Researchers have discovered a massive bloom of phytoplankton beneath ice-covered Arctic waters. Until now, sea ice was thought to block sunlight and limit the growth of microscopic marine plants living under the ice.
The researchers used a seagoing Imaging FlowCytobot to determine what species of algae live in and below Arctic sea ice (Sam Laney / Woods Hole Oceanographic Institution)
According to Woods Hole Oceanographic Institution (WHOI), the amount of phytoplankton growing in this under-ice bloom was four times greater than the amount found in neighboring ice-free waters. The bloom extended laterally more than 100 km (62 miles) underneath the ice pack, where ocean and ice physics combined to create a phenomenon that scientists had never seen before.
In a paper in Science Express, the researchers conclude that ice melting in summer forms pools of water that act like transient skylights and magnifying lenses. These pools focused sunlight through the ice and into waters above the continental shelf north of Alaska, where currents steer nutrient-rich deep waters up toward the surface. Phytoplankton under the ice were primed to take advantage of this narrow window of light and nutrients.
“Way more production is happening under the ice than we previously thought, in a manner that’s very different than we expected,” said Sam Laney, a biologist at WHOI, who was part of the multi-institutional team led by Kevin Arrigo of Stanford University.
Just as a rainstorm in the desert can cause the landscape to explode with wildflowers, this research shows that events like pooling melt water can happen on very short timescales in the Arctic yet have major effects on the ecosystem.
“If you don’t catch these ephemeral events, you’re missing a big part of the picture,” Laney explained.
The unexpected discovery occurred during a 2011 expedition aboard the US Coast Guard icebreaker Healy. It was part of the NASA-funded ICESCAPE program to investigate the impact of climate change in the polar Chukchi and Beaufort seas.
The team gathered images of several kinds of phytoplankton in the bloom, including this image of the alga Phaeocystis, only a few micrometers in diameter, attaching to two cells of a larger algae species, Thalassiosira (Sam Laney / Woods Hole Oceanographic Institution)
The ship conducted transects into the ice pack to determine how far and deep the bloom extended. Measurements of biomass showed the largest part of the bloom occurred far away from the open ocean, under thick ice and close to water upwelling at the continental shelf break, where the shallow coastal shelf plunges steeply into deeper water.
Bob Pickart, another member of the team and physical oceanographer at WHOI, showed that easterly winds churned out by monster storm systems along the Aleutian Islands can reverse the current along the shelf break. The change in circulation drives cold, nutrient-rich water up from the abyss and refreshes the supply of nutrients available to phytoplankton growing near the surface. “Without a doubt the bloom was enhanced at the shelf break,” Pickart said.
As the melt pools introduced light through the ice, phytoplankton in shelf break waters likely experienced something akin to an all-you-can-eat buffet.
“Discovering the under-ice bloom was very serendipitous,” Pickart said, “But the difficulty of exploring the harsh, remote region also leaves it wide open for surprises.”
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Bibliographic information: Arrigo K. et al. 2012. Massive Phytoplankton Blooms Under Arctic Sea Ice. Science. Published online: June 7, 2012; doi: 10.1126/science.1215065
