With a water temperature of 25-26° for several days now, the Mediterranean is also experiencing a long episode of strong heat on the Catalan coast. Still far from the 30° recorded in Corsica, but the observation is alarming. The consequences could be fatal for highly sensitive flora and fauna. Especially if the episodes of sea heat waves intensify further. In the meantime, an episode of the Tramontana would be welcome. Diagnosis and perspective with Pascal Romans curator of the Biodiversarium and Pascal Conan, oceanographer in Banyuls-sur-Mer.
What is the magnitude of the warming of the Mediterranean?
“This Friday, August 5, the water temperature was 16° at 40 meters deep and 24-25° at the surface off Banyuls. We can add one or two degrees for the temperature in the bay. Obviously, there is cause for concern, especially if it lasts for longer”. Curator of the Côte Vermeille marine nature reserve, Frédéric Cadène is on the front line in the face of the warming of the Mediterranean. “Less striking at home, because we are one of the areas where the Mediterranean is the coldest”.
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Off Bastia, the Mediterranean has been sweating between 28 and 30° since May. Over the past 25 years, its global temperature has risen by one degree. “Even if this figure seems small, it is a lot for the ecosystem”, warns Pascal Romans. “We are seeing increases this year of 4 to 6° in the Gulf of Lion. When normals on average are 22°, we have been at 26° since the end of July”, adds the curator of the Biodiversarium of Banyuls-sur-mer. Meteorologists dare to use the term marine heat wave in the Mediterranean.
“With the high temperatures and the absence of the north wind, there is no longer any mixing between the water at the surface and the water at depth. This lack of movement between the different marine strata causes the non-regulation of the temperature of the sea. An episode of tramuntana would make it possible to find the usual 21 to 22°“, he hopes. “We have not yet seen the sea heat wave of 2006 again, when the sea temperature rose here, in Banyuls, to 28°. but, it should not last 15 more days. The danger is the duration and the power of this phenomenon”. “The current event, across the entire Mediterranean, is one of the ten most exceptional in recent years”, reveals in the world, Roman Mcadam, researcher in ocean modeling.
What are the consequences for fauna and flora?
“The temperature of sea water depends on the renewal of its strata. The movement renews the deep waters, bringing oxygen and organic matter. If this renewal no longer takes place, the oxygen becomes scarce, the organisms can no longer feed and the ecosystem is impoverished”deciphers Pascal Conan, oceanographer in Banyuls. “We are reaching the limit. The models have clearly accelerated since the beginning of the 2000s and the simple observation is no longer enough”.
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If they therefore still remain measured on the Gulf of Lion – “where the corals are suffering” – the consequences for fauna and flora are worsening in the rest of the Mediterranean. “They are, for the moment, more marked on the eastern Mediterranean where exchanges with the Red Sea, through the Suez Canal, accelerate the arrival of exotic species, like the lion fish or the flute fish, formidable predators”, Pascal Romans module. Throughout the Mediterranean area, nearly 1,000 non-indigenous species have already been identified, including more than a hundred fish.
But the immediate emergency on the Gulf of Lion hits the Posidonia seagrass. “In summer, it is covered with small organisms – sponges, worms, molluscs – then in September, saupes (a fish very present in the Mediterranean) come to mow it. Today, especially in Corsica and the Balearic Islands, it is suffering , already damaged, its cycle is disrupted. Posidonia grass is a nursery and an enormous storage of the CO2 emitted. Some studies announce a massive mortality for 50 species in other areas of the Mediterranean. This is less the case in the western part. For the moment, there are fewer migrations from the Atlantic. Facing the Sahara, a very cold water zone blocks species from the south”.
And marine nature also has its paradoxes. “The higher sea temperature improves the reproduction of certain species”, don’t forget Pascal Romans. “The growth of juveniles is also faster and therefore protects them more quickly from predators. In 2006, we observed an exceptional reproduction of the dentex, accelerating the repopulation. sea heat wave, their lengths and intensities will have dramatic consequences if they continue”.
“They can only be global in the face of climate change. We can no longer deny the action of the hand of man in this global warming. 25 years of measurements clearly demonstrate that the impact is not natural. The facts are there”, activist Pascal Conan. “The balance is already very fragile. If this balance is broken, the ecosystem collapses. So every gesture counts”, concludes Pascal Romans.