Tackling the oceans’ dead zones

The rancid smell of rotten fish met New York City residents as they strolled along the Hudson River in December 2020. Walkers and environmental action groups reported hundreds of dead fish washing up on the river’s banks from Sleepy Hollow to Yonkers, or sightings of the sea life visibly suffocating in the city’s harbour.

While there were multiple causes of the die-off, one significant factor is hypoxia – which is when the amount of oxygen dissolved in a body of water drops below a certain threshold. Without enough oxygen in the water, fish and many other forms of sea life may die en masse, leaving the surrounding area lifeless.

Hypoxia (or anoxia, where there is no oxygen present in water at all) can happen naturally. There are certain areas of the ocean that contain low levels of oxygen due to their location and limited inflows of new water. However, this phenomenon is greatly exacerbated by human activity; the number of coastal dead zones has increased tenfold since the 1950s, according to a 2018 study. What is more, many natural dead zones have noticeably increased in size.

Besides the damage dead zones cause to our oceans and waterways, they also have worrying implications for coastal economies, with fisheries and tourism particularly affected. Why do they happen, and what can be done about the problem?

Several human activities contribute to the development of dead zones, but by far the biggest culprit is the use of artificial fertilisers in agriculture. Since the middle of the last century, farms across the world have begun spraying phosphorus and nitrogen onto their fields at industrial levels, improving crop yield and making poor-quality land more productive. However, when this fertiliser gets into waterways, it can have disastrous consequences for life in lakes, rivers and oceans.

It works like this. Excess nutrients in the water encourage algae to grow at frenzied rates. If you’ve ever seen a lake or canal covered in a deep green layer of algae, it may well have been caused by an excess of nutrients seeping in.

Eventually, these algae blooms die and sink to the bed of the river, lake or ocean. There, they are broken down and consumed by bacteria, using large amounts of oxygen in the process. The more oxygen the bacteria use, the more the water is depleted of the element and other species struggle to survive. This can then lead to catastrophic collapses of local sea life (of course, the process is more complex than this brief description and varies depending on the local context).

Professor Paul Tett of the Scottish Association for Marine Science (SAMS) in Oban explains that several other factors also contribute to the emergence of dead zones. Untreated sewage means organic matter gets dumped on the seabed, which encourages the growth of seaweed and phytoplankton. When these die, there is greater organic decomposition, more oxygen consumption and, in turn, hypoxia. Fish farms also contribute in a similar way. Tett adds that climate change is an important factor. Warm water holds less oxygen, so as the ocean’s temperature rises, we can expect hypoxia to become more common.

Perhaps the biggest ‘man-made’ dead zone is found in the Gulf of Mexico, which stretches from Louisiana to Texas. It varies in size but can reach some 20,000km2 – an area roughly the size of Wales, where little life can exist during the dead zone’s peak season. The Baltic Sea also has large dead zones reaching as much as 60,000km2 in total (though how much is caused by humans and how much is the sea’s natural state is unclear). The largest dead zone on the planet is found in the Arabian Sea (some 160,000km2). This one is largely natural, although its size is being exacerbated by climate change.

The solution seems obvious at first. If governments were simply to ban the use of fertilisers and prevent effluent entering waterways, the root cause of the problem would disappear.

Indeed, Tett explains that “if we stop the inputs, some dead zones can go away within as little as a year,” especially if they are fairly small and concentrated in size. The ocean’s currents simply move more oxygenated water into areas that have suffered from environmental dumping and the seabed can rebound.

However, the reality of dealing with the underlying causes is complex and far from straightforward. Tett points out that even if we were to completely stop using fertilisers today, the amount that has leached into the soil means that run-off will continue to happen for many years.

The bigger issue is that banning fertilisers would be a formidable political and economic challenge. Countless farmers around the world rely on fertilisers to grow crops, and forcing them to stop would be very difficult, but there are things that can be done to better control the agricultural use of fertilisers. Some of this comes down to education and irrigation methods, which would mean farmers use fertilisers less indiscriminately. Planting cover crops on fields and along their borders to absorb excess nutrients is also an option, while controlling how water gets drained out of fields could also help (see box below for UK rules on fertilisers). Ensuring all sewage is treated would of course be beneficial, too.

“The ultimate solution to stopping dead zones is to stop the causal mechanism” says Dr David Koweek, a marine scientist at Ocean Visions, a centre for research in Stanford, California. “But sometimes you need to address the problem symptomatically,” he argues. “If the ecosystem is on life support, you want to stage a more drastic intervention; there are times when a ventilator is called for.”

In moves to address these symptoms, marine scientists around the world have begun investigating a range of solutions that could provide the ‘ventilator’ that highly stressed dead zones might need to survive.

Dead zones occur because the bed of the sea is covered in decomposed organic matter. At this lower level of the water column, there is almost no oxygen available, which means plant and animal life cannot feed there. As a result, fish and other creatures higher up the food chain have nothing to feed on either.

Surface water naturally dissolves oxygen from the air and therefore contains more of it. Water at the top of the column also moves around more, so is easily replenished with oxygenated water from elsewhere. But water at the bottom of the column moves much less by comparison, so remains hypoxic.

