Undersea story: discover how the decline of a tiny creature could spell trouble for Antarctica's under-ice community

One. Two. Three. The penguins launch headfirst off a pancake-shaped slab of ice and into the frigid Southern Ocean below. Each propels itself downward by paddling its flipper-like wings back and forth. At a depth of about 20 meters (66 feet), the flightless bird slows down as it reaches a swarm of krill. With a snap of its beak, the penguin gulps down a meal of the small reddish crustaceans (organism with an external skeleton segmented body, and paired, jointed, legs).

With swarms of this Antarctic krill, or Euphausia superba (yu-FO-zee-ah soo-PUR-bah), in the waters surrounding Antarctica, you'd think a hungry penguin could surely catch a belly full. But since the 1970s, the krill population in parts of Antarctica has declined by about 80 percent, says Angus Atkinson, a marine biologist at the British Antarctic Survey. The sharp drop could spell trouble not only for penguins, but for other Antarctic dwellers as well. That's because this critter is at the heart of Antarctica's food web. In this interconnected system, organisms eat other organisms to obtain the energy they need to survive (see Nuts & Bolts, p. 20).

Now, scientists are trying to piece together the story of why krill are on the decline and what that could mean for Antarctica's diverse organisms.

VANISHING ACT

During the dark Antarctic winter--from June through September--Earth's southernmost continent resembles a scene from the movie Ice Age. Frigid temperatures cover about 19 million square kilometers (7.4 million square miles) of seawater in ice. Then, as the sun peeks above the horizon in a signal of summer, the sea ice that floats atop the Southern Ocean begins to melt. This leaves pockets of open water amid floating chunks of ice.

But some scientists say global warming, or the average increase in the temperature of Earth's atmosphere, is causing shifts in Antarctica's field of ice. Particularly, the winter air temperature around the Antarctic Peninsula has increased by 6[degrees]C (9.5[degrees]F) over the past 50 years. That warming has melted the areas sea ice. With less ice, krill are feeling the heat.

SEA FOOD

During the winter, krill feed on phytoplankton, or ocean-drifting plants, attached to the bottom side of the ice. "It's like an upturned lawn. The algae are growing on the underside of the sea ice," explains Atkinson. By clinging to the surface ice, these microscopic plants can keep from being carried by ocean currents down to the dark sea bottom. That way, the algae stay at the surface where they soak up winter's limited sunlight through ice cracks to power photosynthesis. This process of capturing the sun's energy to turn carbon dioxide and water into food enables the phytoplankton to grow.

A loss of floating ice means that phytoplankton can't get the needed sunlight. Result: fewer phytoplankton--and less wintertime food for krill. Without enough food, krill populations can plummet.

THE EMPEROR'S NEW COAT

For emperor penguins, snagging large helpings of krill is vital to survival. That's because each year these penguins rely on a diet of krill to pack on an extra 15 kilograms (33 pounds) of body mass.

Why fatten up? Built for life at sea, the 1.2 m (4 ft)-tall penguins are covered with a dense coat of water-repelling feathers. But throughout the year, wear and tear damages the protective covering. So annually, the birds swim 1,200 kilometers (746 miles) through the Ross Sea to their molting grounds. There they lose their feathers and then grow a new coat. "When they molt, they shed all of their feathers at once, so they lose their waterproofing," says Gerald Kooyman, a biologist at the University of California at San Diego. Without feathery protection from the frigid waters, emperors stay icebound during the five-week molt. That means a lengthy fast.

To store enough energy to endure this food-free stint, emperors must gain about a fourth of their body weight. If the calorie-rich krill were to vanish, the penguins might not have enough energy to survive the molt. Kooyman is monitoring emperor populations to learn how they are faring.

DOMINO EFFECT

Penguins aren't the only ones suffering from the menu change. Krill is the primary food for Antarctica's small fish and Weddell seals, as well as the humpback whale, which uses a fringe plate in its mouth to filter the critters from the water. This massive baleen whale can feast on two tons of krill each day.

Even populations of organisms that don't directly feed on krill could take a nosedive. Since krill form such a vital link in this marine food web, researchers expect that many animals will suffer as a result of fewer krill (see diagram, below). For instance, the emperor penguin is prey for both the leopard seal and the killer whale. So a loss of penguins could leave these giant marine mammals hungry.

Scientists are continuing to study Antarctica's ocean ecosystem. They hope that by understanding the reasons and consequences for the decline in krill, they will be able to help protect the diverse ecosystem before it's too late.