News•February 14, 2013

Thinning Ice Is Turning Arctic into an Algae Hotspot

By Lauren Morello

Shrinking, thinning Arctic sea ice appears to be accelerating the growth of algae in polar waters, a new study finds, a development that could alter the region’s ability to absorb carbon from the atmosphere.

Scientists cruising central Arctic waters last summer aboard the research ship Polarstern were stunned to discover dense, shaggy deposits of the algae Melosira arctica clinging to the bottom of sea ice.

The green color of water in a melt pond atop Arctic sea ice hints at the riotous growth of algae under its surface.
Credit: Stefan Hendricks/Alfred Wegener Institute

Though researchers all the way back to 19th-century Norwegian explorer Fridtjof Nansen had noted colonies of Melosira hanging under the ice, they had always assumed the algae prospered in areas covered by thick “multi-year” ice that had survived several summer melt seasons.

That wasn’t the case last summer. The Polarstern crew found large clumps of the algae growing in areas covered by ice that was just 3 feet thick, not 10.

Stranger still, when the researchers sent high-powered cameras to the ocean floor — using a small, unmanned robot and other equipment — they found it blanketed with lush green clumps of algae.

“I was shocked when I sat there on board the ship and these images came up,” said the new study’s lead author, Antje Boetius, a biological oceanographer at the Alfred Wegener Institute for Polar and Marine Research in Germany. “I was really screaming. Even the captain came down to look at what was going on. The sailors said, ‘How is it possible that the sea floor is green?’ ”

The research, published Thursday in Science, adds to a small but growing body of evidence that suggests the ongoing decline of Arctic sea ice is changing life in polar waters.

A significant increase in the growth of oceanic plants like algae could pull more carbon dioxide from the atmosphere, and ultimately trap it in the deep ocean as larger organisms eat those plants, or they decay and sink to deep water.

“I think it’s a good paper,” said Kevin Arrigo, a biological oceanographer at Stanford University. “It’s interesting that it basically provides another example of the importance of loss of sea ice, and potential increases in (ocean plant growth in) the Arctic.”

Arrigo discovered his own polar surprise in summer 2011, when he led a NASA research cruise to the Chukchi Sea. His group found a massive — and unexpected — blooms of phytoplankton growing under sea ice there.

Researchers on the summer 2012 Polarstern cruise sample sea ice in the central Arctic.
Credit: Christian Katlein/Alfred Wegener Institute

The scientists, who published their findings last year in Science, believe that thinning sea ice is allowing more light to penetrate Arctic waters, which in turn spurs surprisingly fervent growth of undersea plant life.

That’s the same cause suggested by the authors of the new study. Researchers also believe that melt ponds that form atop sea ice each summer increase the amount of light streaming to the sea below.

“With multi-year ice, it’s been around many seasons. It’s got lots of hummocks on its surface, so its has deep melt ponds that cover a small surface area,” said C.J. Mundy, a biological oceanographer at the University of Manitoba who was not involved with the new study or the NASA research. “On first-year sea ice, you have a relatively smooth surface.”

That produces shallower ponds with a larger footprint, he said. And those ponds “are like windows to let light through the sea ice,” transmitting about half the sunlight that is visible on the surface.

Boetius and her colleagues on the Polarstern cruise believe that, in the case of Melosira, that increased light spurred unusually rapid growth of algae. And that algae, which would have frozen into multi-year ice in the past, was unmoored in an Arctic with less — and thinner — ice, and sunk to the sea floor in large numbers.

“You get much more light, and you get that light earlier in the (summer) season,” Boetius said. “That’s the new Arctic.”

But researchers still aren’t sure what the ultimate impact of that “new Arctic” will be on tiny oceanic plants like phytoplankton and algae.

A dive into a melt pond on sea ice in the central Arctic Ocean to explore algae growing underneath.
Credit: Mar Fernández-Méndez/Alfred Wegener Institute.

The area covered by Arctic sea ice is growing smaller. It reached a new summer low last year, shattering the previous record set in 2007 by 16 percent.

And the Arctic’s sea ice is also growing thinner. The latest satellite measurements show ice volume decreased 36 percent in autumn and 9 percent in winter between 2003 and 2012.

Those changes in Arctic sea ice could ultimately pull more carbon from the atmosphere, if plankton and algae growth increases over a large enough swath of the Arctic.

But scientists aren’t sure that is what’s going to happen.

“The Arctic has very little in terms of nutrients” in its waters to fuel algal blooms, Boetius said. So as ice thins and shrinks, a few years of explosive algal growth could deplete nutrients in the surface ocean—nutrients that may be slow to replenish.

But that is not the only possibility.

“If, as the ice retreats, we’re increasingly able to tap the nutrients we find in deep Arctic basins, [plant growth] may continue to increase,” Arrigo said. “I think we just don’t know right now.”