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On Thin Ice: Disappearing Zooplankton Could Collapse Arctic Food Chain

As the Arctic Ocean’s spring sea-ice cover shrinks, microscopic zooplankton are declining with serious ramifications for the region’s food chain, from fish to seals to polar bears.

Written by Gloria Dickie Published on Read time Approx. 4 minutes
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The tiny Calanus glacialis may lack the charisma of a polar bear, but these microscopic zooplankton form the foundation of the Arctic's food chain.Image Courtesy of NOAA

On – and under – the surface, Arctic zooplankton aren’t much to look at. For starters, many species are microscopic. Closer examination often reveals spindly legs, bulging black eyes or bodies as translucent as the sea ice near which they feed. But while charismatic polar bears may be umbrella species of the Arctic, zooplankton, scientists say, are the keystone.

These lipid-rich little critters serve as the energy source for all of the Arctic’s wildlife – from fish to seals to polar bears. In turn, zooplankton are dependent on sea ice, feeding on the algae that grows on the underside of the ice. Every spring, the algae blooms, zooplankton chow down, fatten up and then drift off into the bellies of bowhead whales and their ilk. But Arctic sea ice is fast disappearing, and with it, so are zooplankton. Already, scientists have documented a population decline and range retreat in Calanus glacialis, one of the most critical species of zooplankton in the Arctic. Last month, sea-ice extent hit a record low in the 38-year satellite record. Without the ice algae, zooplankton may starve, and that means the whole Arctic food chain could eventually collapse.

These marine robot gliders will be deployed for months at a time to study how diminishing sea ice affects productivity in the Arctic. The four-year project is being undertaken by the Natural Environment Research Council in the UK and Scottish Association for Marine Sciences. (Photo Courtesy of the Scottish Association of Marine Science)

These marine robot gliders will be deployed for months at a time to study how diminishing sea ice affects productivity in the Arctic. The four-year project is being undertaken by the Natural Environment Research Council in the UK and Scottish Association for Marine Sciences. (Photo Courtesy of the Scottish Association of Marine Science)

In order to get a grasp on the changes happening in the Arctic, the Natural Environment Research Council in the U.K. and Scottish Association for Marine Sciences recently launched a four-year study, called PRIZE, on Arctic productivity in the seasonal ice zone. Using marine robot gliders deployed for months at a time, researchers will investigate how diminishing sea ice affects productivity in the Arctic, and, therefore, the region’s ability to support marine life. Another SAMS project, dubbed DIAPOD and led by David Pond, will examine the role of the Calanus in the marine food web.

“The Arctic is often regarded as this uniform environment – cold, icy, dark in winter and light in summer,” says Finlo Cottier, an oceanographer and lead investigator on the PRIZE project. “But the Arctic is not a single entity.” The Barents Sea, for example, takes in water from the Atlantic Ocean, bringing in different nutrient loads; the Bering Sea is connected to the Pacific Ocean; and other regions are connected to freshwater channels. “One of the hypotheses is that production and its limits will vary between different regions due things like nutrient loads and the duration of open water. It won’t be homogenous.”

In addition to looking at sea-ice surface cover, researchers will also examine sea-ice thickness that can affect algae production. The thickness of the ice, and any snow cover above, impacts the amount of sunlight that reaches ice algae. “It’s a fine balance that can be quickly disrupted,” adds Cottier.

Thinner, less snowy ice means more sunlight penetrates through, leading to mega-algal blooms that can come earlier in the season. But zooplankton, and the animals that depend on them, might not be ready for the all-you-can-eat buffet. “Animals are timed to feed at a certain point in the year – some may still be in an overwintering state, or hibernation, and are timed to reemerge later,” explains Cottier. “It’s the equivalent of somebody serving breakfast at 6 a.m. and you miss it.”

Robert Campbell, a zooplankton biologist at the University of Rhode Island unassociated with the project, says researchers expect the Arctic marine growing season will lengthen under a changing climate, and with sea ice melting and thinning out earlier, zooplankton may be forced farther down in the water column to feed on other nutrients, like phytoplankton – possibly easing the detrimental effects of a warming Arctic. “We may also see more southerly species of zooplankton move in the Arctic from the lower latitudes,” Campbell says, noting that though species have been introduced before, they were never able to survive. However, these species aren’t as fatty or as long-lived as dominant Arctic species, like the large copepods Calanus borealis and Calanus glacialis that are excellent at transferring energy up the food chain.

When SAMS’ marine robots launch next year, they’ll record how the water is changing by looking at salinity, stability, temperature and how those affect algae and phytoplankton growth. By figuring out where the new hotspots of productivity will be in the future, scientists hope international governments can adequately plan for how they’ll manage their resources, like placing controls on national fisheries or establishing new Marine Protected Areas.

“These species aren’t charismatic. Most people won’t even know they exist. But they are the main link between the plants of the sea and the higher trophic levels,” says Campbell. “Without them, other species won’t exist. We won’t have polar bears, because there won’t be seals and there won’t be fish.”

The Lillehookbreen glacier is seen from on board the research vessel Helmer Hanssen, northwest of Spitsbergen, Svalbard. (Photo Courtesy of the Scottish Association of Marine Science)

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