Massive Icebergs May Affect Antarctic Sea Life And Food Chain

Scientists say the icebergs appear to have caused a 40 percent reduction in the size of the 2000-2001 plankton bloom in one of Antarctica's most biologically productive areas. The icebergs decrease the amount of open water that the plants need for reproduction.

After the calving, or "breaking off," of the B-15 iceberg in March of 2000, researchers used imagery from NASA's SeaWiFS (Sea-viewing Wide Field-of-view Sensor) satellite and data from the Defense Meteorological Satellite Program to see the effect that large icebergs have on phytoplankton (minute floating plants) blooms. The B-15 iceberg that broke off the Ross Ice Shelf and drifted into the southwestern Ross Sea was as large as the state of Connecticut (approximately 10,000 square kilometers or 3,900 square miles)

"This is the first time that satellite imagery has been used to document the potential for large icebergs to substantially alter the dynamics of a marine ecosystem," said Kevin Arrigo, a researcher at Stanford University, Stanford, Calif. Arrigo and his colleagues are publishing their results in a paper titled "Ecological Impact of a Large Antarctic Iceberg," in an upcoming issue of Geophysical Research Letters.

NASA's Thorsten Markus of the Goddard Space Flight Center, Greenbelt, Md., a co-author on the paper, noted that SeaWiFS satellite imagery enabled researchers to see that large icebergs such as the B-15 restricted the normal drift of pack ice. Normally, when the winds shift, ice is carried out into the Ross Sea, creating open ocean space and a breeding ground for phytoplankton. The icebergs, however, created a blockage that resulted in heavier spring/summer pack-ice cover than previously recorded.

Since the area of sea ice was more extensive, the area suitable for phytoplankton growth was reduced, and as a result, so was the length of the algal growing season. Since the B-15 iceberg was so large, plankton productivity throughout the region was more than 40 percent below normal.

The southwestern Ross Sea is one of the most biologically productive regions in the Southern Ocean surrounding Antarctica. This is partly due to the large and persistent areas of open water that form during the Antarctic spring when pack ice drifts out of the Ross Sea.

In the springtime, winds shift in the area of the Ross Sea and clear away sea ice, forming the Ross Sea polynya (an area of open water surrounded by sea ice) where phytoplankton flourish. However, when large icebergs calve, such as B-15, sea ice is not as easily moved by winds, severely reducing the area of open water.

Phytoplankton are a critical part of the entire ecosystem in the Ross Sea, since they sustain marine mammals and birds in the region. During periods where there are no large icebergs, phytoplankton thrive, and so do those organisms that feed on them. The region also is home to 22 percent of the world population of circumpolar Emperor penguins and 30 percent of Adélie penguins.

This research is a part of NASA's Earth Science Enterprise, a long-term research effort dedicated to help us better understand and protect our home planet.

Images and video are available at: http://www.gsfc.nasa.gov/topstory/20020416iceberg.html

Information on SeaWiFS can be found at: http://SeaWiFS.gsfc.nasa.gov/SEAWIFS.html

Information on icebergs is available on the National Snow and Ice Data Center Web site at: http://www.nsidc.org