Life on a Melting Continent

Every winter an expanse of ice twice the size of the United States forms around Antarctica. Before it disappears in the spring, a vast, unexplored ecosystem comes into being.

By Jane E. Stevens|Tuesday, August 01, 1995
A layer of frozen freshwater thousands of feet thick covers Antarctica. This is the ice familiar to all of us in the form of the huge icebergs that calve from Antarctic glaciers. But around the perimeter of the continent there’s another ice, formed from salty ocean water. In the Antarctic summer months--December, January, and February--there’s only about 1.1 million square miles of it. In March, though, when air temperatures can plummet to -40, the sea ice begins growing at an average rate of 22 square miles per minute. By the end of the Antarctic winter, in September, the pack ice has expanded to 7.7 million square miles--more than twice the size of the United States--in a layer usually no more than three feet thick. And in this ice is one of the strangest ecosystems on the planet.

A decade ago most biologists would have maintained that the winter sea pack ice was a monolithic sheet devoid of life. All organisms, they’d have said, must hibernate or move to warmer climes once Antarctica tips away from the life-giving rays of the sun and the seas freeze. But in July 1986 the German icebreaker Polarstern, a ship slightly longer than a football field, sailed into the winter pack ice of the Weddell Sea and changed the accepted view. On board the Polarstern were some of the most knowledgeable sea ice experts in the world, and they had a collective hunch that more might be going on in the winter pack ice than an annual sprouting of the world’s largest ice-skating rink.

Their hunch was based on findings gathered both from the landfast sea ice that grew along the edge of the Antarctic continent and from the edge of the winter pack ice itself. When biologists first explored the landfast ice along McMurdo Sound in the 1960s, they expected to find frozen, dormant traces of the simple food chain thought to be operating in the Southern Ocean. At the bottom of that chain were phytoplankton, tiny floating plants nourished by the ocean’s nutrients and summer sun; these were devoured by the small crustaceans known as krill, which in turn were eaten by birds and mammals. The researchers, however, found not moribund, quiescent plankton but the living, dynamic organisms themselves, growing in astonishing abundance, sometimes in thick, hairy mats on the underside of the ice. The algae (as phytoplankton are called once they’re locked up in the sea ice) were thriving in temperatures nearly four degrees below freezing, with less than half of 1 percent of the surface light and bathed in water three times saltier than seawater.

We could pump the water out of the bottom of the ice, and it would come out looking like espresso, says biological oceanographer Cornelius Sullivan, now director of the Office of Polar Programs for the National Science Foundation. There were 2,000 to 5,000 times more organisms in the ice than in the equivalent amount of water just underneath the ice. Living with the algae were bacteria, faring better than those Sullivan had found in the sewage outfall of Los Angeles Harbor. And instead of a simple, uniform population of krill, the researchers found a diverse group of tiny crustaceans and fish. Clearly, something was wrong with the old ideas about a simple food chain.

Science’s first explorations of the winter pack ice itself had come in the early 1980s, when a strengthened Coast Guard vessel penetrated 150 miles into the ice edge as it advanced in autumn and retreated in spring. The biologists on board had observed seabirds, penguins, and seals among the floes. Divers had seen krill shuffling along on the bottom of the ice, and bands of algae inside the ice. At that point biologists began to wonder whether this richness of life might span the entire length of the winter pack rather than be restricted to its edges.

The researchers aboard the Polarstern meant to find out, and as the ship sailed toward the pack ice, they gathered on the bridge. On the swells far below, long ribbons of grease ice--a thin, transparent film made from jumbled ice crystals known as frazil--extended from the pack edge. The white pack ice rimmed the horizon like a neon light. Against it the ribbons looked like tatters of black silk. The waves endlessly pushed and pulled at the grease ice and turned it into a soupy mess that soon aggregated into disks known as pancakes. Within moments, the Polarstern sailed into the midst of millions of pancakes, each one or two feet in diameter--a seemingly unending pond of icy water lilies.

The scientists on the Polarstern sailed into a world beyond their wildest imaginings. The icescape sometimes stretched out infinitely before them like the empty expanses of a frozen Sahara. But it wasn’t always solid; it often broke up into wide leads that looked like rivers running through flat, snowy farmlands. The water in these leads--warm relative to the air above--exhaled an eerie fog. And the pack ice was indeed rich with life. Inside the ice, the explorers found algae living between the crystals. The water below the ice, meanwhile, was devoid of life; you could see through it for hundreds of feet. Living among the algae were bacteria and viruses. Along the bottom of the ice, the researchers discovered grazing krill. Milky gray Antarctic petrels and snow petrels cruised the leads, along with Adélie penguins and emperor penguins, their pearly bellies outlined by black wings. The birds were fatter than they were in the Antarctic summer.

