The disappearance of sea ice could disrupt the Arctic food web

Peggy's data was a bit of a shock.

Peggy is a floating buoy attached to sensors monitoring the temperature, salinity and other properties of the Bering Sea in the Arctic. These sensors are over 70 meters deep and are anchored to the ocean floor in western Alaska.

Here, the comings and goings of floating sea ice follows a seasonal pattern. Peggy's data normally show this pattern. But in the winter of 2017-2018, they did not do it. The pack ice never appeared!

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

In summer, the Bering Sea is largely free of ice. In winter, ice forms in its northern parts. The ice also migrates southward through the strait from the Chukchi. Scientists consider the waters to be frozen when at least 20% of their surface is covered with ice.

In most years, pack ice appears in the Bering Sea in November. When this ice forms, it causes the formation of a large mass of cold and salty water near the seabed. In the spring, algae bloom under the ice and around it. At the beginning of summer, the pack ice begins to melt. But the cold pool near the bottom, with an average temperature of only below freezing, persists during the summer.

The deep, cold pool is at the heart of the Bering Sea ecosystem. It is here that Arctic cod has taken refuge. These fish are hiding from predators such as Pacific cod and pollock (which do not like cold weather as much). Arctic cod also grows on large shrimp copepods (KOH-peh-podz) and then breeds. In turn, these fish keep polar bears and seals well fed.

Peggy's data, along with other sensor sets, revealed that the cold pool near the seabed was missing. This and the absent pack ice have alarmed the scientists of the ocean. Researchers met in Washington, DC at the annual meeting of the American Geophysical Union in December 2018. Many shared new data, exchanged stories, and reflected on the meaning of these changes.

Have these results been a stroke of luck? "We do not have enough data yet," says Jacqueline Grebmeier. She works at the Center for Environmental Science at the University of Maryland in Solomons. But Grebmeier, who has been studying seabed life in the Arctic for over 30 years, has a strong sense that ice-free winter is not an isolated event: "I think it's the beginning of change" , she says.

If last year's events herald a new normality for the Bering Sea (and that the very low sea ice cover in March this year suggests), a cascade of changes announces for its complex ecosystem – algae at the bottom of the water. the food web for humans at the top.

Signs of warming

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

Phyllis Stabeno is a physical oceanographer with the National Oceanic and Atmospheric Administration (NOAA). She works at the Pacific Marine Environmental Laboratory in Seattle, Washington. The air temperatures above the waves were typical of November, she explained at the December meeting. But a sustained wind blew from the south. This wind prevented the ice from drifting from the Chukchi Sea as it would normally do.

The wind calmed down in December and January. At this point, the air temperature was warmer than normal. The Chukchi Sea, instead of being mainly covered with thick ice, still had wide bands of open water. This means that less ice was available to migrate south across the Bering Strait.

Mooring M8, Peggy's cousin, is about 800 km northwest of Peggy. He has read the water temperatures just above the sea floor since 2008. And this water was over 3 degrees Celsius (5.4 degrees Fahrenheit) warmer than normal. In fact, M8 had never recorded so little ice in winter here.

In February 2018, strong southerly winds erupted. The strange wind direction lasted until March. Scientists believe that these winds kept the Chukchi Sea warm. They pushed warmer waters north of the Bering Sea.

This hot water also prevented the sea ice from forming. The ice that formed in the Chukchi and Bering Seas was thin and easily pushed north by the winds.

By the summer of 2018, Peggy was already recording the highest temperatures in the water. And the waters close to the seabed have never fallen below zero.

Feeding time

Sea ice is an anchor of the Bering ecosystem. This ice helps to determine when and where food becomes available for creatures living in the water or on the seabed.

When the migrating sea ice moves south, it melts. This meltwater is less salty and less dense than the surrounding water. As a result, the water forms layers. The cooler water full of nutrients remains at the top.

The melt water also contributes to the proliferation of algae in the spring in the southern Bering Sea. These blooms in turn feed on copepods and other small floating creatures. And when these algae die and sink on the sea floor, they are an important food source for the animals that live there.

But the absence of sea ice meant that the water did not form layers until late in the spring. So, the algae bloomed later, too. Not everyone in the Bering Sea can adapt quickly to these delays in the food chain.

"Timing is important," says Grebmeier. "It's a matter of speed [the animals] can adapt. "

Due to winds and heat, a small pack ice has formed in the Bering Sea. It meant no deep, cold pool near the seabed.

A changing Arctic

The dramatic absence of sea ice last winter was surprising. But Arctic waters have experienced a warming trend that has lasted for several decades. Since 1981, the southern Chukchi Sea has frozen about half a day later every year. Stabeno and his colleagues shared the results last November Search on the high seas.

In the northern Bering Sea, M8 data show significant changes over the last four years. From 1981 to 2014, the Bering froze on average at the end of December. But since 2014, it has not frozen before January or February. In 2018, the Bering Sea has not frozen at all.

This warming trend has affected the inhabitants of the seabed. Species such as the bivalves that used to line the seabed around St. Lawrence Island in the Bering Sea have shifted northward, Grebmeier said.

