Vacationers in northwest France are being warned to stay away from beaches, which are growing a bumper crop of a seaweed that releases a potentially toxic gas. The culprit: Up-stream releases of manure from intensive farming that overload the near-shore waters with nitrates.

The seaweed (sea lettuce, Ulva lactuca) must be removed within 48 hours of washing ashore — because as it rots, it releases so much hydrogen sulfide that swimmers and strollers are endangered. The French ministry for health and the environment has warned visitors to avoid areas with overgrowth, and told workers scooping up the seaweed that they must wear monitors to alert them they have entered especially toxic pockets and must clear out within minutes.

The potentially toxic weed shows up every year in Brittany, but this year’s overgrowth is at least half again the size of last year’s, according to coverage in the The Telegraph and Radio France International.

The pressure group France Nature Environnement is running a campaign to raise awareness of the slime attack. It pins responsibility for it on concentrated swine and poultry farming: Brittany raises half of the pigs and chickens grown in France. According to the group (French), the manure from those farms is as much would be produced by 50 million people.

When farm effluent washes downstream in the United States, it creates a dead zone in the Gulf of Mexico — but that zone is offshore and never seems to get much attention. In France, in contrast, the algae deposits are cutting deeply into tourism, one of the area’s economic engines.

Predictably, the issue has been politicized. President Sarkozy, who’s up for re-election, jumped in two weeks ago to defend farming, even though his government last year launched an algae-reduction plan that ascribes responsibility to farm effluent. On a visit to Brittany, he dismissed campaigners against industrial farming as “fundamentalists” (intégristes), called farmers the “first victims,” and said they were “not responsible for economic decisions made a long time ago” (…ne sont pas coupables de choix économiques qui ont été faits il y a bien longtemps).

The government’s interim proposal is to build additional plants to process the seaweed for biogas. Meanwhile, the agriculture ministry has proposed relaxing regulations on agricultural spraying of manure from pig farms, an action likely to make nitrate concentrations — and the seaweed blooms — even worse.

For more, here’s a BBC video with construction equipment removing mountains of seaweed, and one from France2.fr on Sarkozy’s speech. (I can’t find embed codes on those sites; if you spot them, let me know in the comments.)

Image France Nature Environnement