This map “gives an idea of ​​what the Atlantic Ocean might look like in 100 years in a warmer place”

dead zones. This is what scientists call regions of the ocean – also lakes or estuariesestuarieseg — lack of oxygen. They also talk about oxygen minimum zones (OMZ). There is. They are usually rare. It is confined to the deep ocean floor. However, the term is now more commonly used to designate areas that are unusually low in oxygen. Enough to practiceasphyxiationasphyxiation of wild naturewild nature. And this only at depths between 100 and 1000 meters below the surface.

Did you know?

In the dead zones of our oceans, scientists observe a decrease in oxygen concentration of up to 20%. In some, the deficit is even around 50%.

Dead zones are mostly found along the coast of the United States. Also on the West African side. And in the western part of Indonesia.

It was the first identified cause of oxygen depletion in the oceansAgricultureAgriculture intense. More specifically, air pollutionnitrogennitrogen. Also other pollution, pesticides or metalsmetals heavy In the last 10 years, scientists have begun to doubt the role of climate change. Because the increase in the number and size of dead zones coincides with the increase in global temperature. And because now there are dead zones in the open water. For example, areas where agricultural runoff cannot reach.

Researchers show that when water temperature increases, oxygen dissolves less well in said water. However, work on the dead zones of our oceans has remained piecemeal. Thus, the study published today by researchers from North Carolina State University (USA) is timely. Examines the data available for the Pliocene period.

The Atlantic Ocean is in danger

Why PliocenePliocene ? “Because the Pliocene – between 5.3 and 2.6 million years ago – is the last time we had a globally stable, warm climate. Average temperatures then were 2-3°C warmer than today, which scientists predict will be in about 100 years. they do.Catherine Davis, a professor of marine, earth and atmospheric sciences at North Carolina State University, explains in a statement.

These are small planktonplankton fossilized, single-celled organisms the size of a grain sandsandhe called foraminiferaforaminifera, informed scientists about the location of dead zones during the Pliocene. More a speciesspecies , Globorotaloides hexagonus. Because it is found only in regions where there is little oxygen.

The resulting map shows that low-oxygen waters were more common in the Atlantic Ocean during the Pliocene. Especially in the North Atlantic. “This map of Pliocene dead zones can give us an idea of ​​what the Atlantic Ocean might look like in 100 years. PlacePlace hotter. Because there is less oxygen in warm water. We’re already seeing it.”Comment by Catherine Davis.

The spread of dead zones in the global ocean is worrying

If nothing is done to finally limit the progress of anthropogenic global warming, the consequences will be many, the researchers say. From carbon storage to era food itemsfood items in the ocean through management peachpeach and marine species. So the dead zones consist of a floorfloor» WHO « crush sea animals — those who don’t die of asphyxiation — on the surface”. Fishermen can therefore have the illusion of abundance FishesFishesand the latter will simply be forced to live in reduced places. “Fisheries will have to consider the impact of dead zones in population management. »

Global ocean dead zones are expanding at a surprising rate

Expansion of “dead zones,” regions of the ocean where oxygen becomes too rare for living organisms, has already been reported. But a new study reveals that the short-term effects are far more severe ecosystemsecosystems .

Article published by Jean Etienne on 04/10/2008

Since the late 1990s, the “dead zones” or OMZ observed in the USA and especially in the Baltic Sea (open ocean oxygen-minimum zones , or Minimum Oxygen Zones) are the subject of special attention. This term refers to large areas known as hypoxic, i.e. lacking dissolved oxygen. Some of them are of natural origin, especially in the Black Sea deficiencydeficiency in oxygen for millions of years, even in the great depths of the sea. But human activity and its indirect consequences are causing it to spread today on a scale our planet has never known, except for certain climatic disasters marked by massive extinctions.

Already in 2004, the number of dead zones in the world was considered equal to 150, while in 1970 their number was insignificant. Today, they are growing not only in number, but especially in terms of territory, by about 5% per year.

The formation of the first dead zones was mainly associated with the input of agricultural fertilizers, especially nitrogen, as well as the flow of food and nutrients. issueissue organic matter from agricultural land degradation. other plants intended for production biofuelbiofuel and because it is not intended for human consumption, it is not subject to regulations, it is subjected to particularly intensive treatment and aggravates the phenomenon, e.g. corncorn Mexican responsible for the hypoxia observed in the northern Gulf of Mexico.

But there are other reasons…

The chemical pollutionchemical pollution It is not the only reason and it is accepted that turbidityturbidity water, in other words, loss transparencytransparencyby or as a result of other pollution eutrophicationeutrophication prevents penetration solar radiationsolar radiation hinders photosynthesisphotosynthesisplanktonic and increased lack of oxygen.

After the first dead zones in the Chesapeake Bay (eastern USA) and fjordsfjords Scandinavia, others have appeared off South America, but also in the seas around Ghana, China, Japan, Australia, New Zealand, Portugal and, more recently, Great Britain.

If the biological and chemical hypothesis is indeed the most consensual, global warming also plays a role in the creation of dead zones. Oxygen is actually less soluble in hot water than in cold water, so increasing the temperature accelerates the formation of oxygen. hypoxiahypoxia .

Less resistant species than previously thought

Marine organisms are more sensitive to oxygen deprivation than previously thought, especially fish and shellfish“, insists Raquel Vaquer Suñer of the Mediterranean Institute for Advanced Studies (Spain), the main author of the article published in Pnas.

Overly optimistic estimates

Until recently, most scientists believed that a dead zone could occur when oxygen levels in seawater fell below 2 milligrams per liter. But now it seems that some species suffer more from this deficiency. Some of them larvaelarvaeCrayfish in the eastern waters of the United States and Canada experience this deficiency as soon as the level falls below 8.6 mg/liter, which is well below the normal level for the region.

Although it is difficult to distinguish the effect of chemical pollution on aquatic fauna from the effect of hypoxia, it was possible to determine, especially experimentally, that oxygen deficiency seriously affects the reproductive abilities of animals.infinityinfinity, but also by reducing the size of the reproductive organs and the number of eggs. Hormonal phenomena are also emphasized.

The researchers, in their PNAS paper, are calling for an increase from 2 mg/liter to 4.6 mg/liter, which is considered incompatible with marine life.

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