Life on Ice
Scientists are discovering that icebergs, long considered barren masses, actually sustain all sorts of ocean life, from seabirds above to krill below. They may even play a role in fighting climate change.
You never forget your first iceberg.
The mass of drifting ice that dwarfs your ship is so beautiful, yet so improbable looking, that you simply gaze in wonder. It seems that nothing that large could be natural—and then it strikes you that something so enormous could only be natural.
Another realization soon dawns: These rock-hard floating mountains are dangerous. Think not only of the 1912 Titanic disaster but of hundreds of other accidents, most recently the sinking (with no loss of life) of the Antarctic tour ship Explorer in November 2007.
I’ve seen thousands of icebergs during more than two decades of visiting Antarctica as a journalist, author, and guide. Each one is unique, and all of them fascinate me.
One of the earliest to see an Antarctic iceberg, Anders Sparrman, the assistant naturalist on Captain James Cook’s HMS Resolution, was similarly struck by their attractive menace when he saw a spectacular example in December 1772. It was partly snow-white, partly crystal-clear, and partly sapphire-blue. “This, against the brightness of a lovely day,” Sparrman wrote, “formed the most majestic and imposing sight I have ever seen on sea or land, and I believe that nature seldom has it in her power to produce anything finer. Yet it was fearful too, when one thought of the great dangers we might encounter in such places, in darkness, fog, storm and sea swells. From spectators we might become actors, hurled on to that appalling stage, to be smashed to pieces.”
Now, as climate change is raising global temperatures, more icebergs are being born. Antarctica generates far more of them than Greenland, the source of bergs in Arctic waters. Antarctica’s are also much larger, sometimes reaching the size of small countries. Recent data show the average atmospheric temperature has increased about 4.5 degrees Fahrenheit in the western Antarctic Peninsula since the 1940s, making the region, along with northwestern North America and Siberia, among the fastest warming on earth. This jump has been implicated in the recent collapse of major ice shelves along the Antarctic Peninsula, including the Wilkins Ice Shelf in 2008. As a result, thousands of new icebergs have calved, or broken off, from ice shelves into the Southern Ocean at an accelerated rate.
Even as more icebergs are being created, scientists are learning that these dangerous beauties are far from sterile, inert masses of ice. In fact, they dramatically alter their environments biologically, chemically, and physically, making them islands of life in the open sea. Knowledge of icebergs’ crucial role in the Antarctic ecosystem has come only in recent years. Observers at sea had long remarked that they attract seals, penguins, and seabirds, and divers had noticed that fish are more numerous near them than in the surrounding sea. Now scientists are learning just what the attraction is all about.
Depending on their size, location, and the season, icebergs can be nurturers or destroyers. During their existence—typically years from calving from an Antarctic or Greenlandic glacier to their gradual melting as they drift into lower latitudes—they support animals on, around, even inside their magnificent ramparts. They fertilize the ocean with nutrients, boosting plankton production. Grounded bergs can shelter areas of the seafloor, protecting bottom-dwelling creatures from free-floating icebergs, which can be bottom-scouring marauders, furrowing the seabed at depths of more than a thousand feet like gigantic plows, destroying all marine life unable to move out of the way. Large bergs can also trap sea ice, impeding its annual breakup and thus depriving phytoplankton (algae that take their name from the Greek words for “plant” and “wanderer”) of life-giving sunlight, breaking the food chain at its first link.
It took the calving of the largest iceberg ever recorded to spur more serious study of their biological influence on surrounding waters. The megaberg B-15—named, like all those in the Antarctic, by the National Ice Center in Suitland, Maryland—broke from the Ross Ice Shelf in March 2000. It measured 185 miles by 23 miles—or 4,255 square miles, roughly the size of Qatar. (Eight years after its birth, B-15’s largest remaining piece was 72 miles by 10 miles and still drifting through the Ross Sea.)
“It was too much to resist,” says Gregory Stone, now vice president of global marine programs at the New En-
gland Aquarium in Boston. With diver and filmmaker Wes Skiles, Stone organized a National Geographic expedition to the Ross Sea to investigate B-15 and its offspring in January 2001.
