Troubled Waters "Ocean Deserts" are Expanding, Disrupting Habitats and Suffocating Marine Life

The world’s oceans are heating up. And like a bottle of pop left out in the sun, they’re going flat. Except instead of carbon dioxide, they’re losing oxygen. As with most liquids, water’s ability to dissolve oxygen largely depends on temperature. The warmer the water, the less oxygen it can hold.

Though oxygen-starved or hypoxic zones have always existed, warmer waters may be causing these zones to expand, according to new research published in the journal Science. The study, led by Lothar Stramma at the University of Kiel in Germany, addressed changes in different regions of the oceans, and found that oxygen levels in tropical oceans hundreds of feet below the surface have declined over the past 50 years.

These low oxygen areas are generally found in the Pacific, Atlantic and Northern Indian Ocean, according to coauthor Gregory C. Johnson, an oceanographer with the federal Pacific Marine Environmental Laboratory in Seattle. Though located deep within the ocean, these oxygen-minimum zones have the ability to affect coastal areas.

“Along California, there are undercurrents that carry these low oxygen waters forward,” Johnson says. “These oxygen-depleted zones are spilling onto the continental shelf off the coast of California and are starting to [expand] near the coast of Peru.”

The low oxygen levels suffocate some species while driving out others. “As these areas expand, certain species’ habitats become more and more limited,” Johnson says.

More significantly, surface warming increases stratification, creating a barrier between the lighter warm water and denser cold water in the ocean. It’s like putting maple syrup at the bottom of a container and lighter fluid at the top, explains Jack Sobel, a scientist at the nonprofit Ocean Conservancy. The two can’t mix. “There’s a stronger contrast in density between warm waters above and cool waters below,” Johnson says.

Warmer water means less oxygen and serious threat to ocean species. The Hawaiian monk seal is in danger of starvation. © U.S. Fish & Wildlife

This cap keeps oxygen-enriched surface waters from mixing with the colder water below and also decreases the likelihood of nutrient-rich, colder water making its way to the surface. The process, known as upwelling, is crucial to the productivity of marine life.

“The mixing of nutrients and oxygen is what drives an ocean’s productivity,” Sobel says. “Without this process, it’s difficult for ocean wildlife to survive.”

The Northwest Hawaiian Islands are being heavily impacted by changing ocean conditions. Already faced with threats from over-fishing and human disturbance, the Hawaiian monk seal may now face possible starvation due to a lack of productivity in these areas of increased stratification. “For the monk seals, this may be the last nail in the coffin,” Stobel says.

According to researchers at the National Oceanic and Atmospheric Administration (NOAA) and the University of Hawaii, these biologically unproductive areas——often referred to as “ocean deserts”——are expanding much faster than predicted. The study addresses areas in the subtropical gyres——large-scale ocean currents in the middle of the ocean. The researchers, who published the study in Geophysical Research Letters this March, found that between 1998 and 2007, these expanses of saltwater with low surface plant life in the Pacific and Atlantic Oceans grew by 15%, encompassing a size larger than all of Asia.

“We’re seeing an increase in the low surface productivity areas in the world’s oceans,” says coauthor Evan Howell, a research oceanographer at NOAA. “Though it’s difficult to say what will happen to marine life in these areas, what’s certain is that these zones represent a large change in habitat.”

Between 1998 and 2007, ocean deserts grew by 15%, encompassing a size larger than all of Asia. © Michel Detay

Other oxygen measurement studies across the globe have found similar results. Steven J. Bograd, an oceanographer with NOAA in Pacific Grove, California, recently conducted research on dropping oxygen levels near California. The results of the study, which have been accepted for publication in Geophysical Research Letters, are consistent with those of Stramma.

Bograd used historical data from a long-term sampling program called CalCOFI (California Cooperative Oceanic and Fisheries Investigations) from 1984 to 2006, and found significant declining trends in dissolved oxygen throughout the region.

“There was a trend of declining oxygen pretty much everywhere,” said Bograd. “The results were pretty significant.”

He also found that over the 23-year time period there was an expansion of continental shelf area off of California exposed to low oxygen conditions. Like previous studies, Bograd emphasized that an expansion of oxygen-minimum zones has important ecosystem implications. As the low-oxygen areas increase, habitats get smaller, until species suffocate, move to another region or adapt. “Most marine species have minimum oxygen thresholds that they need for survival,” Bograd says. “As oxygen decreases, these animals will suffer and/or be compelled to move to other areas.”

Though the overall outlook is grim, there is good news for certain ocean species. The jumbo squid (or Humboldt squid) can handle low-oxygen waters better than most of its competitors. William Gilly, who coauthored Bograd’s report, has observed a broad expansion of jumbo squid in the eastern Pacific, including the first recorded observations of this species in Alaskan waters.

This expansion could be deadly for other species, as jumbo squid are voracious predators. Says Bograd, “Although many fish and invertebrate species will be negatively affected by an expanding oxygen-minimum zone, the one thing that you can say for sure is that we would expect there to be a change in the community structure of local ecosystems.”

On what’s causing the expansion of these hypoxic areas, researchers have yet to reach a consensus. One obvious culprit is global warming, which has caused the ocean’s temperature to increase about .1 degrees Celsius to date, according to Sobel. “This number sounds small,” he says, “but it is significant given an ocean’s massive size and enormous heat capacity.”

Though all three studies’ findings match up with global warming models, which predict an overall decline in dissolved oxygen concentration and an expansion of oxygen-minimum zones, head researcher Lothar Stramma and others are hesitant to rule out other causes just yet. Says Stramma, “For me, it is too early to blame the expanding oxygen-minimum zones on global warming,” he says, “but we have to find out soon.”