier this week that intensive farming was "in large part responsible" for the invasion of the noxious weed. "This has been going on for 20 or 30 years," she said. "It is time to put an end to it." Ms Kosciusko-Morizet (known as NKM) ordered that, in future, any Breton beach that could not clear away the algues vertes within 24 hours would be closed. Up to 50 beaches, mostly on the north Breton coast, but also stretching around as far as the Quiberon Bay in the south, could be at risk.
Farmers' groups were furious. Jacques Jaouen, president of the Breton chamber of agriculture, said that her words were "revolting" and "insulting". Farming was crucial to Brittany, he said, and farmers had already made "considerable efforts" to reduce their nitrogen emissions.
This is true. The average nitrogen level in Breton rivers has fallen from 38mg a litre in 1998, to 30mg today. Government scientists put the danger level at 10mg.
Local politicians are also puzzled by NKM's "24-hour" rule, complaining that some communes do not have the resources to clear beaches so quickly.
In any case, the threat comes not just from the fresh weed, but the deposits of noxious mud that have been built up over decades.
There are, so far, no plans to clear that mud away.
Last week, the Breton branch of the main French farm union, the FNSEA, organised a football match for young farmers. The match took place on the "sand" of the Plage Saint-Maurice to "prove there is no danger to health".
"They should have organised a proper match, farmers versus ecologists," Mr Le Lay said.
"They would have beaten us easily. We would have had to wear our gas masks."