As the Antarctic Ice Pack Recedes, a Fragile Ecosystem Hangs in the Balance

Feature Open access, freely available online H arrowing tales of starvation and endurance epitomize Antarctica's " heroic age, " when men equipped with little more than fortitude struggled against a landscape seemingly designed to thwart their intentions. A menacing sea ice fi gures prominently in these improbable survival stories. Daunting to early-20th-century explorers—trapping (and ultimately crushing) Ernest Shackleton's Endurance and derailing Robert Scott's 1901 Discovery expedition—the seasonal pack ice is the lifeblood of Antarctica's marine ecosystem. But as winter temperatures continue to climb in the Antarctic, the once-forbidding winter sea ice is starting to deteriorate. The ice pack is forming later and retreating earlier—and it's having a serious impact on the abundance of krill, the backbone of the Antarctic food chain. " Sea ice is the engine that drives Antarctic ecosystems, " says William Fraser, a principal investigator for the Palmer Long Term Ecological Research program (LTER) and president of Polar Oceans Research Group in Montana. " Many of the key species that govern ecosystem dynamics in the Antarctic have life histories that depend on the availability of winter sea ice. The most important species in most sectors of Antarctica is krill. " A major food source for Antarctic fi sh, penguins, pelagic seabirds, seals, and whales, krill (Euphausia superba) look like shrimp, but weigh just a gram as adults and measure about six centimeters long (Figure 1). Norwegian for " whale food, " krill aggregate in super-swarms that can reach a density of 30,000 individuals per square meter, attracting whales, which can eat three tons of krill in a single feeding, and fi sheries, which net on average 100,000 metric tons per year. The waters off the Antarctic Peninsula favor high krill concentrations. " Once you get into extreme environments such as the Southern Ocean, diversity will decrease but the number of individuals will increase because the production can be very high, " says Scott Gallager, a marine biologist at Woods Hole Oceanographic Institution (Woods Hole, Massachusetts, United States). Production is particularly high along the sea ice edge, he says, because the ice is thinner, which allows more sunlight to penetrate, and because ocean mixing processes along the continental shelf cause an upwelling of nutrient-rich deep water. Increased nutrients support increased primary production along the ice edge. " If ice forms too late, " says Gallager, " you don't get this higher production, which impacts zooplankton populations like the …