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Climate change is altering species distributions across the world, and this is particularly marked in the Arctic where warming is occurring most rapidly.

Researchers at Durham University, UK, and the Fisheries and Marine Institute at Memorial University in Canada have explored how future expansions of new species into an area of Arctic Ocean could impact the existing marine communities. They focused on the North Water Polynya, a large area of open water surrounded by sea-ice between Canada and Greenland, which is one of the most productive marine ecosystems in the Arctic.

The team explored future potential changes to the local food web—the network of interactions that occur between species in a community, due to predation and competition. Possible future scenarios included changes to the plankton communities as the ocean warms, declines in Arctic cod as the area is colonized by capelin (a warmer water species), and the novel establishment of killer whales in the area.

The team analyzed how changes to the amount of one species in an area will impact other species, whether they interact directly or indirectly. Direct interactions, such as an increase in prey resulting in an increase in the predator population that eats them, are the most obvious responses to be expected in a food web.

However, there can often also be indirect responses where changes in one species can alter the populations of other species that seemingly have little interaction with them.

The team projected that capelin, a small fish, would become more abundant in the study area, while plankton numbers could either increase or decrease depending upon how water circulation and temperature change in future. Killer whales were predicted to be more common in future, particularly as the sea-ice free season lengthens.

When these changes were added into food web models to simulate future species population changes, some unexpected findings emerged. Polar bear numbers halved in response to declines in phytoplankton numbers, the latter being tiny photosynthesizing organisms in the open ocean.

Changes to the numbers of larger, energy-rich copepods (tiny crustaceans) had the largest effect on the entire ecosystem. Their decline was linked to reduced numbers of Arctic cod, ringed seals, beluga whales and polar bears. Increased colonization by capelin was also projected to have a similar effect, while the addition of killer whales to the ecosystem led to more substantial declines of narwals and seals.

The findings have been in the journal PLOS Climate.

Beth Gillie, of Durham University's Department of Biosciences, co-led the study and said, "The research sheds new light on how novel species moving into the Arctic due to climate change could have cascading effects on the entire food web.

"I find it fascinating how a subtle shift in the relative abundance of large and small crustaceans could lead to a four-fold decline in polar bear numbers in future, highlighting the complexity of species' potential responses to climate change."

Study co-author, Andrea Bryndum-Buchholz from the Fisheries and Marine Institute at Memorial University in Canada, said, "These findings highlight the sensitivity of Arctic marine ecosystems to climate change and the need to reduce carbon emissions as quickly as possible."

The study presents one of the first assessments of how climate change could lead to cascading effects across Arctic marine food webs and may pave the way for future related research in these and other ecosystems.