A large ‘dead zone’ in the Gulf of Oman is increasing in size, according to scientists who warn the oxygen-scarce area is worse than previously thought and poses a threat to the environment. But what does that mean?

The dead zone in the Arabian Sea is now the world’s biggest Oxygen Minimum Zone (OMZ). About the size of Scotland or Florida, the dead zone almost covers the entire Gulf of Oman, which borders Iran, Oman, Pakistan and the United Arab Emirates.

What is a dead zone?

A dead zone is an area of the sea or a large body of water that’s almost entirely devoid of oxygen. The low-oxygen areas are called dead zones as they can’t sustain marine life. Fish, animals and plant life in the zones suffocate as a result of low oxygen levels, while some marine life manages to swim away from the area, leaving it empty.

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Scientists began noticing increasing areas of dead zones in the 1970s. In 2008, 405 dead zones around the world were noted by Sweden's Göteborg University.

Conditions in Gulf of Oman

The existence of the Gulf of Oman dead zone has been known for about 50 years, but the extent of the area has only recently been confirmed thanks to the use of underwater robots, called Seagliders.

“The Arabian Sea is the largest and thickest dead zone in the world. But until now, no-one really knew how bad the situation was because piracy and conflicts in the area have made it too dangerous to collect data,” Dr Bastien Queste from UEA’s School of Environmental Sciences said. “We barely have any data collected for almost half a century because of how difficult it is to send ships there.”

The remote-controlled submarines are the size of a small person and can spend months underwater at 1km. They were deployed by researchers from the University of East Anglia to the Gulf of Oman for eight months.

The team’s findings have now been published in Geophysical Research Letters. They found a strong decrease of oxygen in the afflicted zone compared to pre-1990 levels, and also charted how the oxygen is spread around the area across different seasons.

“Our research shows that the situation is actually worse than feared – and that the area of dead zone is vast and growing. The ocean is suffocating,” Queste said.

What causes dead zones?

Dead zones can occur naturally, but also grow as a result of excessive nutrient pollution from human activities, the National Oceanic and Atmospheric Administration explains. Many chemical, physical and biological factors combine to create dead zones, but nutrient pollutions are the primary culprit. Nitrogen and phosphorus in fertilizers run into the water, and then act as nutrients which fertilize algae.

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The algae eventually dies and decomposes in the water. This then feeds bacteria which consume oxygen around them, depleting the supply. Climate change has exacerbated the issue, as warmer waters contain less oxygen.

The Gulf of Mexico is home one of the largest dead zones, which occurs each spring when farmers fertilize their land and the rain washes the fertilizer into rivers and into the sea. An area in the Baltic Sea is another large dead zone.

Why are dead zones bad?

Dead zones are “a real environmental problem, with dire consequences for humans too who rely on the oceans for food and employment.” Guete said.

More worryingly, dead zones produce nitrous oxide, which is more damaging to the environment than carbon dioxide.

Scientists warn that computer simulations of ocean oxygen reveal levels will decrease over the next 100 years, with oxygen minimum zones growing. The next step for researchers is further study to determine all of the contributing causes of the increasing dead zone.

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