Bering Sea Ice

Studying the Bering Sea from the Bottom Up

Residents in Nome and farther north in Alaska have witnessed climate change with their own eyes. Thinning ice sheets, fewer gray whales in the region and coastal villages sinking into the sea are telltale signs that the world is warming. But what about the communities of animals below the surface?

Scientists aboard the US Coast Guard Icebreaker Healy have just completed a month-long study of the Bering Sea. Their mission was to gather as much data as possible about the plants and animals in the water. “The main project was looking at what happens to the animals on the bottom as we see the ice recede in the springtime,” said Lee Cooper, one of the chief scientists leading the research cruise entitled, “Climate-driven Changes in Impacts of Benthic Predators in the Northern Bering Sea.” In this case, the benthic predators are walruses, sea ducks, gray whales, seals and crabs that feast on the food living on the seafloor. By monitoring these meals in the mud, researchers hope to better predict how climate change will affect the movement of predatory animals into and out of the Bering Sea. No one has previously investigated this aspect of climate change.

“One of things we’ve been seeing in the last few years is that the ice is leaving about three weeks earlier than before,” said Jackie Grebmeier, the other chief scientist aboard the Healy. “That has an impact because the animals are adapted to a certain timing.” The Bering Sea is one of the most, if not the most, productive oceans on Earth, but Grebmeier and the other scientists onboard have all said they see signs of the system slowing down.

Grebmeier, a researcher at the University of Tennessee, studies the flow of nutrients from the surface to the seafloor. Carbon, for example, is the main component of any diet. It starts at the surface in the form of algae. The algae bloom when the ice melts in the spring. If the timing is off due to an early ice melt, the plants and animals in the water must also adjust so they can get food. Grebmeier’s team is trying to see if who can adjust and who can’t.

Pieces of the Puzzle
Understanding changes in a whole ecosystem, such as the Bering Sea, involves investigating all the angles. Cooper and Grebmeier were joined by a small army of researchers from several universities. Each group was studying a different – but related -- piece of the climate change puzzle.

Beth Caissie and Kinuyo Kanamaru from the University of Massachusetts were collecting sediment samples to understand the climate as far back as 20,000 years. They sent down a device that took cylinder-shaped cores of the mud at the bottom.

Grebmeier also used the sediment samples. She set them in a dark, cold room that mimicked conditions on the seafloor and measured how quickly the tiny animals in the mud used up oxygen and gave off carbon dioxide to see how quickly they metabolize food.

Water samples collected at different depths were filtered through several different machines. Marjorie Brooks had one device that captured microscopic algae on thin paper discs. In the coming months, Brooks will analyze the algae in her lab at the University of Wyoming. She is looking for chemical “biomarkers” that she can trace all the way through the food web. “This will link the food web chemically,” said Brooks. “It will help us determine who is eating the algae and which animals are eaten by which predators.”

Brooks is working with Jim Lovvorn, also from the University of Wyoming. Lovvorn has been studying spectacled eiders for many years. These sea ducks dive to the bottom of the Bering Sea, some 50 to 80 meters down, to feed on the clams and other bivalves along the seafloor. But no one knows where these birds go just before they head for their springtime breeding sites farther north. “This is a critical time when females have to maintain and even gain weight before they arrive at breeding areas,” said Lovvorn. He took to the skies in on-board helicopter, but had no luck in locating the spectacled eiders. Their whereabouts remain a mystery.

Less of a mystery, however, is the food available to the diving birds, walruses, whales and seals in the region. Lovvorn’s team did the first trawls in the Bering Sea to build a database of all of the species living at the bottom. Brooks will also trace her biomarkers through these samples to develop a food web model.

Of course, the food web has to begin somewhere. In this case, algae form the foundation for life in the Bering Sea. Karen Frey used the Healy’s satellite system to get snapshots of the sea ice from space. “I’m interested in the timing of when sea ice melts and when we get blooms of plants, or algae, in the water,” said Frey. “How long after the sea ice melts plants start to grow in the ocean?” Frey tracks when the sea ice melts, but she also follows the flow of algae down to the bottom by tracking the amount of chlorophyll (the chemical plants use to convert sunlight into energy) at different depths. “And when it falls to the bottom of the ocean, there are lots of critters down there that love to eat these algae species, so it is a very important time of the year for biological productivity,” she said.

Tying it All Together
So what does all of this research tell us about the Bering Sea? “I think the importance of the results we have found is that the biology and the physics are tied together,” said Grebmeier. “We’re seeing a decline of the prey source and the warming of the temperatures that will have large impacts that are beyond the one study area that we’ve been in.” Those shifts are already pushing whale and walrus populations north. Subsistence hunters have felt this blow. Residents of Gambell failed to catch a whale this year.

The Healy research cruise, supported by the National Science Foundation, ended June 5, 2006, but Cooper said they plan to return next year to continue their work. “It is easy to show that the water has warmed up a few degrees and the ice has pulled back earlier,” he said. “But we’ve shown that under all that ice that things are changing on the bottom, too. The other point is that this is a very special system. As the climate changes, it is possible that the system we have now, with walrus and whales and seals that are associated with ice will be gone and we won’t have a special a place up here anymore.”