In 2010, Grebmeier helped set up the distributed biological observatory. It is a global program to monitor the long-term changes of ecosystems in the Arctic. Researchers visit designated "hot spots" year after year. These include the Bering and Chukotka seas. One of these hotspots is just south of St. Lawrence Island, where a very cold pool normally forms.

Here, bivalves from the ocean floor provided a nutritious and fatty meal for walruses and seals. And the eiders with glasses – a kind of sea duck – plunged for the mollusks. The birds used the pack ice as a safe and stable launch pad.

But over time, these bivalve areas have given way to less consistent worms, says Grebmeier. The change of food and the loss of sea ice are a blow to the birds. Today, she says, they are at risk of disappearing. "It takes less energy to sit on the ice and feed on the underlying prey than to swim," she says. "So, the impact on these bodies is dramatic."

Other creatures from the Arctic have also been affected. The researchers measured an increase in the number of small copepods consumed by the newly hatched pollock. But larger, more nourishing and nutritious copepods are declining. This is bad news for young fish that need larger copepods to survive during the winter.

These changes in prey have had cascading effects on the food web. For example, changes in location and types of fish have sentenced thousands of seabirds. Last summer, it was the third year in a row that seabirds were disappearing, notes Calvin Mordy. He is a biology oceanographer at NOAA's Pacific Marine Environmental Laboratory in Seattle. Particularly noticeable: seabirds showed signs of starvation and not disease, he reported at the AGU meeting in December.

Feel the heat

Past recordings suggest that the duo of wind and heat that led to the Bering Sea record icepack last winter was unusual. The question, says Stabeno, is how these conditions will be typical in the future.

The Arctic is warming twice as fast as the rest of the Earth. It has an average temperature of about 3 ° F (1.7 ° C) above the long-term average from 1981 to 2000. This is the 2018 Arctic Bulletin. This is NOAA's annual report. about the state of the Arctic.

In fact, the five years since 2014 are the five hottest ever measured by scientists, said Emily Osborne. She is the editor-in-chief of the 2018 newsletter and a climatologist. Osborne works for the NOAA Arctic Research Program in Silver Spring, Maryland.

Last year was the second hottest year ever, only in 2016, she said. The increase in temperature has had the visible effect of a sharp decrease in sea ice cover in summer. The last 12 years are the 12 lowest recorded.

The rising temperatures of the air and the oceans are not the only signs of change in the Arctic Ocean. Runoff from local rivers also has an impact. The Arctic Ocean is "the most land-dominated ocean in the world," says Karen Frey. She is a polar scientist at Clark University in Worcester, Massachusetts. "It's a dump for everything" of algae fluvial sediments, she adds. "All roads lead to the Arctic."

Arctic overload

By 2018, the volume of water discharged by the eight largest rivers flowing into the Arctic Ocean was about 20% higher than that of the 1980s. The rise was due to a number of factors related to global warming. They included the decline in permafrost and increased precipitation in the High Arctic.

All this material spilled into the ocean brought more nutrients. That, the warmer waters and more sun shining on the surface all led to a greater proliferation of algae, says Frey. Some of these algae release poisons, but scientists do not yet know how toxic they are. The algal blooms resemble the deadly red tides that have recently occurred along the Florida coast. Some poisons of algae kill the fish and can cause brain damage in humans.

Cases of paralytic shellfish poisoning have increased seven-fold in Alaska over the last 40 years. The state now has one of the highest rates of shellfish poisoning in the world. And the harmful algal bloom spurt may be partly to blame, says Frey.

The frequency and number of sites affected by these nuisance algae have skyrocketed in recent years. This sparked the first special section on harmful algal blooms in the 2018 Arctic Monitoring Bulletin.

The culprits may be algae "present in many areas at low concentrations that would not be harmful. But as soon as you start warming up the seawater … and removing the sea ice, they adapt, "says Frey. "We are just beginning to measure harmful algal blooms and understand how these Arctic species react to light. This is a new question that still needs to be answered.

A vision of the future

Frey went to the Bering Sea last July as part of the distributed biological observatory program. It measured how much the thinner and thinner sea ice together changed the amount of light reaching the water's surface. More light means more blooms of algae. She finds that even the thinnest ice could have a significant effect on the amount of light transmitted.

Thin ice is often covered with melt ponds. These areas "are basically skylights" for the waters below, she says. Melting ponds on the surface of the pack ice can increase the amount of light reaching the water, from about 10% to 60 or even 70%. Frey reported on his team's findings at the AGU meeting in December.

The increase in flowers was spectacular in 2018, notes the Arctic report card. Less sea ice and more melt ponds meant more light was reaching Arctic waters earlier in the year. This caused flowers in the northern Bering Sea (although the south bloomed later).

The waters of the north have blossomed since the month of March. They normally see flowers in May. The amount of biomass of algae in March was about 275% higher than the average biomass of March for the years 2003 to 2017.

Scientists are still studying the role that harmful blooms could play in seabird mortality. Proliferation-related poisons have been associated with other massive deaths in wildlife in recent years. Animals, including walruses, seals and whales, probably eat contaminated fish and shellfish, just as people do, as the Arctic Report Card notes.

"Historically, we should not see another year as [2018]Stabeno said in December. In fact, "what we saw this year was to happen in 2050," she added. So, last winter, she said, "gives us a vision of the future". And this low ice-covered future seems to be coming faster than anyone suspected.