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February 27, 2014

CCPO Researchers Predict Ice Melt in Antarctic

topstory3-lgJohn Klinck and Eileen Hofmann
topstory3-lgBMichael Dinniman

The Ross Sea, a major, biologically productive Antarctic ecosystem, will be extensively modified by climate change in the coming decades as rising temperatures and changing wind patterns create longer periods of ice-free open water and affect the life cycles of both predators and prey, according to a paper published by Virginia researchers.

The authors, from the College of William & Mary's Virginia Institute of Marine Science (VIMS) and Old Dominion University's Center for Coastal Physical Oceanography (CCPO), are collaborators on National Science Foundation-supported research.

While conceding that "predicting future changes in ecosystems is challenging," the researchers write in their paper published Feb. 26 in Geophysical Research Letters that the changes predicted by their computer model have the potential to create "significant but unpredictable impacts on the ocean's most pristine ecosystem."

The wind and temperature changes, the authors write, will affect the ecological balance at the base of the Antarctic food web - including changes in distributions of algae, shrimp-like krill and Antarctic silverfish - which, in turn, may be expected to cause disruptions in the upper portions of the food web, including penguins, seals and whales, which depend on those species for food.

The paper was written by Walker Smith, a professor at VIMS; CCPO professors Eileen Hofmann and John Klinck; and CCPO research scientist Michael Dinniman. Klinck is the director of CCPO.

Hofmann said, "The model results suggest that the current trend of increasing sea ice in the Ross Sea will shift and that it will become more ice free in the summer, which would have a major effect on the food web, especially on organisms that depend on sea ice for their life history, such as Antarctic krill and Adélie penguins."

Added Smith: "The Ross Sea is critically important in regulating the production of Antarctica's sea ice overall and is biologically very productive, which makes changes in its physical environment of global concern."

The researchers used information drawn from a computer model of sea-ice, ocean, atmosphere and ice-shelf interactions - the Regional Ocean Modeling System - to make their predictions.

They note that over the past 50 years the distribution and extent of Antarctic sea ice, or ice that floats on the ocean surface, have drastically changed. Among these changes is a documented decrease of ice in the Bellingshausen-Amundsen sector, but also an increase of sea ice in the Ross Sea sector of Antarctica.

Observations show, they write, that "the duration of ice-free days on the Ross Sea continental shelf has decreased by over two months over the past three decades," which may have had effects on the current balance of biological productivity and the roles of various creatures and microscopic plants in the ocean ecosystem.

But, they note, "future projections of regional air temperature change, however, suggest that substantial warming will occur in the next century in the Ross Sea sector" while wind speeds are predicted to change. "These changes are expected to reverse the sea-ice trends in the future; however, the projected changes in heat content on the continental shelf and ecosystems dynamics that will occur as a result of such changes remain far from certain."

The model indicates that summer sea ice in the Ross Sea could decrease by more than half (56 percent) by 2050 and by more than three-quarters (78 percent) by 2100. At the same time, the summer mixing of shallow and deep waters in the region as a result of other changes is expected to decrease.

While increased open water will benefit diatoms, the preferred food source of many plant-eating predators such as krill, some krill species, such as crystal krill, prefer a habitat with more ice, which they use as a refuge from predators.

In turn, minke whales, Adélie and Emperor penguins and crabeater seals that feed on crystal krill would have less food available if the crystal krill population were reduced.

With less sea-ice cover, however, more humpback whales could enter the Ross Sea in the summer, increasing krill predation. Or Adélies, which prey on silverfish at the ice edge, would have to travel farther from their nests and, as a result, be potentially more vulnerable to leopard seal predation.

While it is difficult to know specifically what changes the Ross Sea ecosystem will face, the model predictions, if accurate, suggest they are likely to be far-reaching.

"Regardless of the exact nature of the alterations," the researchers write, "substantial portions of the food web that depend on ice in their life cycles will be negatively impacted, leading to severe ecological disruptions."