Food-chain alarm from a low-ozone zone
Amid the chill, the krill, an occasional thrill, researchers under the Antarctic ozone hole say it may alter an ecosystem - and life on earth.
ELEPHANT ISLAND, ANTARCTICA
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.
It also damages the DNA of marine bacteria and the larvae of starfish and urchins, they say. And it even alters ocean chemistry, creating potentially dangerous substances in the water itself.
"It's really dramatic what the changes in ozone levels will do to rates of DNA damage and inhibited development," says biologist Deneb Karentz from her office at the University of San Francisco.
"If you have a 30 percent decline in ozone, that doesn't mean a 30 percent decline in a given biological process - it could be a lot more than that," she says.
The tricky part is estimating how damage to one group of organisms might affect others above or below it in the food chain. Ocean ecosystems are extremely complicated and poorly understood.
Phytoplankton concentrations may not be the main factor influencing, say, krill abundance, which also depends on ocean currents, predator-prey relationships, and the timing and extent of annual sea ice.
"It's a tough game to call," says Langdon Quetin of the University of California at Santa Barbara. Krill live in areas usually affected only every few days by the wobbling ozone hole. "So if you average the amount of time phytoplankton are exposed in those areas, their decline may be insignificant for the krill."
"On the other hand, increased UV may affect female krill directly through damage to their ovaries," he says. "It's too early to tell."
Scientists studying UV effects on different portions of the food chain now collaborate closely. Here aboard the National Science Foundation's ice-going research ship Laurence M. Gould, four separate teams coordinate experiments and sampling to begin teasing out relationships between effects on ocean chemistry, bacteria, phytoplankton, fish larvae, and krill.
Wade Jeffrey of the University of West Florida in Pensacola knows that bacterial growth is inhibited in water exposed to increased UV.
But it's not clear how much is caused by direct UV damage to the bacteria's DNA, as opposed to indirect damage from the way UV changes the chemistry of sea water.
So his team works closely on ship with chemical oceanographers Kenneth Mopper of Washington State University and David Kieber from the State University of New York College of Environmental Science and Forestry, in Syracuse.
Surface waters are laden with dissolved organic matter, from which UV can produce hydrogen peroxide and other substances hazardous to living tissues. But Dr. Kieber and Dr. Mopper have found that these substances can also help bacteria indirectly by breaking organic matter into forms the bacteria can consume.
'Good UV, bad UV'
Dr. Jeffrey calls this the "good UV, bad UV syndrome." UV may harm bacteria during short-term exposures, but that could be compensated for by increased food sources over the long haul. Or the organisms may adapt to the new conditions over time.
Determining the net result will require more analysis, further experiments, more hours on deck in the frigid Southern Ocean.
"The Southern Ocean runs around the world, connects all the other oceans, and drives many of their currents and nutrient supplies," Jeffrey says.
"The broader concern is that a significant change in the Southern Ocean can impact production in other oceans."