Oxygen is disappearing from our seas. Although climate change is the primary antagonist in the open ocean, coastal regions are suffocating too as agricultural runoff is triggering biochemical cycles that draw away vast reserves of oxygen.

One so-called “dead zone” in the Gulf of Mexico, already worrying for being larger than the state of New Jersey, is a grim microcosm of this coastal problem. As reported in a new Science study, this 22,739-square-kilometer (8,780-square-mile) region will persist for several decades, even if we do everything we can to reverse course.

The paper suggests that if we’re going to even attempt to shrink the record-breaking dead zone to just 5,000 square kilometers (1,930 square miles) by 2050, we’d need to bring nitrogen levels in the water to zero. They consider this to be not just “unrealistic, but also inherently unsustainable.”

The damage, as they say, has already been done.

“These coastal dead zones are considered by many to be one of the primary threats to marine species,” lead author Dr Kimberly Van Meter, a postdoctoral fellow at the University of Waterloo (UoW), told IFLScience.

“The Gulf of Mexico’s summer dead zone forms right in the heart of one of North America’s most important fisheries, and continuing damage to fish habitat could severely threaten the regional economy.”

Although not unique, the cause of this dead zone is pretty much the same as elsewhere in the world: Nitrate and phosphorus-rich runoff from near-shore agriculture and industry is lapped up by the phytoplankton adrift in the nearby waters.

This triggers a population boom, which ultimately leads to larger die-offs down the line. As they become tiny corpses, they sink, and the bacteria that break them down consume vast amounts of oxygen as they do so.

This process is known as eutrophication, and you can commonly see it in lakes or rivers flush with algae. Sometimes this leads to a brief depression in dissolved oxygen levels in the region, but if the agricultural runoff is persistent or potent enough, it can create a lasting patch of hypoxia.

Consequently, marine life in the region is forced to evade the oxygen-deprived section of the water column, or it fails to adapt and escape and dies off. The proliferation of the algae itself also generates toxicity in the water, which can prove fatal to fish, shellfish, marine mammals, and birds.

The colloquialism of “dead zone”, then, is decidedly apt. The Gulf of Mexico’s dead zone is particularly galling, though.

The result of the near-constant flow of agricultural runoff in the region – particularly from the machinations of the meat industry there – over the past few years, the dead zone shocked scientists as it outmatched all and any predictions of its growth given by the National Oceanic and Atmospheric Administration (NOAA).

Several task forces designed to stem its growth were quickly set up in the 2000s, but each comprehensively failed. As the new UoW-penned study notes right from the outset: “In August 2017, the Gulf of Mexico’s hypoxic zone was declared to be the largest ever measured.”

It’s been previously estimated that if the level of waterborne nitrogen was reduced by 60 percent, it would be enough to stem the region’s devastating eutrophication, but little work has been done to properly assess this idea. Using modeling, the team peered into a variety of possible futures: One in which runoff levels remained unchanged, and others in which 25, 75, and 100 percent reductions in nitrogen loads took place.

Their conclusion is, frankly, grim. Even if agricultural nitrogen use becomes 100 percent efficient, the Mississippi River Basin’s “legacy nitrogen” – the compounds that have impregnated the environment over the past few decades – will persist and not filter out for several decades to come.

Can anything be done? The study’s senior author, UoW associate professor Nandita Basu, maintained that we must maintain current conservation measures, but “we may need to consider more large-scale changes in our crop production system,” including picking crops that “can dramatically reduce the amount of nitrogen running off of fields.”

“If we value water quality, we need to think creatively, stay committed, and be prepared for change.”