Although heavy pack ice (sea ice) prevented Stone’s expedition from reaching the biggest chunk of B-15 by sea, team members made many scuba dives around icebergs floating in the Ross Sea. They noted increased phytoplankton near those they studied, causing them to speculate that icebergs act as “biological engines” by churning and mixing the water around them, spurring growth. The researchers saw juvenile icefish living in finger-sized holes in icebergs. “To this day,” Stone says, “I’m not sure if they excavated those to stay away from predators or if the holes were already there.” The divers also counted more ctenophores (comb jellies) near the bergs than in surrounding waters, presumably attracted by prey. They marveled at the translucent creatures’ slow movements, propelled by cilia that reflect light in a magical-looking rainbow display as they move.
Going beneath one of these frozen behemoths, Stone says, is like visiting “an underwater cathedral, with the sunlight piercing through the water and dancing before your eyes. Antarctica is my favorite place to dive.”
It is also a very perilous place to dive: Bergs can overturn or collapse without warning and are constantly melting. The expedition measured a freshwater lens nearly 200 feet deep extending more than a mile from one large iceberg. This melting created a treacherous downward-rushing flow of denser, heavier saltwater that quickly pushed Stone and his dive partner into the depths before they regained control.
Elsewhere in the Ross Sea, in an extremely risky pioneering dive on an iceberg, Skiles and two other team members with extensive cave-diving experience swam through cracks and tunnels reaching 350 yards into the berg. The feat was possible only because they used advanced rebreathing equipment that scrubbed carbon dioxide from their exhalations, allowing them to remain submerged for three hours. The divers also wore heavily insulated dry suits with electric heating pads over their kidneys to keep hypothermia at bay.
One hundred and thirty feet down, the trio made a remarkable discovery: a hidden garden of invertebrates living on the seafloor. Starfish, sponges, feather-duster worms, sea cucumbers, and other filter-feeding organisms carpeted the bottom. The location was made still more inviting to marine life by the iceberg’s tunnels and caverns, which channeled plankton-bearing currents of seawater through the site: dinner delivered.
Their dive’s danger manifested itself in a stomach-churning way just hours afterward. The berg the three had penetrated—and to which their ship had been moored—suddenly exploded into a sea of small fragments. “We thought that it was a calculated risk that was worth taking because we were trying to do exploratory science,” says Stone. “We weren’t doing it for fun or excitement. But after seeing the iceberg collapse, that was never done again.”
Such hidden havens may become even more important as the planet heats up, new work by scientists at the British Antarctic Survey (BAS) suggests. Seabed scouring by icebergs, BAS researchers have found, increases as a result of shrinking winter sea ice. This is because sea ice holds icebergs fast during the winter, making winds and currents less likely to drive them into the seabed. Winter sea ice has declined dramatically—in both extent and duration—in the west Antarctic Peninsula over the past few decades due to climate warming. This increase in iceberg disturbance on the seabed, where 80 percent of all Antarctic life occurs, could have severe effects on marine creatures living up to 1,600 feet underwater.
In Antarctica’s Ross Sea, Kevin Arrigo, a Stanford University biological oceanographer, has found that big icebergs can have another destructive impact on the marine ecosystem. Winter sea ice, which in a normal spring breaks up and gets blown out of the Ross Sea, was blocked by the calving of B-15 in 2000 and by the birth of another enormous berg from the Ross Ice Shelf in 2002. “These were state-sized icebergs, and they didn’t break up,” Arrigo says, explaining that megabergs melt more slowly than the smaller ones generally found in the Antarctic Peninsula region. Since the Ross Sea is farther south than the peninsula, it is also much colder. “Because the sea ice couldn’t move away, it was really tough on the whole ecosystem,” Arrigo says. With sea ice blocking sunlight, phytoplankton production dropped by 40 percent in 2000 and by 70 percent in 2002. That left little food for krill, which in turn are eaten by penguins, seabirds, and seals. Penguins had to forage much farther from their nesting areas, and fledgling success plummeted. “Basically,” Arrigo says, “the food chain shut down. It was a catastrophe.” No one is certain whether this will occur more frequently as temperatures climb.
