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Figure 3. Oxygen concentrations . . .Click to Enlarge Image

Recent evidence suggests that one predicted consequence of global warming—reduced oxygen levels in parts of the world’s oceans—is now happening. If the trend continues, the number of hypoxic areas, or dead zones, in the oceans may increase. In fact, although little is known about them, there are already permanent hypoxic zones along a number of continental margins; these are found in areas of upwelling where masses of phytoplankton decay as they sink, consuming oxygen in the water column and thereby starving the middle depths, where circulation is more sluggish. Levin and other ecologists find life here, but it is different: dense mats of large sulfur-oxidizing bacteria and worms mining loose soils rich in decayed organic material, surrounded by fringe areas full of shrimp, vertically migrating zooplankton and fish adapted to low-oxygen conditions. Paleontologists have reconstructed the environmental history of some of these regions, but we have only a sketchy understanding the adaptations that allow a few present-day marine animals to cope with permanent oxygen starvation.

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