Science

How plankton changed after WWII

The Brest harbor is currently undergoing construction. A new 400-meter-long loading dock will be used for the logistics required for the construction of offshore wind turbines and their maintenance. Doesn’t this 12 hectare polder run the risk of promoting the blooms of toxic microalgae? Alexandrium minutum, a real scourge for shellfish farmers in the harbor? This is the question asked in 2017 to Ifremer, the Brittany Regional Council, owner of the port of Brest since 2007 and the Engineering Department of the Ports Directorate of the Brittany Region responsible for the polder work. This investigation carried out within the framework of the project “Paleoecology of Alexandrium minutum in the harbor of Brest” (Palmira) gave birth to an astonishing scientific result published in Current biology : it is the impact of man after World War II that made it prosper Alexandrium minutum.

To answer the basic question, the researchers wanted to know the history of the proliferation of this toxic species. “We carried out three cores in the harbor which we cut out centimeter by centimeter, immediately proceeding to the analysis of ancient DNA of the plankton communities., explains Raffaele Siano, researcher in molecular ecology of marine plankton at Ifremer Brest and main author of the article. We were thus able to reconstruct the evolution of these communities until the Middle Ages, 1,400 years ago “. The result is spectacular. Until the 1940s and 1950s, the different genera of dinoflagellates and protists coexisted and toxic species were in the minority. And after, the pollution of the Second World War and the gradual development of agricultural activities on the coastal zone of the Rade de Brest everything changed. The dinoflagellate Alexandrium minutum became progressively more abundant in the harbor, causing, during its regular blooms, bans on the sale of shellfish. Absorbed by oysters, it makes them unfit for consumption due to a powerful paralyzing effect which can in rare cases lead to the death of a human.

Heavy metals, traces of old bombardments

The 400-meter-long vessel will be used for the logistics required for the construction of offshore wind turbines and for their maintenance. Doesn’t this 12 hectare polder run the risk of promoting the blooms of toxic microalgae? Alexandrium minutum, a real scourge for shellfish farmers in the harbor? This is the question asked in 2017 to Ifremer, the Brittany Regional Council, owner of the port of Brest since 2007 and the Engineering Department of the Ports Directorate of the Brittany Region responsible for the polder work. This investigation carried out within the framework of the project “Paleoecology of Alexandrium minutum in the harbor of Brest” (Palmira) gave birth to an astonishing scientific result published in Current biology : it is the impact of man after World War II that made it prosper Alexandrium minutum.

To answer the basic question, the researchers wanted to know the history of the proliferation of this toxic species. “We carried out three cores in the harbor which we cut out centimeter by centimeter, immediately proceeding to the analysis of ancient DNA of the plankton communities., explains Raffaele Siano, researcher in molecular ecology of marine plankton at Ifremer Brest and main author of the article. We were thus able to reconstruct the evolution of these communities until the Middle Ages, 1,400 years ago “. The result is spectacular. Until the 1940s and 1950s, the different genera of dinoflagellates and protists coexisted and toxic species were in the minority. And after, the pollution of the Second World War and the gradual development of agricultural activities on the coastal zone of the Rade de Brest everything changed. The dinoflagellate Alexandrium minutum became progressively more abundant in the harbor, causing, during its regular blooms, bans on the sale of shellfish. Absorbed by oysters, it makes them unfit for consumption due to a powerful paralyzing effect which can in rare cases lead to the death of a human.

Heavy metals, traces of old bombardments

How to explain it? “We looked for the heavy metal contents in the sediments and we thus found nickel and chromium concentrations which are the signature of a pollution linked to the war, probably the bombardments, these same metals being found in similar contents on d ‘other naval warfare sites like Pearl Harbor in Hawaii, where the Japanese air force destroyed the American fleet “, poses Raffaele Siano. These contaminations have completely upset the planktonic communities of the ecosystem and favored the development of new assemblages of species and among them, the most toxic. As a reminder, the Brest harbor suffered 165 bombings between 1940 and 1944 with more than 30,000 tonnes of bombs dropped. The city of Brest was completely destroyed and when it was liberated in September 1944, only four buildings remained standing in the city center.

Since the end of the war, the plankton of the harbor has not regained the equilibrium of the time of the Middle Ages, which calls into question the ecological theory which states that a polluted natural environment is quickly restored as soon as the disturbance has ceased. In the case of the Brest harbor, after the war, the pollution continued, but of a different nature. “The development of Breton agriculture was based on the use of pesticides and chemical fertilizers, which, leached into the rivers flowing into the harbor, brought other pollutants such as copper, manganese, zinc“, continues Raffaele Siano.

Agriculture, responsible for the proliferation of toxic algae

This scientific result therefore raises societal questions. The only real way to reduce the risk induced by toxic plankton is to drastically reduce the use of pesticides and chemical fertilizers, and therefore to change the practices of Breton agriculture. The pollutant curves show that since the first efforts made since the 1990s by the sector, pollution has been decreasing. But this may be insufficient to reconstitute the planktonic ecosystem from before the Second World War. To confirm the effect of pollution on marine plankton after industrial developments, Raffaele Siano’s team will study planktonic changes in a very different environment and climate, the marine areas bordering nickel mines in New Caledonia. .

In fact, what is the response that Palmira gave to the question of the Regional Council of Brittany? The work actually presented a real risk of favoring the resuspension of forms of resistance of Alexandrium minutum and therefore of promoting its multiplication. Ifremer recommended that site managers only carry out their dredging and marine development work in winter, a season unfavorable to the proliferation of this species.

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