Decreasing sea ice has impact on food chain in marine ecosystem

The large-scale die off of gray whales, walrus and certain seabirds has alarmed residents of northwest Alaska as well as the scientists who study those creatures. The scientists use the term “unusual mortality event,” UME for short, when large amounts of the animals wash up on the shore and no cause of death can be identified.
The Arctic is in a state of change and warming waters and receding sea ice are obvious places to look for answers. On Wednesday, Feb. 5 two scientists presented their findings at the regular Strait Science series at UAF Northwest Campus in Nome. Dr. Jackie Grebmeier was first to do her presentation. She is a research professor at the Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science. Of special interest is the appearance of algal blooms that can cause paralytic shellfish poisoning in the Chukchi and Bering Seas, a result of warming water and receding sea ice.
“Lessening ice will have an impact on the type and quality of food available for the zoo plankton and the animals that live in the water column,” said Dr. Grebmeier. The research being done takes samplings of temperature and salinity, currents, and chlorophyll, which is the plant life living in the water column. “And we’re working with NOAA and the U.S. Fish and Wildlife Service on their marine mammal and seabird surveys.” NOAA does bottom trawls, which provide data on the benthic portion of the environment. The benthic zone is the ecological region at the lowest level of a body of water. Data on the pelagic portion come from gliders, autonomous drones which gather and transmit information in the sea, and moorings which send signals year round. Pelagic refers to that part of the sea that is separate from the bottom and the shore. “A lot of countries are involved and we just had our fifth international meeting in Seattle to share those data sets,” said Dr. Grebmeier. “The objective is to track what the changes are and how they are occurring in the Pacific Arctic.”  
The surface water is warming. “But what was amazing for us was the bottom water,” said Grebmeier. They found the water near the sediment to have warmed about 8°F. They are finding this warm water from just southwest of St. Lawrence Island all the way north into the Chukchi Sea. “So the warming water is helping to melt the sea ice. It’s having a big impact on the quality of the food for the animals that live here,” she said.
 The threatened spectacled eiders are keyed to the sea ice and specific bivalves living in the sediment. Their food supply is dependent on the extent and duration of the cold pool. “This is the cold water produced in the winter south of St. Lawrence Island. All of these animals are adapted to living at these cold water temperatures. And as a result a lot of the animals that live in the sediment that walrus, whales, bearded seals, and diving ducks feed on have been in abundance.” In 2018 the sea ice went away in March and didn’t return. That is when the thermal barrier, the cold pool of very cold water, broke down.
“We have this colder winter going on but as the locals say it’s very thin ice,” said Dr. Grebmeier. “The question is how much it will build up this year, how thick it will be, and how quickly it will go out of the system. Without the sea ice we won’t have the nice production of organic matter that is important to get the clams going. The production may not be as high in certain regions south of St. Lawrence Island. This will have an impact on the type of zooplankton later on in the spring.”
What is happening is the high biomass regions are moving northward. The feeding patches that the seabirds and the walrus go after have “contracted” northward. The bigger clams are to the north and the eiders have a set length clam they can get down their esophagus into their stomach. The area where they are found is smaller and the composition has changed. “When we talk about changes it’s not just about how much is out there but who is out there. Some of the animals being consumed have more lipids in them.” More lipids means they provide more energy as food.
NOAA’s Oct. 28 2019 trawl survey shows more Pacific cod and Pollock in the Bering Sea. The fish are moving north as sea temperatures rise. Their movement coincides with the contraction of cold pool movement and the decrease in ice formation. The cod are coming in and feeding on the marine worms.
“If a duck has to dive down 60 meters and come back up, if it comes back up with a clam that has less fat it’s going to expend a lot of energy for less,” said Dr. Grebmeier. “This is something we’re also seeing with walruses that are coming ashore. They have to spend more energy to go out and feed on these richer clam beds. In the past they could ride the ice. That’s one example of what we’re seeing on the bottom. There are going to be winners and there are going to be losers in this region.”
“What we’re looking at is four decades of data. Amphipods, small shrimplike creatures, are what the gray whales feed upon. Back in the 90s we were already seeing changes before the thermal barrier went away. So something was already going on in the ecosystem. Studies in the 90s showed this area moved from being a feeding ground for gray whales to feeding for sculpin. And getting into the 2000s one of the hotspots changes to clams. There’s more contraction northward. The amphipods like to have sand. They need fine particles to glue them together with their mucus. One thing we found out is the eiders usually overwinter here but some because of the lack of sea ice are migrating earlier and they’re being found on the Russian side. They are spreading their range out when they find good conditions.”
