Why has the sea turned red in part of Brittany?

Residents in Brittany reported seeing unusually red water at the bay of Brest on October 25 and 26.

The sightings were reported in Logonna-Daoulas and Landévennec, Finistère, and led to investigations by Phenomer, a marine observation programme run by national research institute Ifremer.

Ifremer's Concarneau station collected samples following the reports and has now identified the cause as a red seaweed called Rytiphlaea tinctoria.

“The first hypothesis ruled out was phytoplankton,” Phenomer said in an Instagram post. “Laboratory filtrations revealed a persistent red colouration in the sample, even after filtration. Microscopic examination detected no phytoplankton bloom.”

The analysis then focused on red macroalgae. “The culprit identified? A red alga: Rytiphlaea tinctoria, also known as Dyer's Rytiphlaea!” it said.

The seaweed contains a reddish-purple pigment called rytiphline, which is easily released into the water.

According to Phenomer, a large amount of the seaweed is thought to have washed ashore, probably as a result of recent weather conditions, releasing the pigment as it decomposed.

No health risk linked to it

Although seeing the water turn red can be disturbing, researchers said that nothing indicates a health risk linked to it and that no particular precautions are necessary.

The phenomenon is different from the green algae blooms sometimes seen along the Breton coast.

Moreover, Rytiphlaea tinctoria is of historical interest. The Romans and Celts used its pigment in antiquity as a cheaper alternative or source of dye for producing purple colouring and red make-up.

Phenomer is now encouraging people to continue reporting unusual changes in seawater colour and, when possible, to collect a sample of the coloured water in a bottle or another container to help researchers identify the organisms responsible.