Above Scandinavia, on the Atlantic side of the Arctic Ocean, mackerel, cod, and other fish native to the European coast are migrating through increasingly ice-free waters, heading deeper into the Arctic Basin toward Siberia. Thousands of miles to the west, above Alaska, kittiwakes and other polar seabirds are being supplanted by southern birds following warm waters streaming north through the Bering Strait. And midway between, above Canada, sea ice-avoiding killer whales from the Atlantic are increasingly making themselves at home in a thawing Arctic.

As the Arctic heats up faster than any other region on the planet, once-distinct boundaries between the frigid polar ocean and its warmer, neighboring oceans are beginning to blur, opening the gates to southern waters bearing foreign species, from phytoplankton to whales. The “Atlantification” and “Pacification” of the Arctic Ocean are now rapidly advancing. A new paper by University of Washington oceanographer Rebecca Woodgate, for example, finds that the volume of Pacific Ocean water flowing north into the Arctic Ocean through the Bering Strait surged up to 70 percent over the past decade and now equals 50 times the annual flow of the Mississippi River. And over on the Atlantic flank of the Arctic, another recent report concludes that the Arctic Ocean’s cold layering system that blocks Atlantic inflows is breaking down, allowing a deluge of warmer, denser water to flood into the Arctic Basin.

Because the oceanographic conditions in the Atlantic and Pacific sectors of the Arctic are distinct, the physical mechanisms behind these widespread changes differ. But scientists say that the growing intrusions on both sides of the Arctic Ocean are driving heat, nutrients, and temperate species to new polar latitudes — with profound impacts on Arctic Ocean dynamics, marine food webs, and longstanding predator-prey relationships.

“You’re really changing the system in terms of its capacity for production with the loss of sea ice, influx of nutrients, and also this ‘highway of prey,’” says Sue Moore, a biological oceanographer with the National Oceanic and Atmospheric Administration (NOAA) Fisheries Office of Science and Technology who studies marine ecosystems in the Pacific Arctic. “It’s a whole resetting of the table.”

Striking ecosystem effects are becoming evident across the high north. Some species appear to be gaining new habitat, while others seem to be rapidly losing ground.

On the Pacific side, humpback whales, a sub-Arctic species, have been observed in recent years as far north as Utqiaġvik (formerly called Barrow), off Alaska’s North Slope, where they had not been reported before. Moore thinks the whales are feasting on pulses of Pacific krill and other food streaming into the Arctic. Reduction in sea ice also allows whales to arrive earlier and stay later, giving them more time to feed and reproduce.

And this past winter, when sea ice in the Alaskan Arctic formed up to three months late and disappeared at the earliest date on record, “We had bowhead whales in the Chukchi Sea pretty much overwinter —we have never seen that before now,” says Moore, who is co-leader of the Synthesis of Arctic Research project, an international collaboration of polar experts looking at the effects of a changing Pacific Arctic on marine life.

Along the Arctic coast of western Canada, indigenous fishermen report salmon venturing unusually far north, says Carolina Behe, the indigenous knowledge science advisor for the Inuit Circumpolar Council-Alaska. And scientists conducting an Arctic ecosystem survey in 2008 were surprised to find walleye pollock and Pacific cod in the Beaufort Sea, said Elizabeth Logerwell, an ecologist with the NOAA Alaska Fisheries Science Center who worked on the survey. If these more southern species extend their range north, they could potentially supplant native Arctic fish. She cautioned, however, that the region lacks long-term historical fish survey data, making it hard to know for certain whether these were actually new arrivals or just being scientifically observed for the first time.

Scientists do say that the intrusion of Pacific waters is taking a toll on seabirds in the Alaskan Arctic. Traditional summer denizens in the Chukchi Sea — such as black-legged kittiwakes, thick-billed murres, and glaucous gulls — are now being supplanted by least and crested auklets, northern fulmars, and short-tailed shearwaters, which had not previously ventured so far north. The shift reflects a change in the birds’ prey, with new prey species being swept north through the Bering Strait. The change is causing an overall decline in seabird numbers, a recent paper concludes.

Signs of ecosystem shifts are even more dramatic on the Atlantic side of the Arctic, where Atlantic Ocean inflows overall are far warmer, larger, and saltier. Moore likens the incursion across the long boundary zone above Iceland and Scandinavia to a “firehose,” compared to a “garden hose” on the Pacific side, where incoming waters are constrained by the relatively narrow Bering Strait.

“When I came to Norway in 1974, the northern border for mackerel was western Norway,” says Paul Wassmann, a professor of marine ecology at the Arctic University of Norway in Tromsø and an expert on ice-edge ecosystems. “Now you can see [mackerel] in Svalbard fjords,” he says, referring to a group of Norwegian Arctic islands that lie nearly 600 miles above the mainland as far as 81 degrees North. Buoyed by Atlantic intrusions, warm water species are spreading northeast into the Arctic Ocean, and resident cold–adapted species, such as Arctic cod, are attempting to retreat further north. In the past half-decade, says Wassmann, “There has been (Atlantic) cod in fishable amounts north of Svalbard, which had not been observed before.”

