An explosive cocktail ravages the plankton of the Brest harbor.

"Plankton (microbes, viruses, small marine animals) represent the basis of the food chain in the oceans and provide all kinds of ecosystems. This is why it has a particularly important role and why it is necessary to understand its metabolism . ”

This statement by Chris Bowler, research director at the CNRS, shows the interest of the studies carried out on plankton in various places on our planet. Microbial distribution in the oceans is mainly conditioned by water temperature. Indeed, plankton is not the same depending on the geographical location and its study provides valuable information on the impacts of human activity on its health as well as on the entire surrounding ecosystem.

In this article, let's take a look at the western end of the Breton coast.

Very recently, Ifremer (the French Research Institute for the Exploitation of the Sea), published the results of a study on plankton carried out off Brest. To do this, the researchers took sediment cores several meters in length in the bay of Brest. After analyzing their DNA, they were able to determine the different species present in the water for centuries.

The results reached the researchers showed once again with certainty that the various types of pollution linked to human activity have a lasting impact on the composition of marine plankton. As might be expected, they report that intensive agriculture causes irreversible damage to plankton.

But more astonishing than that, they discovered that World War II marks a significant stage in the process of plankton pollution in this area. Indeed, the water seems very heavily loaded with lead and PCBs.

As a reminder, “PCBs or polychlorinated biphenyls are toxic, ecotoxic and reprotoxic (including at low doses as endocrine disruptors ). They are ubiquitous and persistent pollutants (half-life of 94 days to 2,700 years depending on the molecules ” - Source: Wikipedia.

Samples taken from the sediments showed that the most significant changes appear from this date. "We expected to find a change in microalgae communities over the last decades, but not necessarily such a drastic change going back to WWII!" reported Raffaele Siano, biologist at Ifremer. “We were able to trace the cocktail of planktonic species that have been present in water for around 1,400 years. Throughout this period dating back to the Middle Ages, the most radical variations do not appear until the Second World War ”.

Mr Siano explains: "The harbor of Brest was marked by extreme pollution events during the Second World War, with in particular the bombardments of the Allies, we found traces of it with high levels of heavy metals in the layers of sediments. period. And since then, the harbor has been the receptacle of chronic pollution with contaminants resulting in particular from intensive agriculture, this is what we find in the more recent sediments of the 1980s and 1990s. "

It was also from the 1980s that researchers noticed a proliferation of toxic microalgae resulting from "extreme then chronic" heavy metal pollution.

In the end, the study clearly highlights “the cumulative effect of pollution from war and agriculture” in the damage caused to the seabed in the harbor of Brest. But more worrying still. It is to be feared that these biological alterations will have wider consequences and generate, as the authors of the study fear: "cascading effects on other biological components of the ecosystem, affecting the entire network. marine thropic ”.