Icebergs take a bite out of Antarctic biodiversity

The icy waters off the Antarctic Peninsula conceal one of the most fascinating ecosystems on the planet: a crystalline seascape populated by foot-long sea spiders, centuries-old glass sponges and long, bony brittle stars.

But that may not be the case much longer, scientists say.

According to research published this week in the journal Current Biology, declining sea ice driven by rapid regional warming has created a perilous and ever-changing environment where rogue icebergs plow the seafloor and decimate bottom-dwelling organisms. Only a single species of hearty invertebrates has managed to hang on.

“Anyone who knows the story of the Titanic knows that disturbance from icebergs is not small,” said David Barnes, a marine biologist at the British Antarctic Survey and lead author of the study. “When an iceberg hits the seabed, it makes a real mess.”

Two decades of diving surveys over a 1,000-square-foot area of seabed near Britain’s Rothera Research Station revealed the surprising ascendance of one otherwise ordinary variety of bryozoan. Starting abruptly in 2007, the crusty white organism, called Fenstrulina rugula, began out-competing other creatures for real estate and resources.

Other species remained in small numbers, but they no longer played important roles in how the submarine ecosystem functioned.

What changed?

Barnes and his colleagues found the answer in daily sea ice observations stretching back to the 1980s and a 14-year history of how often icebergs smashed into the seafloor at the study site. Ice impacts were gauged by the destruction of concrete markers installed on the seabed for just this purpose.

The scientists noticed that years with less ice extent and fewer frozen days allowed more icebergs to roam free, and corresponded to more seafloor collisions. Bad ice years have increased steadily since observations began — “it used to be 1 in 4 was a bad year, now 1 in 5 is a good year,” Barnes said.

But 2007 and 2008 stood out as two of the worst. Barnes thinks iceberg scouring during those back-to-back winters may have wiped out everything except F. rugula, which reproduces quickly and recolonized the seafloor.

“It’s the only team in town because it can withstand disturbance,” Barnes said.

Claudio Richter, a biologist at the Alfred Wegener Institute in Bremerhaven, Germany, who was not involved in the study, agreed with the authors’ conclusion that the ecosystem may have crossed a tipping point.

“The situation is analogous to the notorious disturbance by humans in polluted and overfished areas, where a formerly well-balanced and diverse system is degraded to one dominated by a few seaweeds or jellyfish,” Richter said.

Reducing biodiversity makes ecosystems more vulnerable to collapse, and dramatic changes to the bottom-dwelling community may ripple up the food web.

“Most of the Antarctic fish species are associated with the seafloor, and some of the seals feed on bottom fish,” Richter said. “So increased scouring will take its toll on these species.”

The biggest question is whether Barnes’ observations hold true in other places around the Antarctic. Because of the intense nature of the work, Barnes’ study covered only a small area. Conditions just a few miles up the coast could differ dramatically, where glaciers, currents and winds strongly shape the environment.

“It might be that this isn’t a big deal; it just happens to be where we found it,” Barnes said. However, it’s equally likely that “it could be a bit of a canary in a coal mine.” The Antarctic Peninsula is one of fastest-warming places on the planet, and the organisms that call it home are extremely sensitive to change. Barnes said what’s happening could be a harbinger of changes to come elsewhere on the globe, albeit mediated by some mechanism other than icebergs.

Regardless, conditions will probably continue to shift as temperatures rise and the ice sheet sheds its frozen mass into the sea.

“Changes in ice will increase scour in some areas, but will also slow it down in other places,” said Jim Barry, a marine biologist at the Monterey Bay Aquarium and Research Institute. It will be hard to predict what’s in store for marine ecosystems, he said, but one thing is for sure: “This isn’t the end of the story; this is the beginning.”

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