One of the most striking trends is a rapid poleward spread of phytoplankton, the microalgae at the base of the marine food chain. A new study finds that Emiliania huxleyi, a phytoplankton native to more temperate, Atlantic seas, are blooming 5 degrees latitude, or roughly 350 miles, farther north in the European Arctic today than in 1989.

The northward spread of Atlantic phytoplankton could have a drastic effect on the Arctic marine ecosystem, says co-author Laurent Oziel, a polar oceanographer at Laval University in Quebec City. In the Arctic, the zooplankton that consume phytoplankton are specialized to store large reservoirs of fat to withstand harsh winter conditions. This makes them especially rich food sources for the fish that eat them, the seals that eat the fish, and on up to the top predators, such as polar bears. But the influx of southern phytoplankton is bringing in new types of zooplankton that are leaner and provide less nutrition for local species, Oziel says.

Southern zooplankton may not be able to survive the cold northern conditions. But Wassmann says that thinning and disappearing sea ice is allowing more light to penetrate the Arctic Ocean’s depths, stimulating algae growth that could allow fish and organisms to become established in places that previously lacked sufficient nutrients.

Atlantic influences are also being felt along the Canadian section of the Arctic Ocean. “In Hudson Bay we’ve seen a huge climatic shift in what we call subarctic species,” says Jennifer Provencher, a postdoctoral fellow in northern research at Acadia University in Nova Scotia. “We have documented that shifting occurring from cold-water species to the warmer water Atlantic species. In our case, it’s going from Arctic cod to much more capelin and sand lance.”

Another species appearing with increasing regularity in the Canadian Arctic is Orcinus Orca — the killer whale. These large, carnivorous members of the dolphin family used to be rare in the normally ice-covered waters, which orcas avoided because of their tall dorsal fins. But as sea ice retreats and their prey moves north, killer whales are now showing up more frequently in parts of the Canadian Arctic, where they pose a threat to narwhals, Beluga, and bowhead whales. As Nature Canada blogger Rebecca Kennedy puts it, “Killer whales are poised to become a major Arctic predator.”

Scientists are trying to better understand what’s driving the growing invasions from the Arctic Ocean’s neighboring waters. Historically, the northward flow through the Bering Strait is driven by a height difference — the Bering Sea is higher than the Chukchi Sea on the north side of the Bering Strait, causing the Pacific waters to run “downhill” into the Arctic, explains Robert Pickart, a physical oceanographer with the Woods Hole Oceanographic Institution in Massachusetts who studies Pacific Arctic circulation. Prevailing northerly winds normally oppose the flow, especially in winter, when they retard Pacific water from crossing into the Arctic. Woodgate’s research finds that the increased Pacific flux is not due to a change in local winds, however. Instead, it appears linked to increased atmospheric pressure deep in the Arctic Basin, Pickart says.

“You have all this warm Pacific water coming into the Arctic and what is that going to mean?” says Pickart. “Not only is there more water going through, but there’s an increase in the amount of heat going through.” Adding more heat, he says, is “going to change the composition of the water and its likelihood to melt sea ice.”

In addition, waters from the Atlantic that have long entered the Arctic Ocean and circled down deep are being driven higher onto shallow sea shelves north of Alaska by increasingly intense storms. (The Arctic’s extensive sea ice cover used to tamp down storms.) “What we see at the Chukchi slope in the last few years are these pulses of Atlantic water, which is dreadfully warm, above zero degrees (Celsius), and salty,” says Phyllis Stabeno, a physical oceanographer at the NOAA Pacific Marine Environmental Laboratory and a co-director of the Synthesis of Arctic Research.

On the other side of the Arctic Ocean, between Scandinavia and Russia, a strong layering system that normally prevents warm, Atlantic water from mixing upwards is weakening. Called the halocline, this thick band of cold Arctic water overlies deeper circulating Atlantic water and blocks its heat from reaching the surface of the Arctic Ocean.

But the halocline is losing its strength, according to Igor Polyakov, a University of Alaska Fairbanks oceanographer. He is lead author of a recent study finding that the Arctic Ocean is becoming more like the Atlantic, as reduced Arctic stratification lets more Atlantic water mix in and prevents sea ice from forming in the winter.

“The halocline has grown much weaker in recent years,” Polyakov says, “allowing the Atlantic water heat to penetrate upward and reach the bottom of sea ice.” The phenomenon, which began near Svalbard in the late 1990s, is now accelerating and spreading east into Arctic waters above Siberia.

Researchers point out that data from the remote Arctic Ocean are sparse and long-term observations are lacking, making it hard to pinpoint exact times and rates of change in polar marine ecosystems. The satellite record is only 39 years old and ship-based surveys have been infrequent, so scientists can’t always say for certain how much a population is increasing or extending its range.

“You need a time series in places where we don’t have a time series,” says Wassmann.

All signs are, however, that the Atlantification and Pacification of the Arctic Ocean will only intensify in the coming decades as the world continues to warm and the Arctic becomes increasingly ice-free. Arctic temperatures this past February soared to more than 45 degrees Fahrenheit above normal and hovered well above average all season. Winter sea ice was the second-lowest on record across the Arctic Ocean as a whole. And summer ice cover in the Arctic Ocean has declined by about 40 percent since satellite monitoring began.

“It’s all linked,” says Moore. “Sea ice is such an important component of the system that once it is radically changed, you’re going to have other changes following that.”