Food-chain alarm from a low-ozone zone

On one side of the ship looms Cape Valentine, the hostile, glaciated point where, in 1916, Ernest Shackleton's party miraculously survived for 137 days after their ship was crushed in the pack ice.

On the other, a parade of house-sized icebergs drifts with the cold Antarctic winds, occasionally compelling the bridge crew to change course.

But Geoff MacIntyre and Richard Davis are out on the aft deck in orange survival suits deploying a torpedo-shaped instrument into the icy Southern Ocean, letting its cable out by hand as it drops into the depths.

They're here among the icebergs trying to find answers about a newer, invisible threat from up above.

They're standing under the ozone hole.

Miles overhead, the protective ozone layer has been destroyed by chlorofluorocarbons (CFCs), chemicals used in air conditioners, refrigerators, and aerosol cans. On this October day, the hole is enormous - one month previously it had achieved the largest size ever observed. Satellite images revealed what looked like a spinning blob covering 10.5 million square miles, occasionally wobbling an arm over southern Chile and Argentina.

Under this hole, increased levels of ultraviolet (UV) radiation bombard the earth's surface. Scientists are worried that increased UV, especially UV-B, might be undermining the marine food chain from the bottom up.

"We know UV-B inhibits photosynthesis, but we're a long way from knowing how that will affect fisheries yields, penguins, or whale populations," says Patrick Neale of the Smithsonian Environmental Research Center in Edgewater, Md., who is the chief scientist for the cruise. "My basic feeling is that at higher levels [of the food chain], we'll see effects that aren't huge, but still significant."

A polar problem

Ozone depletion is worst at the poles, where cold stratospheric temperatures promote ozone-destroying chemical reactions. In the 1980s, scientists discovered that an ozone hole formed over the Antarctic every spring. More recently an ozone hole has appeared over the North Pole.

The ozone hole appeared first over the colder Antarctic because the ozone-destroying chemical process works best in cold conditions. The Antarctic continent has colder conditions than the Arctic, which has no land mass.

Although the production of CFCs has since been restricted under an international agreement, the degradation of the ozone layer is expected to continue for many years.

Fortunately there's little life in the Antarctic interior and much of the Arctic Ocean is covered in a protective cap of ice and snow, though recent research suggests Arctic ice is in decline.

But the Southern Ocean is one of the world's most productive marine ecosystems, home to huge numbers of penguins, seals, and bottom plants, and a major supplier of nutrients carried to other parts of the world by undersea currents.

Altering an ecosystem

Little is known about the effect of UV-B on marine life, particularly the microscopic algae called phytoplankton that form the foundation of the undersea food chain. These tiny plants capture the sun's energy through photosynthesis, providing food for microscopic animals. They are eaten by krill, which sustain the Antarctic's abundant seals, penguins, and baleen whales. Less phytoplankton means less food for these animals to eat.

Researchers say it's clear that UV-B harms Antarctic microbes. Dr. Neale has predicted that phytoplankton photosynthesis declines by as much as 8.5 percent under the worst conditions.

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