Microbes flourish under Arctic sea ice

Scientists shocked to find phytoplankton thriving under frozen surface

Secret gardens may hide beneath floating slabs of ice in the Arctic. A pea soup of plantlike plankton has been uncovered that extends more than 100 kilometers under ice off Alaska’s coast.

PLANKTON HAZE A camera lowered beneath an ice slab off Alaska’s coast spotted this thick green haze of phytoplankton thriving in waters thought to be inhospitable to the microorganisms. Karen Frey/Clark Univ.

The explosion of microscopic life, spotted last July, could cause problems for other critters in the Chukchi Sea, researchers report online June 7 in Science. Seasonal blooms this big traditionally happen later in the summer, and only in open waters exposed to the sun after ice melts.

“I’ve been in this field for almost 30 years now, and I would have said this was impossible,” says Kevin Arrigo, a biological oceanographer at Stanford University. “The assumption has always been that where you’ve got ice, nothing will grow in the water beneath it.”

Light doesn’t penetrate ice well, especially the thick ice historically found in the Arctic. Snow covering the ice can add an opaque blanket, making the water beneath a dim, dismal place for phytoplankton, which need light for photosynthesis.

But climate change has altered the character of much of the ice. Gone in many places are the meters-thick grand old slabs that persisted year after year. New ice born every winter that tends to fade away during the summer is thinner and allows more light through.

Warmer air also melts snow and small grains of ice on top of young ice. This melt darkens the surface, like water poured on a sidewalk, allowing the ice to absorb more light.  

More than half of the light striking a young slab of ice can reach the water below, the researchers found.

“The pictures of the ice on the surface are amazing,” says Walker Smith, a biological oceanographer at the Virginia Institute of Marine Science in Gloucester Point. “There’s no snow, and the ice is incredibly transparent.”

After cracking open young ice more than a meter thick in places with a ship and peering beneath with underwater cameras, the researchers found phytoplankton growing at extraordinary rates. Fed by the light and by a steady stream of nutrients coming from the Bering Strait, the organisms thrived to depths of more than 50 meters.

What this prosperity means for the rest of the local food web isn’t clear yet. The hardiness of the microorganisms suggests that the Arctic Ocean could harbor more life than scientists had realized, concealed beneath the ice.

But Arrigo worries that under-ice blooms could make life difficult for other creatures visiting the area. Migratory birds make pit stops in this area every year as the ice melts to feed on bounties made possible by phytoplankton. With blooms occurring earlier and under ice, the creatures could arrive too late for dinner.

Confirming these fears will require more fieldwork, especially since satellites can’t see through the ice.

“We’re really lucky they saw this bloom, but we don’t know how widespread this is right now,” says Jean-Éric Tremblay, a biological oceanographer at Laval University in Québec, Canada.

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