The water of the Loire suddenly turned red: we explain why

The Breton coast turns red at the end of summer, a phenomenon as natural as it is impressive.

While the swimmers have mostly left the coasts of Brittany and Loire Atlantique, the ocean is gradually regaining its rights. But the last holidaymakers present were able to witness a strange spectacle. The water has indeed taken on a very unusual color, red or brown depending on the location.

If this phenomenon is entirely natural, it is nonetheless impressive. At the end of summer, the North Atlantic coasts take on color, an episode that has occurred almost every year for several decades.

Faced with this riddle of mother nature, scientists sought a rational explanation. Quickly they turned to microalgae. The latter were indeed responsible for the coloring of the water, but in an indirect way. Because according to the first studies carried out on the subject, it would be Lingulodinium polyedra, which would be responsible for these changes.

The latter feeds on phosphorus and nitrogen, two nutrients present in quantity in the fresh water of rivers. So when the summer is bad, as was the case this year, the flow of the Loire and the Vilaine increases, which increases the number of microalgae at the mouths of the two rivers. It is also here that the marine color change is most impressive.

If the water turns red in the presence of these microalgae, they are blue and bioluminescent (they produce their own light). They are not the ones who are directly responsible for the color change.

A toxin that kills seashells

The culprit is a toxin released by microalgae during their life. This yessotoxin accumulates in shellfish of all kinds and then gives them this reddish color. so unusual in the oceans.

If the sight of this color on our shores is cause for concern, it is not. Microalgae, present in abundance in the water of our oceans, are not dangers in themselves. Their toxin can become dangerous when eating contaminated shellfish, but according to very regular readings from the health safety authority, yessotoxin levels are still well below alert thresholds.

If the risk for humans is thus quite low, the presence of this microalgae remains a problem for its ecosystem. Nourished by agricultural fertilizers (the main cause of the presence of phosphate in the oceans) these microalgae have proliferated for several years off the French coast, a place where they are found without the slightest predator.

In addition to releasing toxins, harmful to the shellfish they colonize, these microalgae consume a large part of the marine oxygen present on our coastline which can, in some extreme cases, kill the rest of the aquatic ecosystem by asphyxiation.

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