Scientists have pinpointed how crucial the microscopic algae that grow under and around sea ice are to the survival of polar bears, underscoring the ways in which climate change is upending the region's food web.
As sea ice melts in the Arctic, multiple studies have documented the ice's importance as a hunting platform for polar bears to snag seals and other prey, without which the top predators will starve. What's been harder to measure is the ice's vital role as a food factory, according to marine ecologist Thomas Brown of the Scottish Association for Marine Science.
In a study published in the journal PLoS One on January 23rd, Brown's team analyzed the tissues of 55 polar bears hunted by Inuits in Hudson Bay and Baffin Island in Canada and reported that, on average, 86 percent of those bears' diet comprised species such as seals and walruses that ultimately depended on ice algae, as shown by a chemical marker called IP25 in the bears' tissues.
Ice algae "are the basis of the food chain," said Lars Chresten Lund-Hansen, a sea ice ecologist at Denmark's Aarhus University. "If there are no ice algae, then there are no zooplankton, no fish, no seals, no polar bears."
Brown says his research shows how the ice itself is crucial to producing the very food that the ecosystem requires to sustain large mammals such as bears. "This polar bear study represents the top of the food chain," he said. "It's the first time that I've demonstrated that we can not only observe and measure IP25 in the very top trophic level, we can also use it to quantify the proportion of energy received from sea ice."
But as sea ice melts and disappears—its maximum extent hit record lows in 2017—ice algae's role as a key marine food source is also in flux. Since NASA satellites first began tracking Arctic sea ice in 1979, summer coverage has declined by almost 50 percent, and the timing of ice algae blooms has been observed starting earlier, according to previous studies.
Changes in the timing of ice algae blooms may spell trouble not just for bears but for many animals, such as seabirds, which have evolved finely tuned breeding seasons synchronized with the availability of plentiful food. Ice algae are far richer in essential omega-3 fatty acids than pelagic phytoplankton, which flourish in open waters later in summer.
Lund-Hansen's own recent research has documented another way in which changing Arctic climate patterns could affect the timing of spring ice algae blooms in complex and hard-to-predict ways. At his study site in northern Greenland, warmer air temperatures in spring also heat the snow that sits on top of sea ice, changing the snow's optical properties, his research has found. As the grains become larger, sunlight more easily penetrates through to the ice below—boosting the photosynthetic growth of ice algae that usually kicks off the summer explosion of marine life, he said.
His study, published this week in the Journal of Geophysical Research: Oceans, documented ice algae on the underside of the sea ice thriving in minimal sunlight in April and photosynthesizing with more than six feet of ice and snow above. "We assumed that the spring bloom would start much later in May, June, or even July when there's no snow on the ice," Lund-Hansen said. Even though the snow depth hadn't decreased, he said warmer temperatures may change the snow's optical properties just enough to allow the minimum amount of light needed—just 1/5,000th of the daylight that bathes northeast Greenland in April—to penetrate and allow algae to grow earlier in the spring.
That could potentially reduce the amount of food available to fish and, therefore, higher-level predators such as seabirds, when they breed later in the season. It's an effect that could be counteracted by deeper snows that block the penetration of light as the Arctic also becomes not only warmer but wetter, Lund-Hansen said. Predicting what will happen quickly becomes complex, especially with sea ice simultaneously disappearing.
"This study provides crucial insight into how ecosystems function in ice-covered seas," said Jorgen Berge of UiT the Arctic University of Norway, who specializes in low-light marine biology during the polar night (but was not involved in the research). "It's an important issue, with massive shrinking of the ice cover, and increased rain and snow."
This year, as the sea ice extent continues to decline and scientists race to understand climate change across the Arctic, Brown and his Canadian colleagues will be tracking how ice algae energy is transferred throughout an entire ecosystem's food chain in Foxe Basin, north of Hudson Bay, by measuring the same chemical marker in sediments, plankton, fish, and various marine mammals.
"I try to get across to my students that, in science, we're usually talking about changes occurring over long periods of time, or that this will happen by the end of this century—like I'll have retired by then, if I'm still alive," Brown said. "But in the Arctic, we are the ones who are going to be observing, monitoring, quantifying, and realizing what this change really means. It's happening that quickly."