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Chain Reaction from Large Icebergs

Lots of attention has focused during the past several years on state sized icebergs that break off ice shelves in Antarctica. Discussion has focused on the cause and effect of this in relation to global warming, but research by NASA and Stanford University has shown that there is definitely an impact on plant and animal life when these major icebergs break off.

Researchers used satellite imagery to observe how these icebergs can disrupt an entire marine ecosystem. Using data from three orbiting satellites, researchers have been monitoring a Connecticut-size iceberg called B-15 that broke from Antarctica's massive Ross Ice Shelf in March 2000. Within a few months, B-15 had fractured into smaller bergs that formed dams along the coast - preventing thousands of square miles of pack ice from drifting out of the Ross Sea.

As a result, large stretches of normally open ocean were covered with ice -from November 2000 to March 2001. During these crucial spring and summer months, the Ross Sea usually teems with life, as tons of microscopic marine algae - called "phytoplankton" - undergo reproductive blooms.

The Ross Sea ecosystem depends on phytoplankton - a primary food source for shrimp-like krill, which in turn are consumed by fish, seals, whales and penguins. Phytoplankton need open water to reproduce, but satellite data revealed that last season's abnormally high levels of pack ice caused a 40 percent decline in plankton productivity.

"B-15 was one of the largest icebergs ever recorded from the Ross Ice Shelf," said Kevin R. Arrigo, assistant professor of geophysics at Stanford. "B-15 broke into smaller pieces that prevented the normal movement of sea ice out of the region. Sea ice is very effective at blocking light, so the phytoplankton couldn't grow - there was just too much ice around. "

According to satellite data, extensive ice cover reduced both the area suitable for phytoplankton growth and the length of the algal growing season, causing the observed 40 percent drop in phytoplankton, which affected the entire food chain - from krill to penguins.

"We know for certain that penguins suffered breeding losses because of the icebergs in this region," Arrigo said, noting that the Ross Sea is home to 25 percent of the world population of Emperor penguins and 30 percent of Adélie penguins.

"There was a lot less food nearby for penguins to get to, so they had to go much farther to feed," he added. "In doing so, they left their nests exposed for longer periods of time than they normally would. That made them vulnerable to predators such as the skua - a large gull that feeds on chicks and the eggs. So penguin breeding success was much lower last year."

Current satellite images show that a large iceberg remains wedged against Ross Island - site of several large penguin rookeries.

"Now the penguins have another obstacle they have to get around," Arrigo said. "Not only do they have to go farther to find food, but they have to swim around this enormously large iceberg that has found its way in their path. Some rookeries have been abandoned altogether."

Recent observations show that most of the Ross Sea pack ice has drifted away, and that phytoplankton levels are returning to normal. Researchers plan to continue monitoring the region and to assess the ecological impact of icebergs in other parts of the continent.