Melt ponds fueling massive underice Arctic algal blooms

Small melt ponds are forming over vast expanses of the Arctic allowing fueling phytoplankton blooms beneath the metre-thick ice. Photo by Gert van Dijken, Stanford University.

Margaret Munro, Postmedia News

June 25, 2012

The most intense phytoplankton bloom recorded on Earth occurred under the Arctic ice last summer — a finding that has stunned seasoned polar scientists.

“The ice was over a metre thick,” says Kevin Arrigo at Stanford University, leader of the international team that reported Thursday finding the massive bright green algal bloom beneath the ice.

It turns out that first-year polar ice — long considered impenetrable to sunlight — can create ideal conditions for growing phytoplankton, the single-celled plants crucial to the Arctic food chain.

“It’s like the perfect environment,” says Arrigo.

The team was on a U.S. icebreaker smashing its way across the Chukchi Sea between Siberia and Alaska last July when equipment used to measure phytoplankton went “haywire.”

“We thought there was something wrong with the instruments,” Arrigo told Postmedia News.

Then the scientists made their first scheduled stop to take ice samples and got a good look at the ocean below.

“The water was completely green,” Arrigo said. “It was like pea soup. “

The farther they ventured into the ice-covered sea for their NASA-funded project to study ice, the more intense the under-ice algae bloom, says Arrigo, a veteran of many trips to the Arctic and Antarctic.

“It was shocking,” he says.

Phytoplankton were growing and multiplying at an extraordinary rate under an expanse of ice more than 100 kilometres across, the team reported Thursday in the journal Science.

Arrigo says records indicate it was the “most intense” algal bloom ever seen anywhere on Earth. “We had incredibly high concentrations of algae all of the way down to 70 metres in some cases,” he said.

First-year polar ice, which forms over just one winter, is becoming more common in the Arctic due to the recent retreat of much thicker multi-year ice.

While first-year ice was thought to be impenetrable by sunlight, Arrigo says it can actually create optimal growing conditions for algae.

Melt ponds tend to form on top of the ice as temperatures climb in June and July. And the shallow ponds act like lenses, allowing 50 per cent of the light to penetrate through the ice into the water below.

At the same time, the ice screens out UV radiation that can stunt algal growth, he says.

Add the Arctic’s 24-hour summer sunshine, and phytoplankton populations under the ice can explode.

Another factor at play is a “striking” change in some Arctic winds in recent years, says co-author Kent Moore at the University of Toronto. More persistent easterly winds are bringing up more nutrients in the Chukchi Sea, which appears to have fed the under-ice bloom.

Thursday’s report deals with the bloom under the ice in the Chukchi Sea, but Arrigo suspects algae could be blooming under the ice in about 25 per cent of the Arctic Ocean, including large parts of Canada’s North. Anywhere, he says, with shallow water, lots of nutrients and first-year ice.

As the Arctic warms, the scientists say under-ice phytoplankton blooms could become increasingly common, occur early in the season and consume nutrients that would normally feed open-water blooms.

“The real surprise is all these unanticipated consequences of global warming,” says Moore.

The shift may benefit some creatures, the researchers say, but others may have difficulty adjusting.

“If you’re a seabird planning to get to Chukchi Sea in mid-July to feed, you may be out of luck,” says Moore.


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