Over the last nine years, since the Polarstern voyage, researchers have begun ferreting out the pack ice’s exotic rules. The first rule the biologists learned was simple: they couldn’t understand life in the ice without the help of physicists. To comprehend why an organism lives deep in the winter sea pack ice, they needed to learn how that ice forms, from the moment it begins galloping across the seas in March.

Antarctic sea ice is very different from Arctic sea ice. In the Arctic most of the ocean is surrounded by land, which tamps the waters and the air into polite submission. Sea ice in these quiet waters tends to form in a thin skin made up of flat, floating crystals. This frozen, transparent shrink-wrap thickens, turning gray and then white. Little snow falls to insulate the ice from the frigid air. The entire sheet of ice chills the water below and stimulates the growth of long, graceful columns of crystals from its underside. Over the winter the ice thickens to five or six feet. Some of the newly formed Arctic ice survives the summer, so in four years it doubles in thickness. Each year the ice survives, it becomes less salty as the brine squeezed out of the ice crystals filters toward the bottom. Eskimo know that three-year-old ice, which contains one part per thousand of salt water, can be melted for drinking water.

In the Antarctic, however, the surrounding ocean is unimpeded by the presence of land. Cyclones howl around the continent’s perimeter and churn the seas into disorder. Three hundred feet below the surface, a dense layer of salty warm water rolls into the Southern Ocean from the North Atlantic, mixing with the Antarctic waters and slowing down the formation of ice, as do the insulating effects of snow cover. Consequently the ice usually becomes no more than a few feet thick and consists mostly of frazil crystals. Instead of shrink-wrap, the frazil ice forms pancakes. At first the pancakes are thin and rubbery, and they bend with the waves. Gradually, though, they become thicker and stiffer, and they damp out the smaller waves, making the water more placid.

As the ice forms, researchers believe, life becomes trapped in it. Like the winds that create blizzards on the surface of the pack ice, the ocean waves create a storm of frazil ice crystals underneath. As the frazil crystals then rise to the pancakes at the surface, they sweep up microorganisms floating in the upper layers of water.

For several years it wasn’t clear whether the algae were simply collected in the pack ice, where they remained dormant, or whether they could actually grow in the ice and multiply. The issue wasn’t resolved until 1992, when a group of physicists and biologists from the former Soviet Union and the United States set up a drifting ice camp in the western Weddell Sea at the end of the summer. Researchers set out in February to do the difficult work of monitoring three 20-square-meter patches of ice. Mainly, they thought, they’d be recording the seasonal death of the Antarctic ecosystem. But instead of dying algae, they found algae blooming throughout the autumn and into early winter.

It is not known just how the algae can thrive in Antarctic winter conditions, but some researchers have suggested a way. When the irregular frazil crystals collect, they form something like an icy city filled with streets, elevators, and bridges that link countless small pocketlike abodes. Through autumn, as the air temperature drops, the ice in this porous layer forms from the top down. As the water freezes, the salt is forced into the channels, where it makes the water far saltier and thus denser. This brine sinks through the ice channels, out of the pack and into the sea, and lighter, nutrient-rich seawater moves in to take its place. The influx of food makes the icebound population of algae explode. They do not stop growing until the winter sunlight diminishes and the ice becomes colder and freezes off the channels. No more fresh seawater can get to the algae, and they use up the nutrients around them.

We’ve also found that bacteria are very active, says Gerhard Dieckmann, a biologist with the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. After the ice is formed and when you expect everything to come to a standstill, bacterial production will exceed that of algae. As it warms up during the spring and summer, algae take over again.

Although no one has done similar experiments in the pack ice in July and August, University of Southern California biologist Chris Fritsen, a member of the 1992 Weddell Sea team, believes that algae grow all winter. Huge areas of the pack ice, he notes, do not fall into the 24-hour Antarctic night; many places still get 3 to 4 hours of sunlight a day throughout the winter. The scenario that I feel is most probable, he says, is that alternating periods of warmth and cold cause a cycle of flooding with seawater during warm periods and freezing of the flooded areas during cold periods. The flooding unlocks the productivity of the pack ice, which allows the algae to grow if there’s sunlight available.

