The Atlantic invasion
The Pechora Sea, a shallow arm of the Barents Sea north of Russia, is on the front line of a rapid transformation. This region is a hotspot for a process called "Atlantification," where the fundamental characteristics of the Arctic Ocean are changing. Since 1982, the Barents Sea's surface temperature has been warming at a rate of 0.35 to 0.40 °C per decade. Sea ice concentration has decreased by about 14% per decade since 1979. The result is a summer ice-free period that is now 3.4 months longer than it was just four decades ago.
Traditionally, the Arctic Ocean has a strong layering, with cold, fresh water from river runoff and ice melt sitting on top of warmer, saltier Atlantic water that flows in from the south. This stratification acts like a lid, preventing the deeper heat from reaching the surface and melting the sea ice. Atlantification is weakening this layering. Warmer Atlantic water is now mixing towards the surface, heating the upper ocean and drastically reducing the growth of sea ice in winter. This process is so pronounced in the Barents Sea that some scientists suggest it is shifting from an Arctic to an Atlantic climate system.
A food web in flux
The physical changes are causing a cascade of biological responses, a process known as "borealisation." The entire food web, from microscopic plankton to top predators, is being reorganized. With less ice cover and more light, phytoplankton blooms are expanding northward.
This shift directly impacts zooplankton, the tiny animals that form the base of the marine food web. The dominant Arctic copepod, Calanus glacialis, a large, lipid-rich species, is being displaced by its smaller, less fatty Atlantic cousin, Calanus finmarchicus. This is a significant downgrade in the quality of available food.
Fish populations are also on the move. Atlantic cod (Gadus morhua), a commercially important boreal species, is expanding its territory north and east into areas once dominated by Arctic fish. This puts it in direct competition with the polar cod (Boreogadus saida), a species uniquely adapted to life at the ice edge. Polar cod, which have a form of antifreeze in their bodies, rely on sea ice for spawning and protection. Their populations in the Barents Sea have been below the long-term average for most years since 2012. This ecosystem shift affects predators further up the chain, including ice-dependent seals and polar bears.