Study ties reduced ice cover to drop in Antarctic krill

By Usha Lee McFarling
Los Angeles Times

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The new research, published in today's issue of the journal Nature, is the first comprehensive attempt to estimate numbers of the small, shrimplike creatures that were once so abundant that their swarms colored vast patches of the southern oceans blood red.

Now krill have largely been replaced by salps, which are clear, gelatinous invertebrates that provide so little nutrition to predators that they are considered ecological dead-ends, said Angus Atkinson, a marine biologist with the British Antarctic Survey, who led the study.

Such a steep decline in krill could decimate the region's abundant wildlife, ecologists said. The finding may signal that a shift is under way in one of the world's most productive and pristine ecosystems.

"We're just holding our breath to see what the consequences are," said William Fraser, an Antarctic researcher who was not involved in the study.

Antarctic krill are thumb-size crustaceans that feast on drifting phytoplankton and, in turn, provide food for myriad Antarctic creatures, including the blue whale — the largest animal on the planet.

Atkinson and his colleagues pooled data from nine nations that collected krill in Antarctic waters.

Because krill are a boom-and-bust species that varies dramatically in number from year to year, the group looked for long-term patterns.

The international team found krill numbers had decreased by more than 80 percent since 1976 in the southwestern Atlantic near the Antarctic Peninsula, a hugely productive marine area thought to be a krill spawning ground and home to about half of the region's adult krill.

The area has warmed in the past 50 years by 4.5 degrees Fahrenheit, nearly five times the global average, according to the researchers. Some scientists link the warming to natural climate cycles; others say human production of greenhouse gases, such as carbon dioxide, plays a role as well.

The result is a diminished ice cover in some parts of the waters surrounding Antarctica. Krill larvae require sea ice to survive the winter. Young krill eat algae that grow in cracks on the underside of the ice and hide in the cracks to evade predators.
 
"Without sea ice, (the larvae) all starve to death," said Fraser, who has studied Antarctic penguins for three decades and heads the nonprofit Polar Oceans Research Group in Sheridan, Mont. "Sea ice is the heartbeat of Antarctica."

The Antarctic Peninsula is thought to have seen heavy ice years more often in the past. More recently, these "good" ice years have occurred roughly once every five years. Since krill live six to seven years, they can still get in one good reproductive year even if ice is sporadic.

Fraser said if good ice years occur too far apart, the krill will not be able to successfully reproduce.

"What you would see then is a literal collapse of the food web," he said. "All the predators would suffer some pretty drastic declines."

He pointed to the Adelie penguins, which eat only krill during the summer months. Their numbers in the Antarctic Peninsula have declined by 70 percent since 1974.

Some scientists, however, are skeptical of the study's conclusions.

Krill expert Steve Nicol of the Australian Antarctic Division questioned whether Antarctic krill, with a biomass once estimated topping 1 billion tons, were really down by such enormous numbers.

"Could we really have lost 900 million tons of krill without anyone noticing? I don't think so," he said. "You would expect to see most of the predators in decline and that doesn't appear to be happening."

He said the krill could be vastly underestimated because of the difficulty in tracking the creatures as they migrate and are tossed about through the vast seas.

"Something's happened," Nicol said in a telephone interview from Tasmania. "We're just not quite sure what."