Trapped under ice, light-loving algae grow in the dark Arctic winter

Each winter, Baffin Bay freezes over as polar darkness descends over the top of the world.

Come spring, phytoplankton will bloom in these cold waters between Greenland and Canada, bolstering a bustling ecosystem of beluga whales and narwhals (SN: 4/8/20). But scientists have long assumed that the photosynthetic algae remain largely dormant in winter, blocked off from light by thick sea ice and snow.

New research challenges that assumption, however, finding that phytoplankton under the bay’s ice start growing as early as February, when the sun barely blips above the Arctic’s horizon.

Achim Randelhoff, an oceanographer at Université Laval in Quebec City, and colleagues deployed autonomous submersible floats in Baffin Bay that can measure photosynthetic activity and algae concentrations underwater.

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In February, when light was barely detectable under about 1.5 meters of snow-covered ice, Arctic phytoplankton begin growing and multiplying, the researchers report September 25 in Science Advances. The study suggests that springtime blooms are the culmination of an extended period of growth that starts in winter, not a singular burst of activity as was thought.

“Arctic phytoplankton are superefficient at using every little photon they can find,” Randelhoff says, but he was surprised that they could grow with such little light. As the months progressed and the sun rose higher, the team found that algal growth accelerated, reaching its peak growth rate for the year in April and May, despite the microorganisms still being covered by ice.

How these photosynthetic algae can make do with such little light remains opaque. “So much of winter in the Arctic is still a black box,” Randelhoff says. “This is the kind of study that raises more questions than answers.”