May 9, 2016
By Moises Velasquez-Manoff
Oxygen-depleted oceans have preceded many mass extinctions in Earth’s past, including the worst one of all 252 million years ago. Are hypoxic dead zones from California to Namibia a harbinger of the next extinction?
It was crabbers who first reported something amiss. In 2002, they began pulling in traps full of corpses. (Crabs should be alive when you catch them.) And they mentioned something else: Little octopuses had followed their crab lines to the surface, as if fleeing inhospitable conditions below.
Then heaps of dead crustaceans began washing ashore along a stretch of Oregon’s coast. When scientists sent a robotic submersible offshore, they discovered mile upon mile of dead crustaceans, the water brown and murky with detritus.
The killer was low oxygen, or hypoxia. Nearly all animals require oxygen to live, and, that year, dissolved oxygen had fallen so low off Oregon’s coast that whatever mobile creatures could had fled, while more-sessile life had simply suffocated.
“You look out, all you see is blue ocean. It looks exactly the same as 50 years ago. But it’s not.”
Every year since, those hypoxic waters have appeared in late summer and early autumn. In 2006, they became anoxic, meaning they lost all their oxygen. “You didn’t see a single fish in a day,” says Jack Barth, a professor at Oregon State University, “just the piles of crab carcasses and worms that had come out of the bottom, sort of wafting in the current.” When scientists examined the roughly 50-year record of oxygen measurements from the region, they couldn’t find a single comparable event in the past. The hypoxia, it seemed, was unprecedented.
And then it spread.
In 2013, low-oxygen water showed up off the California coast just north of San Francisco. The following year, crabbers pulled in dead crabs in Half Moon Bay, just below San Francisco. Further south, the Monterey Bay Aquarium registered a decline in the oxygen content of the water it pumps in from the ocean.
“It seems like something has changed,” says John Largier, head of the Coastal Oceanography Group at the University of California-Davis. He tries to remain skeptical, but he suspects “large-scale global change.”
These suffocated patches of ocean aren’t just bad for fishermen and their catch; they represent a change in the ocean that has, at times in Earth’s past, heralded mass extinction.
Hypoxia occurs naturally in fertile waters. Plankton blooms and, as it dies, it rots, leeching oxygen from the surrounding water. Human effluent is often responsible for these low-oxygen expanses, or dead zones. When fertilizer or sewage washes down rivers, it prompts algal blooms and then oxygen depletion.
But the low-oxygen zones appearing along the western coast of North America aren’t due to runoff, at least not entirely. They probably reflect a number of related forces acting in concert. Along the western edge of North America — and the western edges of other continents — winds that cause upwelling have strengthened in some areas in recent decades, pushing water out to sea with greater force. That has intensified the upwelling of nutrient-rich water along the coast, leading to more phytoplankton, and more hypoxia as the blooms die.
The ocean is also changing independently of the winds. Warm waters hold less oxygen than cold, so as the Pacific has warmed, its ability to absorb oxygen from the atmosphere has declined. And how the ocean has warmed is also likely playing a role. The ocean is naturally stratified, like a layer cake. Deep waters are, because they’re separated from the atmosphere, always somewhat oxygen-depleted.
The oceans became hot, stagnant, and inhospitable to animals, reverting to a primeval state where microbes reigned supreme.
As the upper layers have absorbed heat from the atmosphere — about four small nuclear devices’ worth per second, according to one estimate — they’ve become more buoyant. That relatively warm, buoyant surface water acts like a lid on the ocean, impeding aeration of deeper waters. It’s this already oxygen-poor water that may be upwelling along western North America — meaning that the water in Oregon and California’s hypoxic zones is being fed by a much larger, growing mass of low-oxygen water in the Pacific.
“When you look out at the ocean, all you see is blue,” says Lisa Levin, a biological oceanographer at the Scripps Institution of Oceanography in La Jolla, California. “It looks exactly the same as 50 years ago, even though it’s not.”
When we think about the ocean and climate change, we often imagine ocean acidification, which will make life difficult — and, as it progresses, impossible — for some shell-forming organisms. We learn of melting ice caps and sea-level rise, which could require that humans retreat from the coastline and abandon low-lying regions, like Florida. We don’t hear much about hypoxia. Yet in some parts of the ocean, like the northeastern Pacific, hypoxia may be a greater near-term threat than acidification. And its emergence in multiple other places has caught some in the scientific community by surprise.
“It’s been a large potential problem that’s generally been under-appreciated,” says Curtis Deutsch of the University of Washington.
Almost anywhere anyone has looked in recent years — including the Atlantic Ocean — those deep, naturally low-oxygen waters have pushed upwards, in some places by an average of three feet per year. It’s unclear how marine life will respond. Off Southern California, where a northerly current has grown warmer and less oxygenated in recent decades, some sea urchins are moving into shallower water. The ecosystems also seem to be simplifying. Like underdeveloped economies, less diverse ecosystems may be less productive — yielding fewer of the goods, such as fish, that we like. They’re also more prone to collapse.
New creatures have also arrived. The man-sized Humboldt squid typically inhabits waters off the coast of Mexico and points further south. In the early 2000s, they began pushing north, frightening divers along the California coast, and eventually fouling fishing nets as far north as Alaska. The squid, it turns out, are unusually tolerant of low-oxygen conditions. They seem to have surfed a growing expanse of oxygen-depleted water northward.
More broadly, as fish squeeze into a narrower band of ocean, catches may improve, at least for a while. But there’s a potential downside to catching too many fish — overfishing. Some speculate that the decline in recent decades of large fish everywhere may have been driven, in part, by rising low-oxygen waters that have essentially pushed fish toward fishermen.
For those who study the climate of Earth’s past, the mounting signs of ocean hypoxia inspire both unease and a sense of vindication. In the past two decades, paleontologists have developed a new theory about mass extinctions — including the worst of all, the End Permian die-off 252 million years ago. Ocean hypoxia features prominently. Then, as now, the trigger was a rapid accumulation of heat-trapping gases in the atmosphere. The oceans became hot, stagnant, and inhospitable to animals, reverting to a primeval state where microbes reigned supreme.
In 1980, scientists Luis and Walter Alvarez, a father-and-son team, proposed a new idea for the disappearance of the dinosaurs 65 million years ago: an asteroid strike. As evidence, they pointed to a 65-million-year-old layer of clay, found around the world, rich in an element called iridium. Iridium is rare on Earth, but common in asteroids. That layer, they argued, contained the pulverized remains of a six-mile-wide asteroid. A 110-mile-wide impact crater later discovered off the coast of the Yucatan in the Gulf of Mexico lent credence to their idea.
Earth has experienced five such extinction spasms, interspersed with numerous smaller ones, and many argue we’re now entering a sixth extinction — one caused by human activity. Before the Alvarez hypothesis, scientists often blamed volcanic activity for these die-offs. After Alvarez, many suspected that collisions with celestial bodies might have been responsible.
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