Researchers explore the impact of sea ice change in Bering Sea

The Bering Sea is the most productive bottom fishery in the world, especially for salmon, halibut and crustaceans. About half of America’s seafood comes from that area, and the fishing industry is the primary driver of employment in and around the Aleutian Islands. The freezing and melting of sea ice in the area has a major impact on primary productivity, the lower end of the marine food chain.

Research Assistant Professor Jaclyn Kinney, Research Professor Wieslaw Maslowski, and Research Assistant Professor Younjoo Lee — all in the Naval Postgraduate School (NPS) Department of Oceanography — have looked at how variability of sea ice in the Bering Sea over the past decades could affect cold primary production of swimming pools and at sea. Their research has been published in PLOS One in April 2022.

In recent years, the Arctic has become increasingly important to US security interests, and in particular to the US Navy. Dual-purpose research in forecasting sea ice and the impact of its fluctuations is critical to navigation and to understanding how systems work.

In addition, less sea ice may also mean more tourism and commercial activity in the area, which could also lead to more patrols and search and rescue. And how sea ice affects food supplies and local jobs can determine the region’s economic and cultural stability. All of these factors are recognized as potential challenges in the Department of the Navy’s Strategic Blueprint for the Arctic, released in 2021, underscoring the importance for the oceanography team’s detailed study.

The NPS research team looked specifically at the very cold water near the seafloor (less than 2ºC) that forms on the shelf each winter, a cold pool. It is formed by the cooling and sinking of surface water. Vertical mixing caused by salt expelled from the water when it freezes to sea ice in the fall and winter, a process called brine shedding, increases the density of the cold pool water. This cold, salty, dense water sinks to the bottom, forming its own marine habitat in summer that is unique to other parts of the Bering Sea.

Melting of sea ice is usually the first cause of water stratification in the spring, but when there is not much sea ice, stratification is mainly caused by the sun heating up the surface water later in the year. Stratification is necessary for primary productivity, in the form of phytoplankton, to thrive.

“If there’s a lot of primary production in the water early on, the zooplankton is still very small and they can’t absorb much of it,” Kinney explains. “So what happens is those phytoplankton cells eventually settle to the bottom and feed the benthic community. That’s good for walruses and gray whales, which feed on the benthic community.”

But this means the pelagic community isn’t getting as much food, she says. When primary production starts later in the season, which happens when there is less sea ice, it becomes a pelagic dominated ecosystem as the zooplankton has a chance to grow in size. These fluctuations from year to year can have major consequences for fish and shellfish populations.

Maintaining these habitat distinctions is important for maintaining the food chain for the region. Some marine species that live in the cold pool are the snow crab and Arctic cod.

Kinney fell in love with the complexity of the Bering Sea in the early 2000s, and it was actually her first area of ​​research.

“It’s really important for food sustainability and for people’s livelihoods,” Kinney says. “I used to study invertebrates that live in the bottom sediment, and I started that in the early 2000s. So I’ve always really loved that region.”

Of course, she closely follows the research that comes from the region. She recently came across a newspaper where she saw the cold pool shrink to the north.

“The reduction in the cold basin means we have a whole new potential for a brand new ecosystem to move in,” Kinney explains. “If we have much warmer water, we’re going to get these southern fish species north, and that will push the Arctic species even further north.”

How big and wide the cold pool is varies drastically from year to year, and the researchers wanted to find out how this variability compares to the variability of the sea ice. They used the Regional Arctic System Model (RASM), developed at NPS, to examine the variability of the cold pool size and distribution to see how its size and shape is affected by sea ice cover. The researchers developed statistical calculations of past sea ice cover conditions in the Bering Sea from 1980 to 2018. RASM can simulate critical physical processes, feedbacks and their impact on the Arctic climate system using several linked models and components, including the atmosphere, ocean, sea and sea. ice, biogeochemistry and land hydrology.

The RASM confirmed a direct correlation between sea ice size and cold pool, showing a smaller cold pool during times of less sea ice cover. Overall, the researchers found that the cold pool in July 2018 was only 31 percent of what it was from 1980 to 2018. The researchers point to a lack of sea ice, caused by strong winds from the south, causing the typical southward expansion. from sea ice to the shelf break.

And as for how this affects the food chain, the researchers found that years of low levels of sea ice were followed by a later diatom bloom and vice versa. These results follow the Oscillating Control Hypothesis, originally developed in 2002, which states that early ice withdrawal will lead to late flowering, while late ice withdrawal will lead to early flowering.

Diatoms are a common type of phytoplankton that form the basis of the food chain. Diatom levels can be measured by seeing how much chlorophyll-a is found in an area, both via satellite and in models. A comparison of the chlorophyll-a trends in the northern Bering Sea between satellite data and RASM showed similar results, forming the basis of another study co-authored by Kinney, published in the journal Oceanography in May 2022. RASM results also provided insight into the mechanism responsible for the changes by showing the variability in nitrate concentration (a variable not measured by satellite).

The researchers were pleased to see that RASM’s results reflected real-life observations. But the cold pool retreat they observed in 2018 remained an issue in 2019 and 2020, which also saw unusually high temperatures resulting in less sea ice. Then 2021 saw a large population of snow crabs collapse, likely due to a reduction in their preferred habitat for cold pools. Without the cold pool, snow crab predators can more easily eat young crabs. This population is collapsing bankrupt communities that rely on snow crabs for their livelihood. The Central Bering Sea Fisherman’s Association expects sales to fall by about 65% as a result of necessary reductions in crab quotas.

“As scientists, we want to know if this reduction in the cold pool is the new norm? Are we going to see the sea ice come back? And then how will the population recover to the south once we see the sea ice return to normal.” ?” said Kinney.

She points out that this isn’t the first time the area has seen a reduced cold pool, the last being in 2001. It recovered, peaking in sea ice prevalence in 2012. But the extent of the sea ice has since declined. The sea ice of the Bering Sea is difficult to predict because it starts all over again every year, giving in to the vagaries of seasonal and inter-annual variability in addition to the larger climatic trends.

“There is no straightforward linear relationship for ice retreat,” explains Maslowski. But the team is encouraged by how well RASM has been able to predict sea ice trends to date, and sees it as a powerful tool to help the navy look into the future of the Bering Sea.

Provided by Naval Postgraduate School