Biodiversity: Ocean 'dead zones' are proliferating

As the UN’s COP15 talks on biodiversity got under way in Montreal on Wednesday, FRANCE 24 spoke to marine biologist Françoise Gaill about marine “dead zones” and their link to global warming.

One of the main goals of the 15th UN conference on biodiversity, known as the COP15, is to ensure the protection of 30 percent of all marine ecosystems on the planet. Though conservation efforts are often focused on the species found on land, oceans and seas are home to a wide range of species whose survival is threatened by several factors.

An ongoing decrease in oxygen levels underwater is an important component of the loss of marine life. More than 400 "dead zones" – where aquatic life can no longer survive – existed in the world’s oceans in 2007, according to a study led by a scientist at the Virginia Institute of Marine Science, compared to 150 in 2003.

Deprived of essential oxygen, these marine areas span 245,000 square kilometers and threaten vertebrate animals, with more than a third of marine mammals affected. The phenomenon has been ongoing since the 1980s and is proliferating, while research on the subject lags behind.

Françoise Gaill, a French marine biologist and vice president of the Ocean & Climate Platform, who is also a scientific adviser at France’s National Centre for Scientific Research (CNRS), spoke to FRANCE 24.

FRANCE 24: What is a dead zone?

Françoise Gaill: Dead zones are hypoxic areas in the ocean, where the concentration of oxygen is below the norm. This can mean a decrease of up to 20 percent, which is already quite significant, but can reach up to a 50 percent drop in oxygen levels.

The lack of oxygen occurs in the ocean’s surface areas, between 50 and 400 metres deep. The shallowest waters are generally less affected since they have more contact with the air and therefore benefit from oxygenation, which is less available in deep water.

Dead zones are mostly found off the coast of the Americas, from California to Chile. West Africa is also affected, as is the western part of Indonesia in the Indian Ocean.

Although they mostly hug coastlines, we are starting to see some dead zones stretch from the Americas into the middle of the Pacific, far from the shore.

What consequences do these zones have on the planet’s biodiversity?

A lack of oxygen in the water causes a change in environment, which will naturally have an impact on marine biodiversity.

When oxygen levels are reduced, fish – who need it to breathe – may experience hypoxia and are at risk of death. If they survive, they will migrate to areas with higher oxygen levels, which affects the ecosystem at large and takes a toll on local biodiversity.

Animals like crabs and shellfish, which can’t escape from these areas as quickly, can die of suffocation. Some dead zones have even been identified after heaps of dead carcasses were found scattered across beaches.

All animals need oxygen to live, and therefore all animals are affected. Plants less so, since they are less dependent on oxygen.

What causes dead zones?

Dead zones are a naturally occurring phenomenon. Some areas are less oxygenated than others due to ocean currents, but it’s normally quite rare to come across.

At first we thought that the proliferation of these zones was caused by human activity in a process called eutrophication, when organic matter like agricultural products or fertilizers enter a body of water, leading to an increase in planktonic organisms. The organisms multiply until they exhaust the oxygen available in the environment.

But over the last 10 years, we’ve realized that human activity isn’t the only cause of declining oxygen levels. Global warming also plays a role; there is a correlation.

The rising number and amplitude of dead zones goes hand in hand with climate change. Although dead zones are mostly coastal, some now extend into open waters – indicating that falling oxygen levels are not only due to agricultural run-off. Global warming causes water temperatures to rise, and oxygen does not dissolve as well in warm water.

Will ‘dead zones’ remain dead forever?

No, not at all. It’s a dynamic phenomenon. Oxygen levels can be replenished by underwater currents or intense weather events such as storms.

Dead zones are therefore not permanent, but there is a probability they will form again in the same place due to local currents. It’s also possible to limit the impact of human activity by reducing the amount of agricultural discharge dumped into waters.

But the correlation with climate change changes things. A consequence of rising seawater temperatures is that marine currents could be stalled, making these zones “watertight” and preventing them from mixing and therefore reoxygenating.

So dead zones must be monitored for the sake of biodiversity, the fishing industry and even tourists. While it’s relatively simple to reduce the amount of discharge going into our oceans – by limiting agricultural waste, for example – global warming isn’t as reversible.

These dead zones will keep proliferating if nothing is done to curb climate change, which requires cutting back greenhouse gas emissions and limiting global warming to a maximum of 1.5 or 2 degrees Celsius.

This article is a translated version of the original in French.