A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Deep-Ocean Life Where Oxygen Is Scarce Oxygen-deprived zones are common and might become more so with climate change. Here life hangs on, with some unusual adaptations Lisa A. Levin n 1925, the Meteor expedition set out cano 7, a submerged mountain about region of the ocean containing less than Ifrom Germany to measure the ocean’s 200 miles west of Acapulco, Mexico. 0.5 milliliter of oxygen per liter of wa- depth, study marine chemistry and, on The seamount has its summit 730 me- ter. It is surprising that any life exists the side, extract gold from seawater. The ters below the surface and its base where the oxygen supply is so sparse. gold project didn’t pan out, but expedi- 3,400 meters deep. With a group of ge- In addition to the OMZs that lie off tion data revealed a curious feature in ologists, I first visited Volcano 7 in 1984 the western coasts of the Americas, the Atlantic, off southern Africa: a giant to photograph and study the moun- well-established OMZs have also been wedge of oxygen-deficient water 300 to tain’s slopes using Alvin, a submersible discovered in the Arabian Sea, the Bay 400 meters down. vehicle based at Woods Hole Oceano- of Bengal and off the western coast of The surface layers of the ocean gener- graphic Institution (WHOI), and AN- Africa. In the United States, the zones ally obtain oxygen from diffusion and GUS, a camera mounted on a sled. I lie off California and Oregon. Califor- brisk circulation. This water sinks to the noticed the fascinating sea life, but we nia’s Santa Barbara Basin, with its bot- seafloor, supplying oxygen to deep-sea did not measure oxygen on that trip. tom at 585 meters, is right within the life. But sluggish circulation and oxy- Four years later, I returned with a California OMZ and has very little gen-poor source waters can reduce oxy- group of women who were studying oxygen. My group at North Carolina gen concentrations at intermediate plankton and life on the ocean floor. State University and then at Scripps In- depths. In several oceans, this effect is Again, we employed Alvin to explore stitution of Oceanography, in collabo- unusually pronounced. Where primary the volcano’s surface. ration with U.S. and international in- production by photosynthetic life is Because Alvin had space for only a vestigators, has studied OMZs off very high, the decay of sinking organic pilot, two scientists and our gear, we California, Mexico, Peru, Chile and matter consumes so much oxygen that had to squeeze down on our knees and Oman. Using data on seafloor topog- the middle depths contain dramatically peer sideways to look through a port- raphy and oxygen measurements from less of the gas than water above or be- hole. The vehicle’s exterior lights, the National Oceanographic Data Cen- low. Such oxygen-minimum zones, or though powerful, illuminated only six ter, we recently teamed up with John OMZs, turn up in productive open to eight feet of water, focusing our Helly at the San Diego Supercomputer oceans and on continental margins. attention on the fauna at hand rather Center to estimate how much of the They can stretch for thousands of miles than the larger view. At about 800 me- continental margin seafloor is inter- and persist for thousands of years. ters below the surface, dense group- cepted by OMZs. Our global estimate I first encountered an OMZ while ings of crabs, shrimp, brittle stars and was about 2 percent. studying animal communities on Vol- sponges were zoned like invertebrates Seasonal oxygen-depleted zones are on a rocky shoreline. But as we moved also known. Water over the shallow Lisa A. Levin is a biological oceanographer in the up the mountain’s slope, the rocks sud- shelf off Chile becomes depleted dur- Integrative Oceanography Division at Scripps In- denly looked barren, covered with the ing the spring and summer, when it is stitution of Oceanography. She completed her Ph.D. merest fuzz of life fed by the poorly oxygenated Peru- at SIO in 1982 and then, after a year of postdoctoral Wondering what might account for Chile Undercurrent. In fall and winter, study at Woods Hole Oceanographic Institution, the dramatic change, we looked at the it becomes oxygenated again as cold, accepted a faculty position at North Carolina State oxygen data generated by a sensor sub-Antarctic currents prevail. Human University. In 1992, with their children, she and mounted on the submersible’s shell. A activities deplete other bodies of water. her oceanographer husband returned to SIO, where liter of seawater can hold 7 or