Some 200 miles west of Acapulco, Mexico, Volcano 7 rises 8,600 feet from the ocean floor.
A range of odd organisms grows along the seamount's flanks, but the volcano is not exactly a hotbed of biological activity.
As marine biologist Lisa Levin tells it, however, that picture shifts dramatically roughly 200 feet short of the summit. There, a narrow zone bustles with shrimp, crabs, starfish, worms, and rattail fish. Just as abruptly, the scene changes to one of submarine desolation a peak as seemingly barren of life as a Rocky Mountain summit above the timber line.
The abrupt shift remained a mystery until Dr. Levin and colleagues discovered that the peak juts into a layer of water severely depleted of dissolved oxygen. And despite its appearance, the peak was home to thriving colonies of tiny worms and bacteria.
The discovery 14 years ago that this low-oxygen region was anything but dead has underscored how little researchers understand these naturally occurring zones, according to Levin, atScripps Institution of Oceanography in La Jolla, Calif.
First noted off the coast of Africa in 1925, oxygen-depleted zones affect more than 400,000 square miles of ocean floor on time scales spanning thousands of years. These zones "are very large features, but they're very poorly known," says Levin.
Recent calculations based on modeling studies and direct measurements of dissolved oxygen in seawater suggest that the seas may be losing oxygen as a byproduct of global warming. Some models hold that if the climate continues to warm, as many scientists expect, low-oxygen zones may become more widespread, affecting fisheries.
In several areas, fisheries already are being devastated by so-called "dead zones" that appear in shallower coastal waters. One of the most noted lies in the Gulf of Mexico off Louisiana and Texas. This summer, the zone covered 8,500 square miles. Studies have traced its origins to nutrients that escape from farms and homes in the drainage basins that feed the Mississippi River and its tributaries. High nutrient levels trigger large algae blooms along the Gulf Coast. When the algae die, they decompose, depleting the water's oxygen content near the bottom. Shrimp, crabs, and other commercially valuable bottom dwellers die before they can escape.
Meanwhile, heavy rains upriver can increase the flow of more-buoyant fresh water into the Gulf. The fresher water acts like a cap, preventing more oxygen-rich water near the surface from mixing with the deeper, oxygen-deprived water.
Recent tropical storms in the Gulf have forced ocean layers to mix, shrinking the dead zone, according to Nancy Rabalais, chief scientist with the Louisiana Universities Marine Consortium.
In the deep ocean, low-oxygen zones can form along coastal areas where upwelling of cold, nutrient-rich seawater provides fodder for algae that later die and decay. The process, Levin explains, can lead to long-term oxygen-deprived zones at depths of 600 to 4,000 feet. Here, they can cut across seamounts, such as Volcano 7, or continental margins, such as those along the west coasts of North and South America.
Although the zones are long-lived, they can change in location and depth, with "huge economic significance" for local economies, she adds.
Levin and colleagues at Scripps have calculated that during El Niño conditions, the oxygen-deprived zone affecting the continental margin off Peru shrinks by as much as 61 percent. More oxygen in the water along the margin during these periods has led to the growth of scallop and hake fisheries during El Niño years, she says.
"We know that in shallow water, an increase in temperature leads to an increased rate" of oxygen loss, "but oceans are more complex," she says, noting that not everyone agrees that warmer temperatures mean less oxygen.