“Dead zones” are increasing in the oceans

COP15, 15^e On December 7, a conference on biodiversity opened in Montreal, Canada. One of its goals is to require the protection of 30% of marine ecosystems. Because when we often think about terrestrial biodiversity, the ocean is also home to a large number of species whose survival is threatened by many factors.

Depletion of oxygen in seawater is one of them: according to a University of Virginia study, there are now more than 400 “dead zones” in the global ocean, up from 150 in 2003, according to UN figures.

These oxygen-depleted marine areas cover more than 245,000 square kilometers and threaten vertebrate life: more than a third of marine mammals are affected. The phenomenon, which has been known since the 1980s, is accelerating, although there is still no research on this topic.

>> Also read: “Biodiversity as both a victim and a tool in the fight against global warming”

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

France 24: What is the dead zone? ?

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

This lack of oxygen is observed in the surface areas of the ocean at a depth of 50 to 400 meters. Most surface waters are generally less disturbed because they benefit from oxygenation from air contact with less access to deeper waters.

They are mainly located on the coast of America from California to Chile. West Africa, as well as the western part of Indonesia, are also of concern in the Indian Ocean.

These areas are often close to the coast, but we’re starting to see some of them go further up the coast of the Americas and into the mid-Pacific, which is far from the coast.

What are the implications for biodiversity? ?

Lack of oxygenation of water leads to environmental modification, which of course affects marine biodiversity. When oxygen is low, or even too low, fish that need to breathe become hypoxic and risk death. If they don’t die, they migrate to better oxygenated areas, which affects the ecosystem as a whole and therefore the local biodiversity.

Especially animals that cannot escape quickly from these areas are at risk of suffocation. I’m thinking of crustaceans and crustaceans, for example – some of these areas were identified after seeing dead animal dumps on beaches. All animals that need oxygen to survive are affected. Plants are less of a concern 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 oxygen-rich than others due to ocean currents, but this is usually a rare occurrence in the ocean.

It was thought that human activity caused the increase in the area and number of these areas. Then we talk about the phenomenon of eutrophication: the supply of organic matter in seawater – for example, caused by inputs thrown by agricultural products, fertilizers – can lead to an increase in planktonic organisms, which will assimilate and reproduce these organic matters and consume them excessively. depletes oxygen and the natural environment.

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

The increase in the number and extent of these zones goes hand in hand with the worsening of climate change: these mostly coastal dead zones are also extending into the open seas, making it clear that oxygen depletion is not just about oxygen. release of agricultural products. Climate warming does indeed cause seawater temperatures to rise, and oxygen becomes less soluble in warm water.

Dead zones are dead forever ?

It’s not worth it. It is a dynamic phenomenon, renewals can be caused by currents or meteorological events, such as storms, which can renew the oxygen in the water.

Dead zones are therefore not definitive, but the probability that they will reform at the same location due to local currents is non-zero. It is also possible to limit the impact of human activities by reducing the input of organic matter from agriculture.

But the correlation with climate change is a game changer. One consequence of rising seawater temperatures is the potential slowing of marine currents: this makes those areas “sealed” and prevents them from mixing, and therefore re-oxygenating.

As a result, marine biodiversity is a phenomenon to be monitored for fishermen and even tourism. And if a simple action could be taken, for example, by limiting emissions of agricultural products to ocean emissions, global warming would be less reversible.

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

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