COP15, the 15th conference on biodiversity, opened on Wednesday December 7 in Montreal, Canada.

One of its objectives is to require the preservation of 30% of marine ecosystems.

Because if we often think of terrestrial biodiversity, the ocean is also home to a great diversity of species, whose survival is threatened by multiple factors.

Diminishing oxygen in seawater is one of them: More than 400 "dead zones" currently exist in the global ocean, according to a University of Virginia study, compared to 150 in 2003, according to UN figures.

Under-oxygenated, these marine areas extend over 245,000 square kilometers and threaten vertebrate life: more than a third of marine mammals are affected.

The phenomenon, known since the 1980s, is accelerating, even if studies are still lacking on the subject.

>> To read also: "Biodiversity, both victim and tool in the fight against global warming"

Françoise Gaill, Vice-President of the Ocean and Climate Platform and Scientific Advisor Marine and Ocean Environments at the CNRS Institute of Ecology and Environment (InEE), answers questions from France 24.

France 24: What is a dead zone


Françoise Gaill:

Dead zones are hypoxic areas of the ocean, that is to say areas where the oxygen concentration is below the norm: we can witness a drop of up to 20% in the usual oxygen, which is already significant, and it can be up to 50% oxygen below normal levels.

This lack of oxygen is observed in the surface areas of the ocean, between 50 and 400 meters deep.

The waters most at the surface are generally less concerned, because they benefit from the oxygenation allowed by their contact with the air, which has less access to deeper waters.

They are mainly located off the coast of America, from California to Chile.

West Africa is also concerned as well as the western part of Indonesia, in the Indian Ocean.

These areas are often close to the coast, but we are starting to see that some of them, off the coast of America, go further, towards the middle of the Pacific, which is very far from the shore.

What are the consequences for biodiversity


The lack of water oxygenation produces a modification of the environment, and this of course has an impact on marine biodiversity.

When there is less oxygen, or even very little, the fish, which need it to breathe, find themselves in a situation of hypoxia, and risk dying.

If they do not die, they migrate to better oxygenated spaces, which affects the ecosystem as a whole and therefore local biodiversity.

Especially since animals that cannot flee these areas as quickly risk dying of suffocation.

I am thinking of crabs and crustaceans, for example – some of these areas have in fact been identified after dumpers of dead animals were observed on the beaches.

All animals, which need oxygen to live, are affected.

Plants are less concerned, because they are less dependent on oxygen than animals.

What are the causes of these dead zones


Dead zones are originally a natural phenomenon.

Some areas may be less oxygenated than others due to sea currents, but this is normally a rare phenomenon in the ocean.

It was first thought that the increase in the area and number of these areas was caused by human activity.

We then speak of a phenomenon of eutrophication: a supply of organic matter in seawater – caused for example by inputs discharged by agricultural products, fertilizers – can lead to an increase in planktonic organisms, which will absorb these organic matter and multiply, consuming too much oxygen and exhausting the natural environment.

But we have been realizing for ten years that this is not the only cause of the drop in oxygenation of the ocean: global warming also plays its role, there is a correlation.

The increase in the number of these zones and their extent goes hand in hand with the aggravation of climate change: these dead zones, mostly coastal, also extend towards the high seas, which clearly shows that the drop in oxygen is not only due to the release of agricultural products.

The warming of the climate indeed causes a rise in the temperature of the sea water, and oxygen dissolves less well in warm water.

Are dead zones dead forever


Not at all.

It is a dynamic phenomenon, renewals can be brought by the currents or by meteorological phenomena, such as a storm for example, which can renew the oxygen of the water.

The dead zones are therefore not definitive, but there is a non-zero probability that they will reform in the same place due to local currents.

And it is also possible to limit the impact caused by human activities, by reducing the input of organic matter from agriculture.

But the correlation with climate change is a game-changer.

One of the consequences of the increase in seawater temperature is the potential slowing down of sea currents: this makes these areas "sealed", and prevents them from mixing, and therefore from reoxygenating.

As a result, it is a phenomenon to be monitored, for marine biodiversity, for fishermen and even for tourism.

And if it is possible to have a relatively simple action on the discharges of inputs into the ocean, by limiting the discharges of agricultural products for example, global warming is much less reversible.

These dead zones will therefore increase if nothing is done to curb global warming, which means reducing greenhouse gas emissions and limiting global warming to a maximum of 1.5 or 2 degrees Celsius.

>> To read also: "According to the IPCC, humanity has three years to reduce its CO2 emissions"

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