Arctic Sea Ice Decline Shakes Up Ocean Ecosystems / Scientific Frontline

Arctic Sea Ice Decline Shakes Up Ocean Ecosystems

Saturday, November 8, 2008

Red represents regions of open water in 2007 that were ice covered in 2006, much of which has never been ice-free for as long as measurements have been available.

Warm-colored areas show regions that saw larger changes in phytoplankton growth in 2007 (above) and to the ice-free growing season (below) compared to 2006.

Credit: NASA/Stanford University

Uncertain as to how phytoplankton -- microscopic marine plants on which much of ocean life depends -- would respond to Arctic sea ice decline, researchers took advantage of NASA satellite images to show that the microscopic floating plants are teeming in regions of recent ice melt.

The explosion in phytoplankton populations is the result of new open-water habitat and, more significantly, an extended ice-free growing season, biological oceanographer Kevin Arrigo and colleagues from Stanford University in Stanford, Calif., reported last month in the American Geophysical Union's Geophysical Research Letters.

Since phytoplankton cycle carbon dioxide into organic compounds and also form the base of the marine food web, the researchers believe the booming populations could have complex ecological consequences.

"Arrigo and colleagues have brought together the effects of air-sea interaction, warming water, and decreasing sea ice extent," said Paula Bontempi, a program scientist at NASA Headquarters in Washington. "You start to look at all of these interlocking pieces and think: there has got to be an impact on phytoplankton and the ecology of the system."

Phytoplankton, like any plant, require nutrients to survive. However, Arctic Ocean surface waters usually have a limited supply of nutrients, which has led some researchers to assume that new areas of open water would not necessarily promote additional phytoplankton growth.

To find out how phytoplankton respond to diminished sea ice cover, the team calculated changes in the sea ice extent and phytoplankton growth from ten years of chlorophyll measurements -- which are used to estimate phytoplankton abundance -- collected by the Sea-viewing Wide Field of View Sensor (SeaWiFS) instrument on the GeoEye satellite. The team also collected measurements of sea surface temperature and ice extent from other satellite instruments such as NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua and Terra satellites.

The researchers were most interested by what happened between 2006 and 2007, when the summertime minimum sea ice extent made its sharpest annual reduction since satellite measurements began in 1979.

By comparing maps of new ice free areas in 2007 with maps of increasing phytoplankton abundance since 2006, the team could deduce how much of that phytoplankton growth was due to newly ice free regions. In a similar way, the team could compare the maps of ice-free regions with maps that show the magnitude of an extended melt season, to deduce how much phytoplankton growth resulted from the longer season.

The team found that 30 percent of the increase in phytoplankton between 2006 and 2007 was due to large new areas of open water exposed by the extensive melting of sea ice. The other 70 percent of the increase could be attributed to a longer growing season, which in some Arctic regions was extended in 2007 by as much as 100 days, compared to 2006.

"We expected a big phytoplankton increase in the areas that were historically covered by sea ice because the plants now have sunlight." Arrigo said. "But the longer growing season is ultimately what allowed most phytoplankton to grow and increase productivity."

Phytoplankton and all plants naturally remove carbon dioxide, a greenhouse gas, from the atmosphere. Newly open water in the Arctic could therefore act as a new "sink" for carbon dioxide if marine plants and their carbon sink out of the surface waters to the deep ocean. Still, the magnitude of such a carbon sink remains to be seen because further growth could eventually be limited by the supply of surface nutrients. Scientists also wonder if the uptake of carbon into the Arctic Ocean will be temporary or long lasting.

Whales, seals, marine birds, zooplankton, and other marine animals all depend either directly or indirectly on phytoplankton for food. Researchers are uncertain what effect a boost in plant growth will have on the ecosystem, particularly migratory species that depend on the timing of sea ice melt and food availability.

"The Arctic is undergoing so many changes already," Arrigo said. "Nobody knows how this will play out."

Source: NASA / Goddard / Kathryn Hansen