By Michael Hirst
Stretches of French coastline are plagued by the algae
Holidaymakers beware - the picturesque beaches of north-west France are not as safe as they may seem.
The culprit is not a rogue shark or a plague of venomous jellyfish, but an innocuous-looking seaweed.
Ulva Lactuca - more commonly known as sea lettuce - is harmless while living, but when it decays on land it forms a crust under which a deadly gas forms.
This year has seen unprecedented levels of the algae being washed ashore on Brittany's beaches, heightening concerns along a coastline that is visited annually by an estimated 9m tourists - 700,000 of whom come from the UK.
"When you walk into the crust of such accumulation, you make a hole in a reservoir of hydrogen sulphide, and this gas is very toxic," said Alain Menesguen, director of research at the French Institute for Sea Research and Exploitation (Ifremer).
"It can make animals or people unable to breathe, so you can die in less than a minute," he told the BBC.
Environmentalists say decades of misuse of Brittany's agricultural land is to blame for the phenomenon, due to the high levels of nitrates used in fertilisers and excreted by the region's high concentration of livestock.
Despite its small population of 3m people living on just 5% of the country's agricultural land, Britanny is home to 60% of France's pig farms, 45% of its poultry farms and 30% of its dairy farms, said Jean-Francois Piquot of the environmental group Eau et Rivieres de Bretagne.
Nitrates used in farming leach into local rivers, and combine with the summer sunshine to cause an explosion of algae growth around the river estuaries, he told the BBC.
As the seaweed is then washed ashore, tonnes of the slimy green substance are left to rot on hot sand along parts of Brittany's 1,650-mile (2,700 km) coastline.
Certain areas have been blighted by the algae for more than three decades, but the problem is worsening.
Despite annual clearance efforts - some councils spend more than 100,000 euros (£86,000) bulldozing putrid piles from their beaches each year - local authorities acknowledge they are no match for the tide of seaweed.
Mr Piquot said local authorities had wasted 1bn euros in the past 30 years collecting the sea lettuce and trying to get rid of it.
Only a third of the 200m cubic metres of the algae washed ashore had been cleared, and clearance treatment was prioritised for those beaches popular with tourists, he added.
A spokesman from the Brittany Tourist Board said only three beaches in the Cotes d'Armor - one of Brittany's four departments - were affected.
"The British public should be reminded that Brittany's beaches are continuing to be cleaned very regularly and monitored on a daily basis," he said.
Action at last?
Some officials argue they are powerless to deal with an agricultural problem that lies beyond their jurisdiction.
With algae levels continuing to rise, activists and locals alike blame French President Nicolas Sarkozy's government of failing to adequately address the worsening problem.
Locals protested after a horse was killed by toxic fumes on one beach
"Only a major reduction of the use of fertilisers and other nitrogen chemicals will result in a lower green algae tide," said Mr Piquot.
Ifremer's Mr Menesguen told the BBC's World Today programme that despite the dangers, only one beach had been closed since a local vet was dragged unconscious from a metre-deep pile of the rotting algae earlier this month.
Vincent Petit, 27, had been horse riding when his mount collapsed after inhaling fumes from the algae.
His horse died within minutes, and Mr Petit has threatened to sue local authorities for reckless endangerment.
The Brittany Tourist Board said the "isolated" incident was being treated very seriously.
Local communities hope it will finally spur the authorities into action.
On Sunday, a 400-strong crowd gathered on one local beach demanding an adequate response from Mr Sarkozy's government.
Eau et Rivieres said a central programme to cut the quantities of nitrates being used in the area was long overdue, but the government had proved "incredibly passive" in promoting sustainable agriculture.
"The state's indifference is the worst kind of pollution," said the group.