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 College 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.
"The amount of phytoplankton in this bloom -- top to bottom -- is more than has ever been seen in any bloom anywhere in the world, in open water," said Arrigo, who has studied phytoplankton for 25 years.
He compared the plankton-filled water to "pea soup." The dense blooms, more than 150 feet deep in places, extended from the ice edge more than 70 miles into the interior of the ice pack.
Although the researchers examined a small portion of the Chukchi Sea, they believe that in roughly 25 percent of the Arctic, conditions are ripe for similar plankton mega-blooms.
The possibility that the phenomenon is so widespread suggests scientists have underestimated the amount of carbon dioxide pulled into the Arctic Ocean by phytoplankton -- which pull CO2 from the atmosphere to make energy -- and animals that consume them.
An early feast for whom?
And the blooms may play into a growing disconnect between the coming of spring and the annual northward journey of wildlife seeking a tasty phytoplankton meal.
The Arctic's open-water plankton blooms now begin about a month earlier than they did in the 1990s, a consequence of the warming climate. Scientists worry that is throwing off the timing of migratory animals like Arctic terns, snow geese and California gray whales that once arrived to feed at the height of the yearly phytoplankton bloom.
The discovery that massive blooms of plankton begin before Arctic sea ice starts to melt in the spring changes the picture slightly.
"Under-ice blooms are going to be good for anything that eats off the bottom," Arrigo said, because they begin when there are fewer predators in the water column to consume the plankton -- meaning more of the tiny organisms will fall to the ocean floor.