Thinning Arctic Ice Is Breeding Dangerous Phytoplankton Blooms

Not only is the extent of sea ice coverage in the Arctic shrinking, but the ice that remains is also becoming thinner. And that may be leading to significant changes in the ecosystem beneath the ice, with potentially profound consequences aquatic life in the Arctic over the coming years and decades.

Each summer, as sea ice melts, surface water in the Arctic is bathed in sunlight, which triggers blooms of phytoplankton to develop along the ice edge. The blooms reach their maximum concentrations in September — when the amount of sea ice extent in the Arctic is at its lowest — because sea ice cover prevents sunlight from reaching the microscopic plants.

Or so it was thought.

It appears that climate change has created conditions in which greater expanses of the Arctic are conducive to phytoplankton blooms, according to a new study published in the journal Science Advances. Thinner ice means more sunlight can penetrate the water column. And more sunlight means more phytoplankton, said lead author Christopher Horvat of Harvard’s School of Engineering and Applied Sciences.

Hovat and his team were curious about a chance discovery in 2011. Scientists on board an icebreaker in the Chukchi Sea, encountered a massive phytoplankton bloom under the sea ice. The scientists concluded that the amount of the microscopic marine life in some Arctic waters had been underestimated by as much as ten times, and that changing ice conditions were more likely to trigger blooms. Horvat and colleagues set out to confirm and quantify that observation.

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Sea ice prevents sunlight from penetrating to the waters below not so much because of its solidity but because of its albedo — the relative whiteness of the ice surface, which determines the degree to which it reflects or absorbs incoming solar radiation. As ice thins, it darkens, decreasing its albedo, which has contributed to an increase in the amount of melt ponds across the Arctic. And melt ponds, like thinner ice and the open water of the Arctic, absorb rather than reflect sunlight.

Horvat and his team used statistical models to determine whether melt ponds, diminished thickness, or both might be contributing to an environment more favorable to phytoplankton blooms beneath the ice.

They concluded that, although melt ponds were indeed a contributory factor, the thickness of the ice was by far the greater variable. Indeed, they found that conditions conducive to massive blooms beneath the ice existed only in three or four percent of the Arctic Ocean basin prior to two decades ago. That area has increased, however, to greater than 30 percent, suggesting phytoplankton blooms — until recently considered nearly impossible — may in fact be routine.

The consequences for the Arctic marine ecosystem may be profound, said Horvat, although the extent of the damage on aquatic life depends in part on whether early-season under-ice blooms are likely to take place in addition to the usual September blooms, or instead of them. Earlier phytoplankton blooms run the risk of disrupting the Arctic food web, by being out of sync with the reproductive cycles of marine life that have evolved to take advantage of a later bonanza.

“If it’s an addition, it might be beneficial. We might have increased biodiversity, and more critters can live under the sea ice,” he said. “But if it’s replacing the bloom, there might be a shift of several months, from when the entire ecosystem is built, in about September, to about July. And that’s really abrupt because all these Arctic animals that feed on the fish that feed on the zooplankton that feed on the phytoplankton are all tied into a seasonal cycle. Nature’s not really adapted to rapid several-months shifts in that seasonal cycle.”

The Arctic remains a frequently inaccessible environment because of the hight cost and difficulty of organizing expeditions. Horvat said he is hopeful that research initiatives such as the International Arctic Drift Expedition, known as MOSAIC, which will intentionally trap a research vessel in frozen, drifting sea ice, might provide an opportunity to explore changes in Arctic marine ecosystems.

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In the meantime, he said, he marvels at the way in which the chance discovery of the 2011 bloom has underscored how much remains unknown about the impacts of climate change.

“I would never have thought to consider this problem,” he said. “I think it’s one of the greatest and most interesting paradigm shifts we’ve seen in a while, because it was totally out of left field.”

“It makes sense," he added. "But nobody had a clue this was happening. And if this is happening across the Arctic — and there are other observations that are trickling in — this is a dramatic change to our scientific understanding.”

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