Sea Ice Skylights Spur Phytoplankton Blooms

Sea Ice Skylights Spur Phytoplankton Blooms

For much of the winter, the Sun barely rises over the Arctic Circle; some days, it doesn't rise at all. Most of the Arctic Ocean and nearby land is capped by ice. Beneath the ice cover, phytoplankton—the microscopic, plant-like organisms that underpin the entire ocean food web—take a “long winter’s nap.”

When sunlight returns and the ice retreats, it’s like a cover being drawn off the roof of a greenhouse. Along the edge of the retreating ice, the water explodes with blooms of phytoplankton. The images above offer two views of a phytoplankton bloom in the Chukchi Sea (northwest of Alaska) on July 10, 2011. Both images are from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite.

The top image is a natural-color, photo-like view. The bottom image is a map showing milligrams of chlorophyll per cubic meter of seawater. (Phytoplankton use chlorophyll for photosynthesis just like land plants.) Blue indicates areas with low concentrations of chlorophyll, and yellow indicates areas with high concentrations. Land is light gray, and areas where MODIS could not collect data due to clouds or sea ice blocking the satellite sensor’s view are dark gray.

North of Wrangel Island, the sea ice has a dingy look in the natural-color image. This could be from sediment, but its distance from shore and the fact that the ice is fringed by waters with extremely high levels of chlorophyll suggest that the ice has been discolored by algae and other phytoplankton.

According to Arctic oceanographer Karen Frey, the discoloration is consistent with an unusual phenomenon that she encountered while on a research cruise in the Chukchi Sea in July 2012: a massive bloom of phytoplankton stretching up to 100 kilometers (60 miles) under the ice pack. Historically, the ice in this area has been thick enough even in spring to keep the waters below in darkness, Frey says. But in the past decade, ice conditions have changed dramatically, leaving a thinner ice cover laced with shallow ponds of meltwater. The ponds act like skylights, allowing light to filter through and support phytoplankton blooms beneath the ice.

According to Frey, the presence of this bloom is not just a “geewhiz” phenomenon. Previous satellite-based estimates of Arctic phytoplankton productivity have generally assumed that nothing much is happening under the consolidated ice pack. Preliminary estimates of the size of this bloom—and of the area of the Arctic Ocean with similar ice conditions—suggest that phytoplankton productivity could be ten times higher than previously estimated.

Arctic ecology is discussed in more depth in the Marine Ecosystems chapter of the 2012 Arctic Report Card, released by the National Oceanic and Atmospheric Administration (NOAA).

NASA Earth Observatory images created by Jesse Allen, using MODIS data from the Land Processing Distributed Active Archive Center (LP DAAC). Caption adapted from “Melt pond ‘skylights’ enable massive under-ice bloom in Arctic” by Rebecca Lindsey, NOAA ClimateWatch Magazine.