Scientists who traveled to the Arctic on a NASA research cruise last summer were looking for signs of climate change. What they found was a secret world hidden beneath the region's cap of sea ice.

During their travels through the Chukchi Sea off the coast of Alaska, they were stunned to find massive blooms of phytoplankton under the ice -- in water so teeming with the microscopic plant life that it turned an opaque, vivid green. The discovery upends the notion that the sea ice that forms in autumn ushers in a cold, dark and nearly lifeless season for the ocean below.

"This is what you live for as a scientist," said Don Perovich, a sea ice expert at Dartmouth and co-author of the study, published yesterday in the journal Science. "It's unexpected. It's pure discovery."

Lead author Kevin Arrigo, a biological oceanographer at Stanford University, said the findings amount to "a fundamental shift in our understanding of Arctic ecosystems."

"Clearly there are parts of the Arctic Ocean that are much more biologically productive than we thought," he said.

The scientists believe the blooms are a result of changes in the region's sea ice, which is receding and thinning as the climate warms.

"Decades ago, there would have been thick multiyear ice in this area, maybe 3 meters thick -- 9 to 10 feet," Arrigo said. "No way there would have been sunlight getting through ice that thick to have a bloom like this."

But these days, the portion of the Chukchi Sea the scientists studied is covered with 3-foot-thick "first year" sea ice that forms in the fall and melts in the spring.

More sunlight = phytoplankton bloom

More sunlight is able to penetrate that thinner ice, an effect compounded by declines in snowfall and a proliferation of melt ponds on the ice surface.

Those ponds "are windows from the sky to the ocean," Perovich said. "They transmit about 50 percent of the sunlight that's visible on the surface."

And that light, combined with nutrient-rich waters under the ice, is a recipe for a phytoplankton explosion.