Bering Sea ecosystem responds to dramatic loss of sea ice
South winds and warm water are hitting sea ice on arctic waters with a double whammy. Climate scientists and biologists are surveying and sampling to determine the extent that the assault on sea ice is effecting mismatches in prey and predator, altering the food chain, altering the fish distribution and creating chaos in the arctic ecosystem.
Janet Duffy-Anderson, straight off the research vessel USCG icebreaker Healy explained the profound response of fisheries ecosystem dynamics to unprecedented loss of sea ice.
Duffy-Anderson is a fisheries biologist at the NOAA Alaska Fisheries Science Center in Seattle. She is manager the ecosystems and fisheries oceanography coordinated investigations program.
The team comprises about 40 people looking at general oceanography of phytoplankton or algae, zooplankton and young fish as a prey base for marine mammals, fish and sea birds.Two main kinds of plankton are zooplankton and phytoplankton. Phytoplankton are plants, which obtain their energy through the conversion of sunlight in photosynthesis and pull nutrients from the water around them. Plankton are at the bottom of the oceanic food chain and vital to the food chain. They need sun to thrive.
Zooplankton generally feed upon other plankton, including phytoplankton and zooplankton. Zooplankton are tiny invertebrates of which krill and copepods are types, that live in swarms in the ocean. Many animals, including seals, whales, seabirds, fish and squid eat krill and copepods.
This story is going to go up the food chain to fish and birds.
Loss of sea ice is affecting the availability of nutritious plankton in the prey base for ecosystems that depend on them in the Northern and Southern Bering seas.
The lack of winter sea ice in recent years is unprecedented, Duffy-Anderson said.
While April 2012 satellite photos of the Northern and Southern Bering Sea show sea ice coverage as expected, the photos from March 2019 provide a striking observation of almost exclusively open water.
“We’ve seen lack of sea ice in the southeast Bering Sea before but in the northern Bering Sea, this is unprecedented, so unprecedented,” she said. “And so some of the questions that we have are: Can we learn anything about what might happen in the northern Bering Sea based on what we know has happened before in the southeastern Bering Sea?”
Climate scientists have observed that the Bering Sea cannot freeze up without the Chukchi Sea freezing first. They can learn a bit about what to expect in the Bering Sea by looking at patterns in the Chukchi Sea freeze-up.
Scientists have buoys of instruments in the Chukchi Sea that remain for a year recording measurements in the water column, including how many days the ice concentration is less than 30 percent. Over time, between 1980 and present, there are more and more days with sea ice concentration being less than 30 percent.
More and more, winds are coming out of the south to push sea ice northward, producing a drop in the sea ice extent over the whole Bering Sea, Duffy-Anderson continued.
Now add warming sea water.
“The Bering Sea has never been warmer than it is now,” Duffy-Anderson said. “In fact, six of the 10 warmest springs in the past 120 years have occurred in the last six years. So that gives you an idea that we’re kind of facing what I consider a double whammy because we have increasing winds from the south that are working to push ice northward and keeping it off of the shelf.
“We also have increasing temperatures that are making, making it harder for that sea ice to be maintained as it moved south because the water is much warmer,” Duffy-Anderson said.
What does that mean for the ecosystem?
One thing is the timing of the algal bloom with the loss of sea ice, according to Duffy-Anderson.
In cold years, when ice moves southward from the Chukchi Sea down through the northern and southern Bering Sea, it starts melting back in the spring, putting fresh water into the surface layer.
The fresh water stratifies the water column into two layers—fresh on top and dense and salty underneath.
According to Bob Pickart, an oceanographer who spoke at the same session of Strait Science at Northwest Campus UAF, this stratification is important in keeping the plankton, phytoplankton and algae up to thrive in the sunlight and water. (See story page 6)
However, in warm years, there is bad news concerning these nutrients. With little or no ice, there is no spring melting to stratify the water column, just dense and salty water.
“You are getting much later stratification, not getting the algal cells up in the lighted water, so you are not getting the phytoplankton bloom until later,” Duffy-Anderson said. “When you don’t have ice, you also get different types algae with lower lipid content [lower quality nutrient], with less fat in the algal cells.”
So, less ice delays algal bloom containing different and smaller algae.
Take the year 2018, Duffy-Anderson said. In a cold year in the Bering Sea, you would expect stratification and an algal bloom the third week of May.
“But what we saw last year was that that bloom happened three weeks to a month late because there wasn’t ice melt to help stratify the water column and keep the algae cells up in the lighted water,” she said. “So changes in the timing of the algal bloom and changes in the types of algae matter because you are going to have different types of zooplankton feeding on them.
“You have mismatches in the timing of algae and the bloom of plankton and zooplankton– the base of the food web for fishes, birds and animals,” Duffy-Anderson said.
In this case, the zooplankton bloom may have little to feed on, or they may be getting less nutrients as the nutritious algae are associated with ice. Copepods and krill are important zooplankton that are primarily the food source for most whales, sea birds and young fish before they start feeding on other fish. Krill and copepods, present when water is cold, are rich in lipids. When the water is warm and sea ice free, fish and seabirds are feeding on less nutritious prey, according to Duffy-Anderson.