more In some estuaries and coastal regions, both are now professors. She studies the ecology of milliliters of oxygen. (For comparison, fertilizer runoff or sewage outfall pro- animal communities in marine sediments, particu- a liter of air can hold 210 milliliters of motes periodic phytoplankton blooms, larly those of stressed environments, from the inter- tidal zone to the deep sea. Address: Integrative oxygen.) But the water that swathes which rob the water of oxygen as they Oceanography Division, Scripps Institution of the peak of Volcano 7 contains less decay. Such human-inspired hypoxic Oceanography, 9500 Gilman Drive, La Jolla, CA than one-seventieth of that amount— events have taken place in many parts 92093-0218. Internet: [email protected]; 0.1 milliliter per liter. The seamount of the world, including Chesapeake http://levin.ucsd.edu summit protrudes into a vast OMZ—a Bay, the New York Bight and the Gulf © 2002 Sigma Xi, The Scientific Research Society. Reproduction 436 American Scientist, Volume 90 with permission only. Contact [email protected]. phytodetritus seapens foraminifera sponges galatheid crabs serpulid worms upper summit shrimp brittle stars rattail lower summit pillow lava sponges flank xenophyophores bottom Figure 1. Oxygen-minimum zones, such as the one atop Volcano 7 in the eastern Pacific west of Acapulco, Mexico, are permanent features of sever- al oceans. They are found at intermediate depths and can stretch for thousands of miles. With an oxygen content one-fourteenth or less that at the sur- face, they provide habitats where certain marine organisms can enjoy abundant food and freedom from predators. Single-celled creatures in the or- der Foraminifera and minute worms called nematodes dominate the summit of Volcano 7. Farther down the seamount, near the lower boundary of the oxygen-minimum zone, the still-plentiful food supply and slightly higher oxygen concentrations support larger organisms such as seapens, sponges and shrimp. The density of animals at this lower boundary can be several times higher than the density farther down the volcano. © 2002 Sigma Xi, The Scientific Research Society. Reproduction 2002 September–October 437 with permission only. Contact [email protected]. of Mexico. A severe hypoxic event in the northern Adriatic in 1977 slaugh- tered 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 enrich- ment 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. OMZs: What, Where and Why OMZs generally form where winds cre- ate currents that move water along a continent’s western shoreline. Aided by the Earth’s rotation, such currents Figure 2. Extensive oxygen-minimum zones (gray) have arisen off the west coasts of North, sweep water offshore. Replacing the Central and South America, off southwest Africa, and in the Arabian Sea and Bay of Bengal. It lost mass, cold water rises up from the has been estimated that about 2 percent of the continental margin seafloor lies below such ocean’s depths. It brings nutrients, es- zones, which consist of midlevel waters depleted of oxygen by the decay of organic matter pecially nitrogen, to the sunlit surface, from highly productive surface waters. Incoming currents do not replenish them because they where a lack of nutrients would other- are also oxygen-poor. The map also includes hypoxic zones, such as those in the Black and wise limit plankton and nekton growth. Baltic seas, that have arisen by different processes. When the resulting glut of surface- 0 dwelling organisms dies, the bacterial garbage recyclers of the ocean deplete the water of oxygen as they degrade the detritus. If such conditions persist 50 for long periods, middle depths can be- come permanently short of oxygen, < 0.01–0.1 and an OMZ can appear. Areas that lack a constant supply of 1,00 oxygenated water are good candidates 0.1–0.5 for OMZ formation. For example, the eastern Pacific and Indian oceans re- ceive water from the North Atlantic, 0.5–1.0 1,50 where most new deep water forms. As this water travels south for hundreds of years, it loses oxygen. The Arabian 2.0 Sea, which contains a vast OMZ, also receives poorly oxygenated waters, depth (meters) 2,00 which come from the Red Sea, Persian Gulf and Banda Sea. Interested in the adaptations of sea- 2,500 floor communities to extreme environ- ments, I began to study OMZs in 1988, 3.0 when groups led by Karen Wishner (University of Rhode Island), Marcia Gowing (University of California, Santa 3,000 Cruz), Lauren Mullineaux (WHOI) and myself visited the slopes of Volcano 7 aboard WHOI’s research vessel Atlantis II.