Brest liberated, Brest polluted

Destruction of the city of Brest during a bombardment in 1940-1941 • 3EME OEILL

In Brest, plankton tells part of the story of the 20 ^th century. These microorganisms that live in suspension in seawater were indeed modified after the Second World War, and toxic species developed. Scientists from Ifremer (French Research Institute for the Exploitation of the Sea), who had observed a significant presence of a toxic microalgae in the harbor of Brest for a few years, wanted to know its history.

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They carried out three corings in the harbor and recovered the DNA of ancient plankton communities. They were thus able to reconstruct their evolution since the Middle Ages. Until the 1940s and 1950s, different species cohabited, but toxic species were in the minority. After the Second World War, the microalgae Alexandrium minutum has become more abundant in the harbour, leading to bans on the sale of shellfish and oysters. "The change around 1945 is such that it is irreversible:the plankton of the Middle Ages has almost disappeared", underlines Raffaele Siano, researcher in molecular ecology of marine plankton who led the study and worked with historians from the university. from Brest.

Over 30,000 tons of bombs

Analyzes of heavy metals in the sediments showed concentrations of nickel and chromium directly linked to pollution caused by the war, in particular the bombings. Similar levels have indeed been found on other sites such as Pearl Harbor, the American base bombed by the Japanese in 1941. However, Brest was one of the most affected cities during the Second World War. It underwent 165 bombardments between 1940 and 1944, and more than 30,000 tons of bombs were dropped there, in particular by the Allies who wanted to neutralize the German submarine base there. The city, completely destroyed, experienced a renaissance. But the plankton has not regained its pre-war form. Especially since pesticides linked to agricultural pollution have aggravated the changes triggered by the war.