Biologists are examining the dietary problems of the krill and copepods to see if the situation may be handing the Bering Sea fish populations and the sea birds their own double whammies. Fish and seabirds have been feeding on less nutritious prey when the southeast Bering Sea has no ice, Duffy-Anderson said.
“In the southeast Bering Sea we have more observations starting in 1996 and before 1996 and even before 2000,” Duffy-Anderson said. Scientists found that the southeastern Bering Sea would sometimes have no ice. Maybe there would be one cold winter and then an OK winter.
“The ecosystem didn’t really respond so much to one year of no ice; what changes the ecosystem is when you have multiple years of no ice,” she said. Scientists started seeing that in the southeast Bering Sea in 2001. There were warm years, 2001 through 2006; then between 2008 and 2013 or 2014, sea ice was present in the southeast Bering Sea.
What happened to the zooplankton?
“ The first time we had back to back warm years in a row, we saw declines in the abundance of these nutritious plankton for years in a row,” Duffy-Anderson said. “Fortunately, when it got cold again, they came back. When it got warm again for consecutive years, the abundance switched.
“So we had an idea of what happened in the southeast Bering Sea. We had never seen no ice in the northern Bering Sea,” Duffy-Anderson said, and put a slide up showing a chart of cold and warm years in the northern Bering Sea.
The data showed low abundance of nutritious prey in the northern Bering Sea in cold years and then during the recent warm years, the less nutritious prey.
“This is all repercussions of loss of sea ice,” she said.
Take the walleye pollock and the Pacific cod for example, Duffy Anderson said.
The main biomass of walleye pollock historically has been in the southeast Bering Sea. A chart shows the population of pollock in the southeast and northern Bering Sea combined. The population has varied.
“That’s pretty typical of fish populations, but you can see recently in the southeast Bering Sea, you get this dramatic decline in the number of pollock. At the same time in the northern Bering Sea you get more pollock.
“The question is, are these fish moving? Why are we seeing this now in the northern Bering Sea when we didn’t see them before?” Duffy-Anderson asked.
Another illustration on the screen at the presentation shows slow northerly movement of pollock between the years of 1985 and around 2005.
Now, in the most recent years, the pollock are moving at a rate of 30 km a year.
“It’s the fastest ever observed for a commercial species. These fish eat plankton. They’re called zooplankton terrorists,” Duffy-Anderson explained.
Duffy-Anderson directs attention to fish larvae. Ocean scientists have collected from larval communities on Distributed Biological Observatory cruises and other cruises conducted between 2010 and 2017.
“Not only do you see differences in fish distribution when you have lack of ice. You see different fish communities,” Duffy-Anderson said.
In low ice years, the larval communities tend to be made up of warm water species.
“Pollock would be some of the fish larvae in the group of fish we catch, in 2010, 2011 and 2017 when those warm water communities occur farther north,” she said.
In 2012, 2013 and 2015, with ice present, cold water communities tend to dominate, according to Duffy-Anderson.
“This is important because a whole community of fish occurred, not just one. It is also important because these are fish larvae,” she said. “One of the things we want to understand is not just are our fish moving because they can move, but are they colonizing the area?”
Loss of sea ice causes changing of timing algal bloom, changes in zooplankton, changes in production, changes in communities, finding of some southerly occurring species in the north, and also super die offs.
Scientists are concluding that the loss of sea ice and other climatic occurrences lead to a state of phenology— the match or mismatch between predator requirements and the resource [prey] availability.
Studies show the number of sea birds in the Bering Sea is going down in a lot of recent years, except in 2018 when there is a slight increase, Duffy-Anderson said, pointing to a plot of the total number of sea birds in the northern Bering Sea last year.
In the northern Bering Sea last year there were a lot more auklets, but overall in terms of diversity of species and number of birds, it’s been lower.
“What they’ve seen are die-offs of murres, that one of our observers on the Healy tells us has continued in 2019. There’s also been mortality events among puffins,” according to Duffy-Anderson.
“These birds are emaciated. There is evidence they are starving,” she said. “There is this compound problem alluded to by Bob Pickart [also from the Healy cruise and on the same Strait Science program], the harmful algal blooms (HAB) in the systems right now.
“Are the HAB making these birds weak, more susceptible to starvation, things that they might have been able to weather earlier—changes in food source or moderate to some extent of they have HAB exposure? Does all this make them in worst condition? It’s sort of another double whammy,” Duffy-Anderson said.
“And finally, I think the last question, and it’s the one that I don’t have the answer to, but I think it’s all on all of our minds is, is the northern Bering Sea more likely to be ice free in the future?” she asked. “This phenomenon of declining ice in winter time is something that was forecasted but not for another 50 to 60 years. The fact that it happened in the last two years is just unheard of. So I think we’re all wondering the same thing—is this a new normal? Where are we going after this?”
Strait Science is an information program sponsored by Northwest Campus UAF and Alaska Sea Grant.