Veterinarian Vincent Petit was riding his horse on the beach on July 28 when the animal fell ill and suddenly died; Petit has to be carried away from the scene.

The culprit was quickly identified: green algae, more exactly Ulva armoricana, commonly referred to as sea lettuce. It’s common in Brittany. The photo below shows the beach at Saint-MIchel-en-Grève, where the death occurred; the numbers indicate where samples were subsequently taken for analysis.

The science behind what happened is straightforward. The plants wash up on the beach, forming little mounds. The sun bakes the piles, creating an upper crust that covers the plants below. As the plants decompose, they release gases. These include hydrogen sulfide, which is poisonous; it’s what killed Petit’s horse.

What’s more intriguing are the origins of the sea plants. They’re indigenous to the area. But their proliferation isn’t natural; it results from human activity.

Other areas of France are sunnier or more populous, but Brittany is an agricultural powerhouse. It’s home to 60% of French pig farms, 45% of French poultry farms, and 30% of French dairy farms. The regions is the leading producer in France of cabbage, artichokes, green beans, spinach, and potatoes; Brittany is also the runner-up producer in France of peas and tomatoes. With its Atlantic coastline, Brittany is the leading French region for fish and seafood capture.

sealettuce2This bounty comes at an environmental price. Extensive fertilizer use boosts nitrate levels in the water, to which livestock add impressive quantities of waste. This soup flows from streams and rivers to the sea, where it becomes a nutritious brew for sea lettuce. This plant thrives, but as it proliferates it takes up oxygen and pushes out other, slower-growing plants. When the tides sweep the sea lettuce to shore, sunny weather accelerates decomposition and production of dangerous gases.

Here’s the problem, and the paradox: the problem has a human origin, and human action could stop it. Fertilizer use could be curtailed, and animal waste disposed of otherwise than by discharge into waterways. Is there an economic incentive for doing so? Today, there is not. Is there the political will to require behavioral change? Certainly not. Environmental protection is not about aspirational efforts to “save the planet”; it requires tradeoffs and exacts real costs. People in France are prepared to make neither, whether as consumers or as voters. My bet is that sea lettuce will enjoy a good future in Brittany.