Nevertheless, the hypothesis remained: Drifting icebergs impart significant biological and chemical characteristics to the surrounding ecosystem. To test this, Ken Smith, an oceanographer at the Monterey Bay Aquarium Research Institute, and nearly 30 researchers are making the most intensive study yet of individual bergs and their immediate environs. They worked first from the U.S. research vessel Laurence M. Gould in the Weddell Sea in 2005, then from the Nathaniel B. Palmer in the Scotia Sea in 2008. This year they will make a third Antarctic cruise to study icebergs.
Targeting single bergs, the researchers spend seven to ten days studying each one. Their ship can safely approach only to within 100 yards because the icy giants can roll or break up, so the team deploys an array of instruments to measure their subjects. Laser tools find the bergs’ height above the waterline, while pole-mounted sonar systems scan their much-greater underwater portions. Sensors sample water to determine conductivity (and thus salinity), temperature, and depth. A remotely operated vehicle gathers samples and pumps seawater to the ship for chemical analysis, while also sending back real-time color video from beneath the bergs. The team tows nets through the surrounding ocean to collect plankton, and a sediment trap sent beneath the bergs catches pebbles, dust, and other matter released by melting. In a playful—but cost-efficient—first, researchers used a radio-controlled model airplane to photograph one of the icebergs and to drop a GPS locator atop it, which transmitted its location every six hours via satellite.
All this research has revealed the critical role icebergs play in the Antarctic marine ecosystem. The first two cruises found—in what Smith calls a “halo effect”—that the waters around bergs supported much more life than the open ocean, including denser zooplankton communities, larger krill populations, and higher seabird concentrations. The halo of enrichment extends about 2.3 miles, but the bergs’ influence is even greater: Instruments detected changes in the sea as far as 5.6 miles away.
The abundant phytoplankton in the vicinity of icebergs seems to attract predators, including Antarctic krill, jellyfish, and worms. Up the food chain these are eaten by several fish species and by seabirds including cape petrels and Antarctic fulmars. Smith’s team also discovered, for the first time ever, algae growing on rocks embedded in the underwater part of a berg.
The scientists estimate that icebergs are raising the overall biological productivity in the Weddell Sea by nearly 40 percent. The reason lies within the bergs themselves. As glaciers scrape across the land, they accumulate debris and grind up rocks. After the glaciers calve icebergs, melting gradually releases this debris and pulverized rock, along with tens of thousands of years of accumulated dust. The terrestrial material acts as oceanic fertilizer, Smith explains. “These are nutrients required by the phytoplankton.”
One of the most critical nutrients in this meltwater is iron. Parts of the Southern Ocean’s surface water, where phytoplankton live, are iron deficient, a condition some oceanographers call “marine anemia.” Recent studies have shown that a lack of iron restricts phytoplankton’s ability to perform photosynthesis, so iron is a major limiting factor in the ocean’s productivity. Although they are still analyzing their data, chemists on Smith’s team may find significant amounts of iron in the meltwater. The researchers are also keen to explore another aspect of the puzzle. By stimulating phyto-plankton growth, which removes carbon dioxide from the atmosphere, icebergs may play an important and previously unforeseen role in regulating global climate change. In March Smith’s team will return to Antarctica for a 40-day expedition. “We’re interested to see how much of that carbon sinks into the deep water,” he says.
From personal experience, I know that icebergs will continue to astonish and captivate visitors to the polar regions with their size and ethereal beauty. But now—just when these ecosystems are in rapid flux due to global climate change—these frozen masses are taking on a new dimension of wonder as we uncover their critical role in the biology and chemistry of polar seas. No longer can we look at icebergs as mere passive beauties. They are active agents of change, each one an icy oasis trailing a wake of life as it drifts on its inexorable oceanic journey to melting.
Jeff Rubin, author of Antarctica (Lonely Planet Publications, 4th edition, 2008), the best-selling travel guide to the continent, lectures regularly about Antarctica's history aboard tour vessels there.
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