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Climate Warms The Bering Sea

Major Ecosystem Shift In The Region Connecting The North Pacific To The Arctic Ocean

A few years ago, scientists began noting significant changes in the southern Bering Sea. This subarctic gateway typified by great expanses of winter sea-ice was not freezing over. Now, the northern Bering Sea, traditionally thought of as an "arctic” ecosystem, is also beginning to show signs of warming.

A major climate shift that occurred in the region in 2000 may be responsible. Since this time, the cold northerly winds that usually dominate the region have slackened, and air temperatures have increased. The resulting warmer air reduces the overall extent of the ice found in the region and causes the sea-ice to melt three weeks earlier than usual in the spring. Below the surface, water temperatures have also risen.

These changes are beginning to have dramatic effects on the ecosystem of the northern Bering Sea, according to a report published recently in Science. In the paper, researchers link warming water to a decline in the rich bottom food resources that characterize this zone. Many animals including gray whales, walruses, and seabirds depend on this food supply, and as a result, are moving further north in search of their meals.

The stakes are high for any potential change to the Bering Sea, says James Overland, an oceanographer at the Pacific Marine Environmental Laboratory in Seattle and a coauthor of the study. This shallow sea produces 47 percent of the annual fishery catch in the U.S. and supports 80 percent of the U.S. seabird population.

According to Jacqueline Grebmeier, an ecologist at the University of Tennessee in Knoxville and the lead author of the study, warming water and reduced sea-ice impacts this ecosystem by decreasing the amount of energy delivered to the bottom of the northern Bering Sea.

Normally, phytoplankton, the photosynthetic organisms that form the base of the oceanic food chain, bloom in the area in early spring, just as the ice is beginning to melt. These phytoplankton fall to the bottom, providing a rich carbon source for the clams and other invertebrates living there. For reasons not completely understood less of this carbon is reaching the seafloor. Grebmeier suggests the spring phytoplankton bloom is changing, either in magnitude or timing, creating a sort of benthic drought. Whatever the mechanism, bottom-dwellers are left waiting on a meal.

This in turn has cascading effects on the animals that feed on the bottom-living organisms. Diving sea ducks such as the spectacled eider feed extensively on clams living in the northern Bering Sea. A reduction in their prey could have grave consequences for this already threatened sea bird population.

Additionally, gray whales, which are huge consumers of bottom-dwelling invertebrates in the Bering Sea, are now moving farther north than previously observed, presumably in search of food. Walruses have followed suit. They need ice around to haul out on, so when the ice retreats north, they go with it.

Coastal communities such as the Yupik hunters of St. Lawrence Island, located in the northern Bering Sea, provide compelling accounts of these changes. They have witnessed the hard pack-ice change to a thinning sheet, and now have to go farther offshore to reach the mammals they depend on for food.

Warming water may also profoundly change the distribution of fish in the northern Bering Sea. Normally, the cold water curtain of sub-zero temperatures found in the area keeps bottom-dwelling fish out.

However, as temperatures have risen, the curtain has lifted further north, and fish are beginning to expand into this range. Scientists from the Alaska Fisheries Science Center found large numbers of juvenile pollock and pink salmon in the northern Bering Sea in surveys conducted in 2002 and 2004.

Locals also report that pink salmon have begun colonizing rivers north of the Bering Strait. Although it is early to tell, this may open up the northern Bering Sea to commercial fish harvests previously relegated to more southern shores.

Part of what makes Grebmeier's study noteworthy is that few researchers have documented this kind of ecological response to climate change in the northern Bering Sea, says George Hunt, a seabird expert at the University of Washington in Seattle. According to Hunt, previous research focused largely on the southern region where the economically valuable fisheries reside. "What we've learned in the southeastern Bering Sea might not apply farther north,” says Hunt. "The two regions may have different vulnerabilities to climate change.”

The climate in the Bering Sea is known to be a fickle friend. Characterized by great year-to-year variability, this region switched between cold periods observed in the 1970s to warmer periods seen in the 1980s. However, the changes observed since 2000 have perplexed researchers who have studied this area for the past 20 to 30 years. According to Overland, the Bering Sea appears to be entering a new climate state, one with predictably warm years in succession.

The change that precipitated this shift is unknown. "This is the big uncertainty,” says Overland. "The weather pattern that goes along with these changes for the last six years looks substantially different from the main climate patterns that occurred in the 20th century.”

Just how global climate change and its associated increase in atmospheric carbon dioxide play a role in this change is also not known. What is certain is that temperatures are increasing at a rate far exceeding that predicted by models of global warming.

Overland speculates the Bering Sea climate may eventually revert to a cold period or the warming may slow down. However, it is unlikely to revert rapidly because, "the ocean is a great heat capacitor,” says Grebmeier, "once you warm it, it is difficult to reverse.”

David Lawrence is a biologist at the University of Washington and has studied science writing in the UW Department of Technical Communication.

Images

Top: Map of the Bering Sea showing depth contours. Photo: NOAA Pacific Marine Environmental Laboratory

Middle: The sea ice edge near St. Matthew Island in the northern Bering Sea. Photo: NASA Jesse Allen Earth Observatory Goddard Earth Sciences

Bottom: Walruses haul out on an ice flow in the Bering Sea. Photo: Marc Webber, U.S. Fish and Wildlife Service

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