The north Bering Sea, one of the world's richest feeding grounds for whales, walruses, and sea birds, is warming to the point where animals are being forced to adapt or suffer the consequences.

The Bering Sea sits between Siberia and Alaska above the Aleutian Islands (see map). Its northern half was typically covered in solid ice for seven months of the year.

But now there is less ice in general, and the seasonal melt is starting earlier in the spring, said Jacqueline Grebmeier, a marine ecology expert at the University of Tennessee in Knoxville.

Some animals, like gray whales, are moving farther north to follow the cold water. Meanwhile, pink salmon and pollock, fish typically found in the southeast Bering Sea, are moving into northern waters.

Other animals of the north Bering Sea may not be adapting enough to survive. Bearded seals and walruses, which feast on the sea's bottom-dwellers, are struggling with a reduced food source.

Also in trouble are diving Eider ducks, a threatened species under the U.S. Endangered Species Act.

"[The ducks'] population is going down, and their food supply is going down," Grebmeier said. Her team's results are published in today's issue of the online journal Science Express.

Meals on Ice

Warming of the Bering Sea is likely due to global climate change and to a weakening of the cold north winds that blow across the sea, affecting its temperature.

The sea is usually teeming with wildlife thanks to phytoplankton, the starting link in the region's food chain.

The tiny sea plants start to grow under the ice each spring until their population reaches a massive, nutrient-packed overgrowth by June.

As the overcrowded plants suffocate and die, their remains fall to the muddy bottom, creating a carbon-rich food layer for worms, shrimp, and clams.

These bottom-feeders then support populations of birds and marine mammals.

But the current trend of earlier ice melt and warming water is interfering with the life cycle of the phytoplankton. Now there are fewer clams and worms growing on the seafloor, Grebmeier said.

Spring ice cover in the region is melting about three weeks earlier compared to 1997, said Grebmeier, who has been tracking the impact of climate changes on the north Bering Sea ecosystem since the 1980s.

Her team's data show that the temperature at the bottom of the sea increased from about 29° F (-1.6° C) in the early 1990s to 32° F (0° C) in 1998.

In short, Grebmeier said, the region is shifting from an Arctic ecosystem to one that looks more like the subarctic.

Whales at Risk

Large pods of gray whales typically travel to the Bering Sea's northern waters each spring from Baja California. They make the return trip in autumn, and the total migration is the longest known of any marine mammal.

But now the animals are heading even farther northward into the Chukchi Sea, above the Arctic Circle, seeking colder waters and the food they are accustomed to—amphipods.

These tiny shrimplike creatures live in the muck at the bottom of the shallow sea.

The whales feed voraciously all spring and summer in preparation for a three- to five-month fast during their 12,000-mile (19,000-kilometer) journey back to Baja California.

Now some whales are so comfortable in the north that they are foregoing their full autumn migration, going no further south than Kodiak, Alaska.

As the gray whales shift northward, they are moving closer to the territory of the bowhead whale, which feeds offshore on krill.

Alaskan natives hunt the bowhead and are concerned that the more aggressive gray whale may interfere with the quieter bowhead.

"There is the potential for space competition," Grebmeier said.

The elusive and nearly extinct north Pacific right whale also swims to the north Bering Sea to feed.

These animals number as few as two dozen, said John Hildebrand, a professor at the University of California, San Diego, and Scripps Institution of Oceanography, who studies whale song.

"We need to make every effort possible to give them a chance to survive," Hildebrand said.

"I can't predict at all how climate change would play into the lives of right whales. It definitely will play into their lives but how, we don't know."