Thinning ice creates undersea Arctic greenhouses

Light shining through could increasingly let phytoplankton bloom in polar region

phytoplankton blooms and diatoms

GREEN LIGHT  The thinning and melting of Arctic sea ice allows more light to reach the waters below, which may spawn more and more phytoplankton blooms (right). Typically the thick, opaque ice prevents the blooms, which can consist of diatoms such as these (left).

William M. Balch/Bigelow Laboratory for Ocean Sciences; Karen Frey/Clark Univ.

Sea ice skylights formed by warming Arctic temperatures increasingly allow enough sunlight into the waters below to spur phytoplankton blooms, new research suggests. Such conditions, probably a rarity more than two decades ago, now extend to roughly 30 percent of the ice-covered Arctic Ocean during July, researchers report March 29 in Science Advances.

The microscopic critters need plenty of sunlight to thrive, so scientists were stunned by the discovery of a sprawling bloom below the normally sun-blocking Arctic ice in July 2011 (SN: 7/28/12, p. 17). Satellites can’t peek below the ice, though, so scientists at the time didn’t know whether the bloom was an oddity or representative of a shift in the Arctic environment.

Harvard University oceanographer Christopher Horvat and colleagues created a computer simulation of sea ice conditions from 1986 through 2015. Warming temperatures have thinned the ice, the researchers found, and increased the prevalence of meltwater pools on top of the ice that allow more light to pass through than bare or snow-covered ice.

Whether blooms are in fact more commonplace under the ice remains unclear, though, because the study didn’t consider whether there would be enough nutrients such as nitrogen and iron for budding blooms. If more blooms are lurking in the Arctic Ocean, they may already be dramatically reshaping the Arctic ecosystem. A boost in phytoplankton could alter marine food webs as well as soak up more planet-warming carbon dioxide from the environment.

Big bloom

 Increasingly large swaths of the ice-covered Arctic Ocean allow enough light into the waters below to support phytoplankton blooms, new research suggests. Green regions indicate bloom-friendly conditions in July over the last few decades, with darker shades representing longer duration. 

Christopher Horvat

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