For Bering Sea Ice, Timing is Everything

The engines roared as the Healy’s metal hull plowed into the thick white ice surrounding Big and Little Diomede islands. The ship collided with ice for several meters, swallowing ice chunks under the bow and then retreating to the open water it just created to get back to ramming speed. It seemed like we had made good headway through the ice pack. Yet, a satellite image revealed that the Healy had barely made a dent in the icy blanket around the islands. That made me wonder what else satellites are telling us about the Bering Sea.

“We can actually get real-time satellite imagery of where we are in the Bering Sea,” said Karen Frey, a scientist with the University of Tennessee. “The ship has an antenna mounted right above us, so it downloads data directly from the satellites orbiting the Earth.” Frey is using satellite imagery to track the sea ice in the Bering Sea. She is wants to know when the sea ice melts because that triggers a flurry of activity for the plants and animals in the water. If the melt happens too early, everything living in the Bering Sea has to adjust or move north to find more ice.

“I’m interested in the timing of when sea ice melts and when we get blooms of plants in the water. How long after the sea ice melts do plants start to grow in the ocean?” Frey explained. The plants she mentioned are not your regular garden variety, unless you are in the Bering Sea. “The general term for them is phytoplankton; it’s basically one-celled algae, like diatoms.” she explained.

To study the amount and location of this plant life in the water, Frey lowers a device that measures sunlight, temperature and chlorophyll to the seafloor some 30 to 50 meters below.

This time series image show chlorophyll in the Bering Sea during spring bloom in 1994. Black indicates no chlorophyll and red indicates lots of chlorophyll. The algae use chlorophyll to convert sunlight into energy during photosynthesis, so a lot of chlorophyll in the water indicates a lot of plant activity. “We’re really only seeing down to about 20 meters, so light’s not penetrating at certain wavelengths much deeper than that,” said Frey. “The phytoplankton in the ocean grow just like any other plant would; they need nutrients and sunlight. If light’s not penetrating deeper, then the phytoplankton aren’t able to photosynthesize at deeper depths because there’s no sunlight available at those depths. But we do see them at those depths. Basically that’s telling us that phytoplankton are forming at the surface and sinking down through of the water column.”

Sinking phytoplankton means a smorgasbord of seafood is on the way for the animals that live on the seafloor. Clams, sea cucumbers, shrimp-like amphipods and more gobble up the plants that drop to the bottom. In turn, walruses, seals and sea ducks dive to the seafloor to feast on those plant-eaters in the mud. Finally, local hunters from the islands rely on the walruses, seals and birds for food.

The trouble is, Frey and her colleagues aboard the Healy are seeing the springtime blooms earlier as the sea ice is starting to melt sooner than expected. “We’ve seen that spring is coming earlier and earlier every year. So not only are we seeing warming temperatures as a whole, but the timing of all of the things happening throughout the seasonal cycle is getting earlier,” she said. “That’s a problem for the animals that live here. We’ve seen northward movement of a lot of species.” The animals move north where the temperatures are colder, and the ice melts when they expect it to melt.” The image below shows a thick ice cover (red) north of the Bering Strait and a band of thin ice (blue) outlining the edges of Russia and Alaska. The large black portion in the lower left is open ocean.

“One of the reasons that Arctic stays cool is that we have all of these really bright reflective surfaces, really white colored surfaces like ice and snow,” explained Frey. “Once you start melting this ice and snow, you’re replacing really bright surfaces with very dark surfaces, like the ocean. The ocean is actually a very good absorber of sunlight. And when you’re a good absorber of sunlight, you actually warm the air temperatures even more.” Scientists call this trend the positive ice-albedo feedback loop. Once it starts, the whole systems “snowballs” into warmer and warmer temperatures.

“The Arctic specifically as a region is so much more susceptible to warming than anywhere else because of the ice-albedo feedback loops,” said Frey. Researchers say the five most extreme seasons have taken place in the last decade. “We’re seeing warming here more pronounced and earlier than anywhere else on Earth.” Researchers say in time, maybe not much time, we’ll see major changes taking place.

Bering Sea: Back to the Future

If she had a time machine, Beth Caissie from the University of Massachusetts, Amherst, would probably transport herself back about 20,000 years ago to investigate ice in the Bering Sea at that time. Instead, she relies on the next best thing: sediment from the seafloor that holds an archive of life dating back 20,000 years. Understanding how the ice cover has changed over time will give researchers an inkling of what to expect in the future.

“I’m looking at diatoms, which are yellow-brown algae that like sea ice,” said Caissie. “ I’m comparing the species that live in the sea ice now and look down in time to see when those species were prevalent in the sediment.” Looking “down in time” means investigating the deeper layers of sediment. The deeper the layer, the older the mud. The older the mud, the older the diatoms. Unlike rock-based dirt on land, the sediment on the seafloor consists of diatoms and other tiny plants made of silica, the same chemical in glass.

Caissie and other scientists aboard the Healy use a device called a Haps Corer to collect cylinder-shaped tubes of mud from the seafloor. They carefully prepare the tubes for travel back to the lab in Massachusetts for later testing. Each sample is painstakingly recorded, so scientists know where it came from and how long the sea ice covered that spot.

Caissie will prepare microscope slides of the mud so she can see the different species of diatoms that lived 20,000 years ago. “So I will look back in time to 20,000 years ago,” she said. “If I see a species that today lives in an area where there’s eight months of sea ice, it’s probably safe to say there was eight months of sea ice in that same spot 20,000 years ago.” Only time, and research, will tell how the sea ice will change in the future.