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.