Disappearing sea ice could disrupt Arctic’s food web

a photo of a a camera on a tripod standing on a melt pond, there is ice stretching to the distant horizon
Cameras set up in the Arctic’s Chukchi and Bering seas (one shown) record the amount of light that reaches water through the ice. More light means more algal blooms.

Peggy’s data were a bit of a shock.

Peggy is a floating buoy attached to sensors that monitor the temperature, saltiness and other properties of the Arctic’s Bering Sea. These sensors reach down more than 70 meters (about 230 feet) below the water and are anchored to the seafloor west of Alaska.

a photo of a yellow ocean mooring, Mooring M2 (nicknamed Peggy), floating on the ocean's surface
A package of sensors, nicknamed Peggy, has monitored the waters in the southern Bering Sea since 1995.

Here, the coming and going of floating sea ice follows a seasonal pattern. Peggy’s data normally show that pattern. But in the winter of 2017–2018, they didn’t. The sea ice never appeared!

At their closest point, Alaska and Russia are separated by the Bering Strait. This stretch of water is 82 kilometers (51 miles) wide. To the north lies the Chukchi Sea, which is on the edge of the Arctic Ocean. Below the strait is the Bering Sea, which extends south to Alaska’s Aleutian Islands.

In summer, the Bering Sea is largely ice-free. In winter, ice forms in its northern reaches. Ice also migrates southward through the strait from the Chukchi. Scientists consider the waters frozen over when at least 20 percent of their surface is ice-covered.

Most years, sea ice shows up in the Bering Sea by November. As that ice forms, it causes a large mass of cold, salty water to pool near the seafloor. In the spring, algae bloom under the ice and around it. By early summer, the sea ice begins to melt away. But the cold pool near the seafloor, with an average temperature of just below freezing, lingers through the summer.

The deep cold pool is central to the Bering Sea’s ecosystem. It is where Arctic cod take refuge. These fish hide from predators such as Pacific cod and pollock (which don’t like the cold as much.) The Arctic cod also get fat on large, shrimplike copepods (KOH-peh-podz), then spawn. In turn, these fish keep polar bears and seals well-fed.

Peggy’s data, along with that of other packages of sensors, revealed that the cold, near-seafloor pool was missing. That and the absent sea ice alarmed ocean scientists. Researchers gathered in Washington, D.C., at the American Geophysical Union’s annual meeting in December 2018. Many shared new data, traded stories and pondered what the changes may mean.

a map of the Bering Strait showing where Peggy is moored
The Bering Strait is a narrow region of water between Russia and Alaska that separates the Chukchi Sea from the Bering Sea. The black dot on the map shows where Peggy is stationed to collect data on water temperature and saltiness.

Were these findings a fluke? “We don’t yet have enough data,” says Jacqueline Grebmeier. She works at the University of Maryland’s Center for Environmental Science in Solomons. But Grebmeier, who has studied seafloor life in the Arctic for more than 30 years, has a gut feeling that the ice-free winter is not a one-off event: “I think it’s the beginning of change,” she says.

If last year’s events signal a new normal for the Bering Sea (and the very low sea ice cover as of March this year signals they might), then a cascade of changes are in store for its complex ecosystem — from the algae at the bottom of the food web to humans at the top.

Warming signs

There were early signs that conditions in the winter of 2017 to 2018 were going to be different. By November 2017, the sea ice was already late.

Phyllis Stabeno is a physical oceanographer with the National Oceanic and Atmospheric Administration (NOAA). She works at its Pacific Marine Environmental Laboratory in Seattle, Wash. Air temperatures above the waves were typical for November, she reported at the December meeting. But a sustained wind was blowing from the south. That wind prevented ice from drifting down from the Chukchi Sea as it normally would.

The wind died down by December and January. By this point, air temperatures were warmer than normal. The Chukchi Sea, instead of being mostly covered by thick ice, still had large swaths of open water. That meant less ice was available to migrate south through the Bering Strait.

Mooring M8, Peggy’s cousin, is about 800 kilometers (500 miles) northwest of Peggy. It has been reading water temps just above the seafloor since 2008. And that water was more than 3 degrees Celsius (5.4 degrees Fahrenheit) warmer than normal. In fact, M8 had never recorded so little winter ice here.

By February 2018, strong southerly winds picked up. The strange wind direction lasted through March. Scientists think those winds kept the Chukchi Sea warm. They pushed warmer waters north from the Bering Sea.

That warm water also stopped sea ice from forming. The ice that did form in the Chukchi and Bering seas was thin and easily pushed north by the winds.

By the summer of 2018, Peggy was recording the highest water temperatures yet. And near-seafloor water never dropped below freezing.

Feeding time

Sea ice is an anchoring part of the Bering ecosystem. That ice helps determine when and where food becomes available to creatures living in the water or on the seafloor.

As the migrating sea ice moves south, it melts. That meltwater is less salty and less dense than the surrounding water. As a result, the water forms layers. The fresher water full of nutrients stays on top.

The meltwater also helps give rise to springtime blooms of algae in the southern Bering Sea. Those blooms in turn feed copepods and other small floating creatures. And when these algae die and sink to the seafloor, they provide an important food source for animals living there.

But the absence…

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