Nevertheless, most organisms in the winter pack ice are prepared for starvation. Krill can even molt backward and eat themselves. In the wild they’ve been seen with their molts in their mouths, says Robin Ross, a biologist at the Marine Science Institute at the University of California at Santa Barbara. Krill are almost entirely responsible, directly or indirectly, for feeding the Antarctic’s millions of penguins, millions of seals, and thousands of whales. The biomass of krill exceeds that of any other animal species on Earth. Some estimates go as high as 1.35 billion tons--five times the weight of the world’s 5 billion humans. Krill gather in awesome hordes. One swarm was measured covering 58 square miles to a depth of 650 feet. Although adult krill nearly stop eating during winter and their respiration rate drops to as low as a third their rate during summer, they often use the ice as a refuge and feeding area. Krill larvae appear to depend completely on the pack ice, says Ross. Without it, we think they wouldn’t be able to live through the winter, she says.

Support for this hypothesis comes from the fortunes of krill over the past few years. Ross, with her husband, Langdon Quetin, has studied krill larvae off the west coast of the Antarctic Peninsula during six winter cruises. In the winter of 1992, she says, satellite photos indicated that the winter pack ice began breaking apart early, and by September, when it is usually at its peak, it had disappeared from the biologists’ study area. No krill larvae survived that year, says Ross. The fate of krill pulls the fate of larger animals in its wake. Thus, says Ross, a few seasons of abundant pack ice lead to abundant krill, which lead to abundant Adélie penguins.

On the other hand, according to biologist Bill Fraser of Montana State University, a good winter of pack ice appears capable of killing off all the chicks of the South Polar skua. The birds indirectly depend on tiny crustaceans known as copepods, which need openings in the ice to reach the surface, where they can feed on plants and reproduce; if the ice is too extensive, few of them emerge. Silverfish time their reproduction so that their eggs hatch just after the explosion of copepods. If they have no copepods to eat, though, the silverfish die. And without enough silverfish, the skuas in turn cannot feed their young.

These examples aside, however, for the most part the lives of large animals in the winter pack ice remain a mystery. Penguins, seals, and petrels use the pack as resting or breeding places and, along with minke whales, hunt for food in the leads. Here the answers end and the questions begin. Are birds and mammals nomadic during the winter, or do they have areas in which they stay? To what degree do areas in the pack ice vary from year to year? As on land, are there oases, deserts, savannas in the pack ice, each with its own collection of inhabitants?

Even in the act of disappearing at the end of winter, the pack ice has an influence that reaches into the next season. When it melts, it mixes with the ocean to form a slightly fresher layer 40 feet deep. The plankton in that calm skin remain in the light longer than those in the open ocean, which get mixed by currents and waves down to 300 feet. The melted water also contains dust that had been stored up in the ice for six months; this dust, carried into the oceans from the plains of China and from factories in America and Europe, is rich in iron, a fertilizer for the plankton. Until a storm mixes the water layers or clouds block the sun, they grow like weeds in a newly turned garden.

The receding pack ice edges, teeming with life, may help researchers do a better job of balancing their accounting of the biomass of the Southern Ocean. In the Southern Ocean there are a lot more whales, seals, and penguins than the primary production can account for, says Fritsen. The growth of organisms over the winter and the boost the pack ice gives to life when it retreats probably add up to a vast, uncalculated amount of production.

Strangely, beginning in October, the winter pack ice vanishes twice as fast as it appeared, at a rate of 44 square miles per minute. By February it has shrunk back to a rim around the Antarctic continent. How the sea ice melts so quickly has long been a puzzle, but physicists on cruises in the early spring have noticed that as the ships ram through the retreating ice, the floes break along the dark lines where algal growth is thickest. In ice cores, they find that the layer of ice in which algae live is slushy.

Perhaps, Gerhard Dieckmann suggests, life in Antarctica isn’t just a passive beneficiary of the ice. We believe the biology modifies the ice so much that it affects the melting of the ice, by absorbing light and giving it off as heat, he says. He’s now feeding biological data into physical models of the melting ice to see if they will become more accurate. Physicists used to say, ‘Ice is ice. We don’t care about the biology,’ says Dieckmann. But perhaps now it is time for the physicists studying the ocean to listen to the biologists.
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