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  National Geographic 21 Dec 05
Warming Arctic Sees Return of Blue Mussels After 1,000 Years
Amitabh Avasthi for National Geographic News

After a thousand years, blue mussels--helped along by warmer water temperatures--have returned to high-Arctic seas.

Their comeback could have serious implications for Arctic ecosystems and may be a sign of climate change, according to scientists.

"We are heading into uncharted waters in terms of future climate, and indicators, such as these mussels, are telling us clearly that we had better pay attention, because entire ecosystems are going to be disrupted," said Raymond Bradley, a climatologist at the University of Massachusetts in Amherst.

Since the mollusks require relatively warm temperatures to thrive, many scientists consider blue mussels to be reliable indicators of global warming. Some scientists, though, think the creatures can survive in cold waters too.

Climate Pendulum

Over the past 120,000 years the global climate system has oscillated from warm to cold and back again. During one such warm spell?between about 10,700 and 7,700 years ago--summer temperatures on the surface of Arctic seas reached almost 46ºF (8ºC).

Fossil records show that during the subsequent cooling period, the blue mussel population around the Norwegian island chain of Svalbard gradually dwindled. About a thousand years ago it disappeared completely.

Then in August 2004 the blue mussels reappeared in Svalbard, according to findings published in the November 21 issue of the journal Marine Ecology Progress Series.

Based on the size of the newly discovered mussels and the absence of larvae, researchers concluded that the mollusks were at least a year old and therefore had survived at least one Arctic winter.

To solve the puzzle of the mussels' comeback, scientists at the University Centre at Svalbard looked at seasonal changes in ocean temperature and North Atlantic currents. Using temperature data, biological samples, and knowledge of the life spans of blue mussel larvae, biologist Philos Jørgen Berge and his colleagues came up with a theory.

Warm water surging in from the North Atlantic created favorable conditions for the mussels, the researchers determined. Mussels had spawned in the North Atlantic, and their larvae rode the unusually warm currents that brought them to Svalbard.

"The establishment of these mussels was made possible only through a simultaneous occurrence of a set of independent environmental factors such as increased northward mass-transport [of warm Atlantic water], favorable local wind conditions, and a generally high sea-surface temperature in the North Atlantic," Berge said.

Computer simulations by Berge and his colleagues appeared to back up their theory. Berge thinks that ocean currents could be the mussels' only mode of transportation into Svalbard. The population, he says, is too large to have ridden in on floating debris. And the mussels couldn't have been carried in ships' ballast water--Svalbard is too far away from the nearest ports, Berge says.

Ecological Revamp

The return of blue mussels to Svalbard could be just the beginning of a wholesale transformation of the high Arctic.

"Species composition in the area will change," Berge wrote in Marine Ecology Progress Series. If temperatures continue to increase around Svalbard, he explained, larvae brought in by currents from warmer habitats will establish themselves in the region.

"The implications are significant," agreed James Carlton, a marine science professor at Williams College in Williamstown, Massachusetts. "The appearance of blue mussels and the periwinkle snail--a species not cited by the [study] authors--is probably the tip of the iceberg," Carlton said. "Depending on the species, we could easily see the beginning of the reshaping of shallow-water Arctic marine ecosystems within our lifetimes."

According to Bradley, the University of Massachusetts climatologist, "All of these changes simply reinforce the conclusions--that large climate changes are taking place at an unprecedented rate, leading to conditions not seen for thousands of years."
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