Oxygen-deprived zones are common and might become more so with climate change. Here life hangs on, with some unusual adaptations
In 1925, the Meteor expedition set out from Germany to measure the ocean's depth, study marine chemistry and, on the side, extract gold from seawater. The gold project didn't pan out, but expedition data revealed a curious feature in the Atlantic, off southern Africa: a giant wedge of oxygen-deficient water 300 to 400 meters down.
The surface layers of the ocean generally obtain oxygen from diffusion and brisk circulation. This water sinks to the seafloor, supplying oxygen to deep-sea life. But sluggish circulation and oxygen-poor source waters can reduce oxygen concentrations at intermediate depths. In several oceans, this effect is unusually pronounced. Where primary production by photosynthetic life is very high, the decay of sinking organic matter consumes so much oxygen that the middle depths contain dramatically less of the gas than water above or below. Such oxygen-minimum zones, or OMZs, turn up in productive open oceans and on continental margins. They can stretch for thousands of miles and persist for thousands of years.
I first encountered an OMZ while studying animal communities on Volcano 7, a submerged mountain about 200 miles west of Acapulco, Mexico. The seamount has its summit 730 meters below the surface and its base 3,400 meters deep. With a group of geologists, I first visited Volcano 7 in 1984 to photograph and study the mountain's slopes using Alvin, a submersible vehicle based at Woods Hole Oceanographic Institution (WHOI), and ANGUS, a camera mounted on a sled. I noticed the fascinating sea life, but we did not measure oxygen on that trip. Four years later, I returned with a group of women who were studying plankton and life on the ocean floor. Again, we employed Alvin to explore the volcano's surface.
Because Alvin had space for only a pilot, two scientists and our gear, we had to squeeze down on our knees and peer sideways to look through a porthole. The vehicle's exterior lights, though powerful, illuminated only six to eight feet of water, focusing our attention on the fauna at hand rather than the larger view. At about 800 meters below the surface, dense groupings of crabs, shrimp, brittle stars and sponges were zoned like invertebrates on a rocky shoreline. But as we moved up the mountain's slope, the rocks suddenly looked barren, covered with the merest fuzz of life.
Wondering what might account for the dramatic change, we looked at the oxygen data generated by a sensor mounted on the submersible's shell. A liter of seawater can hold 7 or more milliliters of oxygen. (For comparison, a liter of air can hold 210 milliliters of oxygen.) But the water that swathes the peak of Volcano 7 contains less than one-seventieth of that amount—0.1 milliliter per liter. The seamount summit protrudes into a vast OMZ—a region of the ocean containing less than 0.5 milliliter of oxygen per liter of water. It is surprising that any life exists where the oxygen supply is so sparse.
In addition to the OMZs that lie off the western coasts of the Americas, well-established OMZs have also been discovered in the Arabian Sea, the Bay of Bengal and off the western coast of Africa. In the United States, the zones lie off California and Oregon. California's Santa Barbara Basin, with its bottom at 585 meters, is right within the California OMZ and has very little oxygen. My group at North Carolina State University and then at Scripps Institution of Oceanography, in collaboration with U.S. and international investigators, has studied OMZs off California, Mexico, Peru, Chile and Oman. Using data on seafloor topography and oxygen measurements from the National Oceanographic Data Center, we recently teamed up with John Helly at the San Diego Supercomputer Center to estimate how much of the continental margin seafloor is intercepted by OMZs. Our global estimate was about 2 percent.
Seasonal oxygen-depleted zones are also known. Water over the shallow shelf off Chile becomes depleted during the spring and summer, when it is fed by the poorly oxygenated Peru-Chile Undercurrent. In fall and winter, it becomes oxygenated again as cold, sub-Antarctic currents prevail. Human activities deplete other bodies of water. In some estuaries and coastal regions, fertilizer runoff or sewage outfall promotes periodic phytoplankton blooms, which rob the water of oxygen as they decay. Such human-inspired hypoxic events have taken place in many parts of the world, including Chesapeake Bay, the New York Bight and the Gulf of Mexico. A severe hypoxic event in the northern Adriatic in 1977 slaughtered masses of marine organisms.
Understanding the formation and ecological impact of oxygen-depleted zones is critical because their number and intensity are likely to increase as global warming and nutrient enrichment rob the ocean of oxygen. Because such zones are inhospitable to animals we consider edible, their expansion could have a devastating effect on commercial fishing.