Algae embedded in sea ice drive the Arctic food web

For much of the year, a layer of sea ice covers the Arctic Ocean. It builds up in winter. Come summer, the sea ice thins or shrinks in overall size. But 2016 has been a really warm year, which has led to far less sea ice than normal. So the Arctic is on track this year to match or perhaps even exceed the record low summertime sea ice seen in September 2012, scientists say. A new study shows why this could be a big problem for life in and around the Arctic.

Disappearing sea ice is a symptom of a warming planet. It also poses risks to the life associated with that ice. That includes the algae that live in brine-filled channels within this ice.

“These algae are adapted to grow under very low light conditions,” says Doreen Kohlbach. She is a marine biologist at the Alfred Wegener Institute in Bremerhaven, Germany. These algae are some of the species that form the base of the Arctic food web. And they are an important source of food even for species that don’t live under the ice, her team now reports.

Climate change could therefore be a big problem for the Arctic ecosystem. That’s because a loss of ice there will affect more than just the organisms that have a close connection to the ice. It also will affect life in the open ocean, Kohlbach says. This is a region known as the pelagic (Peh-LAAJ-ik) zone.

Zooplankton are tiny creatures that eat algae and other simple ocean species. For its new study, Kohlbach’s team looked at various species of zooplankton during late summer. Some of these tiny critters live near the sea ice. Others are pelagic. The researchers determined how much of their diet had come from sea-ice algae. To do that, they looked for markers of the algae. These were building blocks of the algae known as fatty acids. If they showed up in the zooplankton, it was sign these small animals had dined on the algae.

Animals that lived beneath the sea ice got a lot of their energy — sometimes described in terms of carbon (an ingredient in everything they eat) — from ice algae. Indeed, 60 to 90 percent of their meals had consisted of the algae,. But even pelagic species “showed a significant dependency” on ice algae, notes Kolbach. Those animals got 20 to 50 percent of their dietary energy from algae that had been embedded in sea ice.

The researchers described their findings July 8 in Limnology and Oceanography

Scientists have yet to look for markers of algae higher up in the food web — in bigger animals that may have dined on the zooplankton. So for now, the scientists can’t yet predict how the loss of sea ice algae might affect larger species, such as fish, seals or polar bears. But they, too, will likely be affected, Kohlbach says. If falling summer sea ice affects life at the bottom of the food web, she says, eventually it also will affect those higher up.

There’s also a chance that species could switch to dining on other species if their preferred sea-ice algae disappear. But the concern is that some species might not be able to make that switch, Kohlbach says. “To determine the consequences on a species-level, we need to do more research.”