Better conditions for the worms in the sediment means more worms and that brings in the Pacific cod and the Pollock.
One of the results of the warm water moving north is acidification. With more carbon dioxide in the water it becomes corrosive. Off Utqiagvik and the Hanna Shoal there are a lot of clams. They take up oxygen and when the currents are slow, the carbon dioxide builds up and corrodes the clam shells. “We’ve seen two types of clams that are really important for the walruses when they come out to feed in that area,” said Dr. Grebmeier. “One type has shells get thinner the more time they spend in this acid water. The other clam is what we call resilient. They have thick enough shells and they can survive. So one will become more abundant than the other.”
With the warmer water researchers are finding algae blooms. The algae form a capsule and sink to the bottom where they overwinter. Warm water causes them to rise and to bloom. This increase is significant because fisheries can be shut down if an algae bloom reaches a certain level. “In a cruise we did in the Healy last August north of St. Lawrence Island they found toxicity levels far above what anybody should eat. Mainly the clams were toxic.” HABs, harmful algae blooms, are increasing in the Arctic with the decline of the sea ice, more sunlight and warmer water. The dinoflagellates, which cause paralytic shellfish poisoning are among the toxin-producing fauna.  Two different algal toxins were found following the die-off of the gray whales in 2019. NOAA declared a UME, an unusual mortality event, for ice seals in the Bering and Chuckchi Seas at the same time. And there was a UME for seabirds.
It is apparent that the system is undergoing changes but exactly what is driving those changes is not clear. It could be the change in the food supply. It could be the toxicity, the warm water, or the combination of all of these changes. There is no resolution as to what is killing these animals. When an animal is sick it is susceptible to other types of disease.
Chelsea Wegner Koch, who is working on her PhD with Dr. Grebmeier, stepped up to talk about the sea ice.
There is a close relationship between the sea ice and the ecology of the benthic zone. The early spring algae, which grows on the underside of the sea ice, sinks to the bottom where it supports the rich benthic populations. This includes the bivalves and the worms. They, in turn, are food for gray whales, walrus and sea birds. “As the sea ice trends continue to go the way they are, declining and shifting, into this new structure, we have more pelagic grazing going on. We’re reducing the quality, quantity, and timing of that food producing benthic environment, and we’re displacing food sources for some of those that depended on the benthos,” said Koch.
Ice algae communities of diatoms are rich in lipids, commonly called fats, and therefore more nutritious than the pelagic  phytoplankton. Koch is using a new technique to analyze that hasn’t previously been applied in this region. She uses two biological markers, which are unique to their source. A marker is a characteristic of an organism that can be traced as it is consumed first by one organism and then by another. One of the markers is produced only by the sea ice algae, the other is widely found in the open sea. Koch samples both and compares the proportions of the two. “If we have more sea ice cover, or conditions where more algae is produced, we get the higher value for the blue compound. In a more open water environment we see more of the red compound. So this tells us about the sea ice conditions and where the animals were feeding.”
Walrus was a primary focus of her research because they feed on that part of the benthos which is dependent on the sea ice algae for food. In other studies it has been found that the liver of an animal is the right organ for looking at these compounds. Walrus hunters on St. Lawrence Island supplied livers. A lot of lipids are stored in the liver and analysis represents what the walrus consumed over the previous three to four weeks. “These markers I’m looking at are very stable over time,” she said.
What she found in the walrus livers was markers indicating stronger sea ice in those harvested while foraging in the Chukchi Sea. The sea ice is more plentiful farther north. There was a statistical difference between Chukchi and Bering Sea walrus. “This shows how their diet was impacted by the sea ice, probably the lipid content of what they had been eating.”
She also found the females have a higher ice signature than the males. That means they’ve been feeding more on clams fed by ice algae than by clams that didn’t get the ice algae. The reason for this is not known, although it could be the females are foraging in different areas, or perhaps it’s related to their reproduction and calving. “We do see differences in different types of clams,” she said.
“Is there a connection between the sea ice signature and walrus condition? Some of you are part of this food web as well and these changes in the sea ice and these shifts to the food web also impact you in the quality of the food items you are getting. These differences in the levels that we see don’t necessarily mean that it’s a bad thing. But it is an indicator of greater shifts that are happening,” she said.


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