As another massive "dead zone" forms in the Gulf of Mexico, other patches of low-oxygen waters are expanding elsewhere in the ocean, threatening marine ecosystems as climate change accelerates.

This summer's Gulf dead zone, which the United States National Oceanic and Atmospheric Administration expects to be the third-largest since monitoring began in 1985, is the result of the runoff of agricultural fertilizers and municipal wastewater. Nutrients in the runoff stimulate algae blooms, which absorb most of the oxygen in the water as they decompose and sink to the sea floor. High stream levels this year have carried even higher levels of nutrient pollution from the land out to sea and scientists expect the Gulf dead zone to double this summer, to the size of New Jersey. Such low-oxygen dead zones threaten fish and other marine life, including commercially valuable species such as shrimp.

But there's another type of oxygen-starved ocean zone, one that is present year-round and wasn't caused by human activities - until recently.

Oxygen-deficient zones naturally occur in belts along the equatorial waters of eastern ocean basins. But as more and more carbon dioxide is pumped into the atmosphere from the burning of fossil fuels and as ocean surface temperatures continue to rise, deoxygenation zones are expected to expand - with profound implications for marine life.

Deoxygenation receives less attention than other climate change impacts, such as ocean acidification and rising water temperatures that have triggered the worldwide bleaching of coral reefs. But it is emerging as a major threat to the ocean.

Said Mak Saito, a senior scientist at Woods Hole Oceanographic Institution: "We've heard about the warming effects for a long time. And we've heard about the acidification effects for about a decade or so now. But deoxygenation is one of the newer problems we're realizing is out there."

An ocean with less oxygen means an ocean with fewer fish, depleting fisheries that people depend on for livelihoods and protein. And deoxygenation threatens to disrupt the food webs that shape the ecology of the oceans.

"Oxygen is really a master variable for all life," said Saito. "For some species, we get below a certain concentration and we just die."

The warming ocean will decrease the production of plankton and other microscopic flora and fauna that are "the ultimate food source for virtually all ocean ecosystems," said Matthew Long, an oceanographer with the National Center for Atmospheric Research who has authored several studies on deoxygenation. "As production declines due to warming, marine food webs may collapse."

The extent and timing of those impacts are still largely unknown and there is some lack of clarity about how all the variables that affect and are affected by deoxygenation will interact. Oxygen-minimum zones, for instance, may expand over the next few decades as warming continues and then contract as climate change reduces the production of oxygen-consuming organic matter, allowing oxygen levels to recover, according to Long.

His 2016 study found some parts of the oceans were already showing deoxygenation that is attributable to climate change and that signs of deoxygenation caused by climate change should be widespread by the 2030s.

However, another study published in February concluded that those widespread signs are already in play. Researchers found that the oxygen content of the ocean decreased by more than 2 percent between 1960 and 2010.

That loss of oxygen is happening for two reasons. Warmer water is able to hold less dissolved oxygen than colder water. And the warmer surface waters become, the more buoyant they are, increasing the stratification of ocean layers and decreasing the downward mixing that brings oxygen to marine life in deeper waters.

That type of stricter stratification is seen in today's naturally occurring oxygen-deprived zones, affecting the ability of currents to distribute nutrients and oxygen. Although it's possible those zones could contract in the future, research shows that they are already expanding and will continue to do so over the next several decades, leading to other potential impacts as well, Long said.

"There is a potential double-whammy effect because as temperatures increase, organisms require more oxygen to meet their basic metabolic needs," he said in an email. "Lower oxygen levels could thus lead to a contraction and fragmentation of viable ocean habitat."

Ocean acidification, he says, is pretty straightforward - more carbon dioxide in the atmosphere leads to more CO2 absorbed in the ocean, lowing the water's pH. But deoxygenation involves many subtle, competing mechanisms that interconnect in ways that make the phenomenon difficult to forecast.

"We know with virtually 100 percent certainty what will happen to mean oxygen concentrations in the oceans, but the subtleties are more difficult to predict," Long said.

One way to try to better understand what an ocean with less oxygen will look like is to study current low-oxygen zones.

In 2016, Saito sailed from Hawaii to Tahiti, studying oxygen-deprived waters - what he calls the "microbiome of the extremities of the Pacific" - to try to learn about the ocean of the future.

While most organisms rely on oxygen, the microbes that live in these oxygen-starved waters evolved to breathe nitrogen, converting fixed nitrogen in seawater to nitrous oxide that is released into the atmosphere.

As low-oxygen regions expand, so will the range of these nitrogen-processing microbes, consuming more of the ocean's nitrogen, a vital resource for ocean life, and releasing more nitrous oxide, the third biggest contributor to climate change after carbon dioxide and methane. "If the balance of nitrogen changes in the ocean, that could be big," said Saito.

"There's a lot of implications that we're just starting to unravel," he added.

Matthew Berger is a journalist based in San Francisco specializing in ocean issues. This article originally appeared on Oceans Deeply, and you can find the original here. For important news about ocean health, you can sign up to the Oceans Deeply email list.

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