BY MARK SHWARTZ
For the second time in 26 months, a massive iceberg has clogged a large portion of Antarctica's Ross Sea, causing what could turn out to be a devastating loss of penguins and other marine life, according to a NASA-funded study by Stanford scientists.
Reduced phytoplankton production in the Ross Sea, a result of unusually large amounts of sea ice, spells trouble for Antarctic Adélie penguins, Stanford scientists say. Photo: Michael Van Woert, NOAA/NESDIS, ORA
Using satellite data, geophysicists Kevin Arrigo and Gert L. van Dijken monitored the movements of a giant iceberg named "C-19," which calved off the western face of the Ross Ice Shelf in May 2002. C-19 is one of the largest icebergs ever recorded -- 19.2 miles wide and 124 miles long, or nearly twice as big as Rhode Island.
Arrigo is an assistant professor of geophysics, and van Dijken is a science and engineering associate in the department. In a study published in the American Geophysical Union's Geophysical Research Letters, the authors described how C-19 drifted northward from May until November 2002, when it apparently ran aground. By January 2003, the iceberg had become trapped against a shallow bank, eventually forming a perpendicular barrier that prevented sea ice from moving out of the southwestern Ross Sea for the next three months -- a crucial time of year when tons of microscopic marine algae, called phytoplankton, normally bloom in open water.
Phytoplankton are a major food source for krill, which in turn are consumed by fish, seals, penguins and whales. Without massive amounts of phytoplankton, the entire food chain in the Ross Sea can collapse.
Like plants, phytoplankton grow through the process of photosynthesis, using chlorophyll to convert sunlight into food. But unusually high levels of sea ice will block sunshine, preventing phytoplankton from growing.
To determine phytoplankton production levels in the Ross Sea, the researchers examined data from SeaWIFS -- the Sea-viewing Wide Field-of-view Sensor aboard NASA's SeaStar satellite.
"SeaWIFS measures the amount of light coming out of the ocean at different wavelengths," noted Arrigo. "Phytoplankton contain green chlorophyll, and with SeaWIFS you can measure the intensity of the greenness of the water. The greener the water, the more phytoplankton there are."
According to the authors, SeaWIFS data from November 2002 through April 2003 revealed that the Ross Sea contained only a fraction of the chlorophyll usually seen during those months.
"Phytoplankton blooms in the region were diminished dramatically, and primary production was reduced by over 90 percent, relative to normal years," Arrigo and van Dijken wrote. The likely reason for the sharp decline, they noted, was the unusually high sea ice cover caused by C-19.
Penguins in peril
In the study, the authors cited their 2002 analysis of an even larger iceberg, called "B-15," which fell into the Ross Sea in March 2000. Using NASA data, Arrigo and van Dijken found that the excessive build up of sea ice in 2000 and 2001 had resulted in a 40 percent decline in phytoplankton productivity, which had a cascading effect on other species higher up in the food web -- including Adélie penguins. Nearly a third of the world's total Adélie population is found in the Ross Sea.
According to the authors, the reduced phytoplankton production caused by B-15 "forced Adélie penguins to forage further form their nests, and the increased ice cover resulted in the penguins switching from a diet of predominantly silverfish to one consisting mostly of crystal krill." As a result, many Adélie colonies experienced a sharp decline in 2000 and 2001, the authors said.
"Considering that the calving of C-19 resulted in a far greater loss of production than B-15, it is likely that [penguins and other animals] that rely either directly or indirectly on phytoplankton production as a source of metabolic fuel were even more heavily impacted in 2002-03 than they had been in 2000-01," the authors concluded.
"Calving events over the last two decades indicate reduced primary productivity may be a typical consequence of large icebergs that drift through the southwestern Ross Sea during spring and summer," Arrigo added, noting that, by July 2003, C-19 had finally drifted out of the Ross Sea, having diminished little in size.
According to Arrigo, most of the face of the Ross Ice Shelf has already calved, although another large crack has appeared. However, predicting if and when another large iceberg will fall into the Ross Sea will be very difficult, he cautioned.
In a related study in the Journal of Geophysical Research, Arrigo and van Dijken also found a strong association between Adélie penguin populations and polynyas -- areas of open water or reduced ice cover that provide ideal conditions for phytoplankton blooms. The authors used satellite data to measure chlorophyll content in 37 polynyas between 1997 and 2002. The survey found that the more productive polynyas were able to support bigger penguin populations -- in part because the birds had shorter distances to forage for krill, which reduced their exposure to predators and other dangers.
"This is the first times anyone has ever looked comprehensively at the biology of the polynyas," Arrigo said. "Any changes in production within these polynyas are likely to lead to dramatic changes in the populations of penguins and other large organisms.
Rob Gutro and Krishna Ramanujan, science writers with the NASA Goddard Space Flight Center, contributed to this story.
Stanford Report, October 8, 2003