The summer of 2022 was the hottest on record in Europe.

These exceptional temperatures have also led to the appearance of sea heat waves, excess heat in the water.

In Marseille, for example, the sea was up to 6°C warmer than normal for the season, all over an unprecedented period - locally, 150 cumulative days between April and October.

For underwater life, the consequences are dramatic.

>> Heat wave: in the seas and oceans too, species are suffocating

"It's a genocide"

Nicolas is a diving instructor in Marseille, mainly specialized in deep diving.

This year, he witnessed episodes of absolutely unprecedented water heat, with peaks of 28°C up to 28 m deep.

"A temperature increase of this order is no longer water, it's a lava flow," he worries.

Jean-Christophe, amateur diver, confirms.

On the first fifteen or twenty meters, all the gorgonians are now completely burned.

"It's the destruction of life: it made me think of the images we saw of the fires in Gironde", he says. 

What is a sea heat wave?

"For the Mediterranean we had never, in memory, seen such long periods with such high conditions," says oceanologist Nathaniel Bensoussan.

Marine heat waves are discrete and prolonged episodes of warming of the water - minimum five days above the temperature threshold defined from the seasonal normals established over thirty years.

Several factors cause them: the conjunction of strong sunshine, light wind conditions and, in the case of the Mediterranean, the absence of tides.

A disturbance such that the western part of this semi-enclosed sea was warmer at certain times of the summer than the eastern part. 

The impact of marine heat waves on Mediterranean biodiversity

Jean-Pierre Gattuso is director of research at the National Center for Scientific Research (CNRS) in Villefranche-sur-mer.

He participated in a vast European study, published last July, on marine heat waves between 2015 and 2019. His conclusion: each area of ​​the Mediterranean was affected, each year, by at least one marine heat wave.

Each of these marine heat waves has been associated with episodes of massive plant and animal mortality.

Among the most affected species: gorgonians and corals.

The fish themselves have the possibility of escaping to depths where they find a little freshness. 

"The Mediterranean Sea will not be a dead sea in the future, but it will be very different from what we have known", concludes Jean-Pierre Gattuso. 

The appearance of introduced species

But some researchers see sea heat waves, as impressive as they are, as an epiphenomenon.

This is the case of Thierry Thibaud, professor of marine ecology at Aix-Marseille University.

For him, there is much more worrying.

From the creek of Cannelongue, where he dives regularly, he shows us the enemy: a brown seaweed, which in theory has nothing to do here, but which abounds.

The warming of the Mediterranean is indeed favoring the appearance of new species in areas further north, which is endangering the ecosystems in a lasting and perhaps irremediable way.

Thierry Thibaud notably tracks "transformer" algae species, which will completely change their new habitat.

“We know very well the impacts that there were in the eastern Mediterranean a few years ago and that we will have here soon,” laments the researcher.

And the diver researcher concludes: "It's the global problem of global warming. The only thing we can do is still preserve habitats that are not impacted. But it's a global effort that must be undertaken, otherwise nothing can be done". 

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