Visiting the northern Brittany coast sounds completely delightful — Mont St. Michel, Paimpol, the pink granite coast — but several recent articles have revealed serious problems with Brittany’s northern beaches, especially in the Côtes-d’Armor department. Algae, washing up on shore and decaying, has made some beaches perilous:

Harmless while in water, the algae form dangerous gases — notably hydrogen sulfide, with its characteristic rotten-egg smell — when they wash up on land and decay. A white crust forms and traps the gases, which are released when stepped on or otherwise disturbed. Over time, putrefied algae turns sand into a black silt muck, sometimes containing pockets of poison gas.

The excessive build-up of algae is probably caused by nitrate-heavy fertilizers in use at coastal farms. The deaths last year of two dogs running on the Grandville beach set off warning bells, but the recent death of a horse (and sickening of its rider) on the beach near Saint-Michel-en-Grève seems to have caught the government’s attention.

Until the problem is resolved, be careful about walking with your dog — or letting your dog run — on northern Brittany beaches. Look out for white crusty spots, or spots of black silt, and avoid areas around streams running into the water — in fact, to be on the safe side, I’d first ask a local resident whether it’s okay to walk on a particular beach.