"This year, all the conditions have come together for a growth in green algae," said Alain Ménesquen, expert from the state marine laboratory Ifremer. "The sun and warmth of May have allowed them to carry out photosynthesis, then the June rains brought all the nitrates they need to develop."
By the end of June, some 25,000 square metres of seaweed had been collected from the beaches of Brittany, twice as much as the previous year.
The worst affected area is Saint-Brieuc in the Côtes-d'Armor coast of Brittany, where around half of all the seaweed is washed up.
Earlier this month, two wild boar were found dead on the beach at Saint-Maurice in Morieux. The beach has been barricaded off ever since, with local authorities saying they had detected dangerous pockets of gas linked to seaweed deposits.
The official autopsy on the two animals found that they had died by asphyxiation due to presence of "mud in their airways" and the local government prefect insisted there was "no link" with seaweed.
But André Ollivro of the association, Halte aux marées vertes (Stop the Green Tides) said he was convinced seaweed was to blame. He said "dozens of dogs" die on this stretch of coast but nobody signals their deaths.
Yesterday, he and other anti-algae activists planted a plaque on the beach at Binic near Saint-Brieuc. It was in memory of Thierry Morfoisse who died of a heart attack while transporting rotting seaweed in a lorry in 2009. An investigation is still underway into the cause of his death.
The government launched anti-toxic seaweed plan last year after a horse died from breathing in toxic fumes at the beach in Saint-Michel-en-Grève in 2009 and his rider lost consciousness.
Ecologists say intensive pig and poultry farming is the main culprit.
In a shock campaign to raise awareness, the group France Nature Environnement, FNE, ran a poster campaign this Spring showing a toddler in armbands paddling in a seaweed-infested beach. The poster squarely blamed intensive pig farmers, who took the group to court and lost.
Benoît Hartmann of the FNE said the only solution was a "change in the agricultural model".
The huge concentration of pig and poultry farms in the region had "created nitrate emissions that hugely surpass the absorption capacity of the soil".
A reduction in seaweed requires nitrate concentrations to remain at 10mg per litre at the most, say experts. They are currently at between 30 and 40mg per litre in the region's waterways.
But on July 7, President Nicolas Sarkozy visited the area and in a clear display of pre-presidential electioneering, declared it was time to stop "fundamentalist" ecologists "pointing the finger at farmers".
He said the best solution for now was to step up seaweed collection and to build a factory to convert pig detritus rich in nitrates into biogas.
Mr Ménesquen of Ifremer said he found this approach "astonishing".
"This is backward-forward thinking that doesn't attack the source of the evil. There is no scientific reason to think that the phenomenon will stop by simply removing seaweed," he told Le Monde.
Besides, he added, the biomass factory would not deal with the nitrates, it would in fact leave them in a more concentrated form.
Farmers say they are making huge efforts and have cut the amount of nitrates they spray on the soil by three since 1998.
"More needs to be done, but we've done a lot already," said André Sergent, environmental vice president of the agricultural chamber of Finistère.
"We (farmers) find it hard to understand. Brittany lives essentially from agriculture. If we enter into a logic of destruction, the young will live on what?"