One option might be to pump oxygen-rich surface water down to the depths. This is the idea behind Dr Koweek’s research. The technique he has investigated is known as downwelling and is not an entirely new concept (it has been used in Swedish fjords, for instance). But Koweek and his peers wanted to find out how efficient it is compared to other oxygenation methods such as using fountains or bubbling water up from the seabed.

The proof-of-concept field experiment was fairly simple, he says: “We used a large plastic straw floating at the surface and simply pumped water down through the tube towards the bottom of the water column.” The effects were impressive, and increased oxygen saturation in the water below the pumps by as much as 30 per cent. This was found to be much more effective than other existing water movement techniques. 

Koweek and his colleagues then used computer models to estimate what it would cost to run this kind of pumping on the scale needed to alleviate dead zones like those found in the Gulf of Mexico. While the costs ran into the tens of millions of dollars, they found it would be significantly less expensive than all the necessary upgrades to wastewater treatment plants or fertiliser reduction programmes to achieve the same goal.

He stresses that such a method has limitations, is but a short-term sticking plaster, and that fixing the root causes is the only real solution.

An alternative method would be to intentionally encourage the growth of plant life that would absorb fertilisers before they can feed algae. This is the idea behind a research project being conducted by Phoebe Racine, a PhD candidate at UC Santa Barbara.

Racine and her colleagues have begun investigating how seaweed aquaculture can take up nitrogen, phosphorus and trace metals. Rather than dying and contributing to the dead zone phenomenon, the seaweed is harvested and removed from the ocean altogether, before the next crop is grown.

“We wanted to find out if it would pick up anthropogenic nitrogen and phosphorus, if it could be space efficient and cost effective, or even revenue-generating,” says Racine. Based on a pilot study, the answer seems to be that it is.

Seaweed farms are normally held up in the water column on buoys and planted on plastic lines. Racine’s model research farm was small, but the seaweed appeared to be effective in absorbing nutrients that may otherwise have contributed to the growth of algal blooms. It had added benefits too, such as increasing water oxygen levels, providing a home for certain types of fish and absorbing atmospheric carbon dioxide.

To address the full scale of the Gulf of Mexico dead zone, Racine’s projections indicate that some 2 per cent of the USA’s portion of that sea’s surface area would need to be covered in seaweed farms, which would represent a vast area. This could produce problems of its own, and she positions seaweed aquaculture as “one tool” among others to address the issue.

An exciting possibility is that such farms could feasibly create new industries and generate jobs. “Seaweed is undergoing a renaissance in product development,” she reports. Various companies are starting to explore its potential use as a biofuel, in bioplastics and textiles, agricultural feed and much beyond.

A more direct solution to this problem is proposed by Dr Gang Pan of Nottingham Trent University. Pan has conducted research into the use of a new nanobubble material, which could be used to tackle hypoxia where it occurs, on the seabed.

Pan’s research has investigated how clay blocks can be loaded with oxygen nanobubbles. These can then be sunk to the bed of a river, ocean or lake where they gradually dissolve. The microscopic nanobubbles don’t rise to the surface but remain in situ, gradually oxygenating the surrounding water.

Pan says the technology can either be distributed by gravity – simply sinking blocks into anoxic or hypoxic areas – or by allowing river or ocean currents to distribute them naturally. In this way there would be almost no energy needed to get the oxygen to the bottom of the ocean. “By loading oxygen into the clay and sinking down to the bottom of the seabed it can be a breakthrough way of addressing hypoxia and anoxia,” he reports.

All the researchers interviewed for this article stressed the dangers of so-called ‘geoengineering’. Tinkering with the natural world’s processes could open a Pandora’s Box of unintended consequences.

For instance, adding new nanobubble materials to the ocean floor could change the natural state of the sediment. Or downwelling could alter the balance of the deep ocean – which is naturally low in oxygen anyway. Professor Paul Tett of SAMS notes that pushing oxygenated water to the bottom of the sea could potentially displace anoxic water that contains methane and hydrogen sulphide and bring this to the surface – which would present its own dangers.

However, Dr Koweek from Ocean Visions points out that “while ‘geoengineering’ has taken on a lot of connotations, we actually do it all the time”. Whether it’s through the burning of fossil fuels or by pumping phosphorus and nitrogen into our waterways, many of our actions are changing the world around us. Researchers are simply proposing “to use the best available science and engineering knowledge to counter its effects”.

In places that have already been changed dramatically by human activity, Koweek warns that we need to consider the relative merits of action and inaction. Scientists must work closely with local communities, he says, and be completely transparent about their activities, in order to build public trust before changing the natural environment.

Addressing the problem of dead zones will therefore require walking a fine line. On the one hand the scale of the damage already done demands large-scale intervention. On the other, the dangers of interfering with hugely complex systems require caution, too.

“Humanity maybe has to intervene if we hope to preserve our oceans in the state we want,” says Tett, “but we need to do a thorough risk assessment before doing any of these big exercises.”