Modern Mysteries

Killer seaweed causes spate of deaths along French coastline

By T.K. Randall
May 12, 2026 ·  1 comment

Image: Green Tide - Ulva armouricana
Credit: Thesupermat / CC BY-SA 2.5 (adapted) The coastline of Brittany in France is a picturesque destination with seemingly little in the way of imminent risk, yet all across its beaches, at certain times of the year, there lies a deadly threat that has claimed the lives of several people while posing a scientific mystery that researchers long struggled to solve.

The culprit: Ulva armoricana - a type of seaweed which, due to high levels of nitrates in the water (a consequence of local farming), creates a thick carpet stretching over a large area of coastline.

While this might not seem to be problematic in of itself, this particular seaweed has been found to release large amounts of hydrogen sulphide - a foul-smelling gas that can overpower and even kill.

Over the decades, several deaths have been connected to this phenomenon, with unwary beachgoers ending up being overwhelmed by the gas before being found deceased on the shoreline.The same fate has also befallen a number of animals, including wild boar and horses.

Among the seaweed's victims are some of those tasked with removing it - a long and arduous process involving driving a tractor down to the beach, loading it up with seaweed and carting it away.

The situation has also long been politically charged, with groups blaming one another, farmers arguing that they are being unfairly held responsible and politicians promising solutions that never materialize.

As things stand, the beaches of Brittany are still a risk due to the seemingly unstoppable encroachment of the seaweed - in some places there is so much of it that it measures several feet in thickness.

Unless a solution can be found, it is likely to remain a problem for many years to come.

Source: The Guardian | Comments (1)

Tags: France, Seaweed