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Suffocating oceans: Arabian Sea ‘dead zone’ is larger than previously expected

Suffocating oceans: Arabian Sea ‘dead zone’ is larger than previously expected
10 May
An oxygen-deprived ‘dead zone’ in the Arabian Sea is much larger than scientists have previously expected

The Arabian Sea has never been an easy place to conduct scientific research. Plagued by piracy and mired in regional conflict, researchers have often had little choice but to leave it alone. For almost 50 years scientists have collected only limited data.

However, a team of researchers and their remote-controlled robots have made it to the Gulf of Oman, the northwest arm of the Arabian Sea, and in the process made a surprising discovery. The team, from the University of East Anglia, in collaboration with Oman’s Sultan Qaboos University, intended to study algae but what they found was a dead zone, much larger than anyone had expected.

Dead zones, or oxygen minimum zones (OMZs), are areas of the ocean with depleted oxygen levels. A natural phenomenon in some parts of the world, they occur between 200 and 800 metres deep and are caused by an imbalance between the oxygen getting into the water (via the atmosphere) and oxygen ‘breathed’ by bacteria that feed on sinking organic matter. When the balance shifts in the bacteria’s favour, dead zones occur. They exist in varying degrees – the most problematic areas have almost no oxygen at all and are known as ‘anoxic’.

The researchers already knew that there was a dead zone in the Arabian Sea, but had no idea of its scale. The accepted wisdom, albeit based on very old data, was that the very core of the Arabian OMZ was anoxic, but that oxygen levels increased toward the margins. Over eight months, however, the sea-faring robots deployed by the team built up a picture of the oxygen levels in the Gulf and the wider sea, revealing an anoxic region larger than Scotland, making it the largest in the world. The new data shows that the Arabian OMZ has both intensified and grown, bringing it much closer to where people live and fish.

editfishkillA beach is covered with dead fish, killed by a bloom of red algae

The most obvious problem with dead zones is that it becomes impossible for fish, marine plants and other animals to survive. Anything caught in the void is effectively suffocated to death, reducing biodiversity. Even areas of low oxygen can lead to stunted growth and poor reproduction. In other parts of the world, where coastal dead zones occur, huge fish-kills have resulted in beaches carpeted with dead animals. In 2016, lack of oxygen in the waters off Chile led to the death of 100,000 tonnes of salmon and trout, equivalent to £590million in exports.

In Oman, symptoms like these will be of particular concern to people working in the tuna industry, according to Dr Bastien Queste, who led the study. ‘The tuna fishery near Oman is huge,’ he explains. ‘It’s very affected by the growing OMZ which compresses fish into a thinner layer at the surface. If we’re not careful, we’ll suspect there are more tuna, but in reality there are the same number, simply stressed and squeezed into a smaller space.’

A further problem with dead zones is that low oxygen forces bacteria to ‘breathe’ nitrate instead of oxygen. This process of denitrification releases nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide.

editTunaTuna fishing is big business in Oman but the growing OMZ could limit the space in which they can survive

The potentially devastating impact of dead zones isn’t limited to the Arabian Sea. In January, another study by the Smithsonian Environmental Research Center found that dead zones in the open ocean have quadrupled in number since the 1950s, expanding by millions of square kilometres. In the same period, dead zones in coastal waters, such as estuaries and seas, have increased more than ten-fold. One of the most well-known dead zones, in the Gulf of Mexico, is now larger than the state of New Jersey and covers around 8,776 square miles. The study’s lead author, Denise Breitburg, said: ‘Oxygen is fundamental to life in the oceans. The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth’s environment.’

To an extent, dead zones are naturally occurring, but there’s no doubt that human activity contributes to their growth. The balance between oxygen supplied to the ocean and consumed by its inhabitants is so delicate that it’s not difficult to shift the scales. In the ocean, climate change is a key culprit.

‘Increased warming through climate change means that bacteria breathe more for a start,’ says Queste. ‘It also means the surface layer is warmer, creating a stronger barrier between the surface warm water and the deeper cold water (a density barrier) which reduces the amount of oxygen being injected into the ocean. Climate change will also change wind patterns which changes surface energetic mixing.’

Coastal waters face another man-made problem. Intensive agriculture results in the run-off of fertiliser into rivers and streams. This, in turn, means that nutrients such as nitrogen and phosphorous, along with manure and sewage pollution, enter the water system, providing a feast for algae populations which bloom rapidly. As the blooms decompose and sink, bacteria step-in to process the algae, consuming huge amounts of oxygen as they go. This type of ‘nutrient pollution’ has been largely blamed for the Gulf of Mexico’s gargantuan dead zone.

editalgaeAlgae bloom pollution in Lake Okeechobee, Florida

Dr Queste doesn’t yet know what has caused the Arabian Sea’s OMZ to grow, though he intends to find out. ‘Figuring out whether the supply of oxygen is decreasing, or if it’s the consumption that’s increasing would be hugely interesting,’ he says.

Either way, he is clear that human activity affects dead zones in ways that marine creatures aren’t able to adapt to. According to Queste and others scientists in the field, immediate action is required.

‘We have to dramatically reduce, and possibly even reverse if possible, our ecological footprint,’ says Queste. ‘CO2 emissions would be a great start, but sewage treatment, and managing agriculture run off are key for protecting the marine environment.’

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