Beached iceberg alters Antarctic marine communities

by Adityarup "Rup" Chakravorty
Monday, July 20, 2015

While the extent of sea ice in the Arctic has fallen substantially over the last several decades, the total area covered by sea ice around Antarctica has grown, with increases in some regions outweighing declines in others. This means that some parts of the Southern Ocean that were once ice-free and exposed to sunlight are being covered over, darkening the waters below and causing shifts in local ecosystems.

Studying the effects of expanding sea ice around Antarctica has been challenging, however, as it is difficult to predict exactly where sea ice will expand and whether it will stick around long enough to make a difference on marine life. But a rare event involving an iceberg bigger than the island of Manhattan created just the environment that Graeme Clark, an ecologist at the University of New South Wales in Australia, and his colleagues needed to study how the sudden advent of long-term sea-ice cover affects marine communities.

Before 2010, Antarctica’s Commonwealth Bay “was ice-free all year round” because of strong katabatic winds blowing down the continent’s steep slopes toward the coast, “which pushed the sea ice offshore as quickly as it formed,” Clark says. But in December 2010, a roughly 100-square-kilometer iceberg became grounded in Commonwealth Bay. This beached behemoth prevented sea ice from moving offshore and created a situation of continuous sea-ice cover. It remains there today.

In December 2013, Clark and his colleagues studied five sites across the bay. They lowered a video camera through holes drilled in the sea ice to see how the marine communities, or benthos, living on the sediment-covered bottom of Commonwealth Bay had been affected by almost three years of sea-ice cover. It wasn’t a pretty picture.

The algal communities — including canopy-forming algae and pink coralline algae — on the floor of the bay were “in severe states of decay,” the researchers wrote in the journal Polar Biology. “Approximately three-quarters of macroalgae were decomposing, while the remainder were bleached or discolored.” Along with the decline of light-dependent algae, Clark and his colleagues also observed animals found more commonly in areas under sea-ice cover, such as polychaete fan worms and ice-fish.

The decaying algae stands in sharp contrast with the pictures of healthy algal forests presented in historical records from expeditions from the 1912 and 1913 Australian Antarctic Expeditions of Sir Douglas Mawson, as well as in photographs of the area taken in 2008. More than a century has passed since Mawson’s explorations, but there’s “no reason to think that the area hasn’t been covered by macroalgae the entire time since then,” Clark says, suggesting the changing health of Commonwealth Bay’s algae is not part of a recurring pattern.

Charles Amsler, a biologist at the University of Alabama at Birmingham, who was not involved in this study, concurs. “I’ve made many hundreds of scuba-dives in macroalgal-dominated communities along the western Antarctic Peninsula,” Amsler says. Although Commonwealth Bay is located in the eastern half of Antarctica, marine conditions are comparable between the two regions. “One does not normally see bleached, discolored or decomposing fleshy macroalgae or bleached coralline macroalgae [in this area]; I’m sure those algae did not look like that before the ice-cover change,” Amsler says.

That photosynthetic algal communities that depend on sunlight are negatively affected by permanent sea-ice cover isn’t surprising, Clark says. “What we don’t know is the rate at which these light-dependent algal-based communities change into dark-adapted invertebrate-dominated ones,” he says, noting that the frigid Antarctic temperatures typically slow processes, such as microbial decay, that would occur much faster in temperate waters. It is also unknown how accessible this area is to populations of dark-adapted invertebrates, such as polychaete fan-worms and brittle stars, so it’s difficult to predict how quickly they might replace the decaying algal forests.

Understanding the rates of these changes has implications for polar ecosystems beyond Commonwealth Bay. “Many polar scientists are concerned about what changes will occur in marine communities, from the plankton to the benthos, as sea-ice cover changes,” Amsler says. As these changes in sea-ice cover will not be uniform in all locations, studies like this one are valuable because it documents a shift under the opposite conditions from the declining sea-ice cover that many polar areas are experiencing, he says.

Clark says he hopes to revisit Commonwealth Bay at regular intervals to better quantify ecological change over time. “At the moment, we only have one time point,” he says, “and the more we can go back and re-sample, the more fine-scale understanding we can get of the rate and nature of the changes.”

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