The Barents Sea is a sub-Arctic shelf ecosystem located between 70° and 80°N. It connects with the Norwegian Sea to the west and the Arctic Ocean to the north. The dynamics of the system are strongly influenced by the inflow of warm Atlantic water from the west. This water mass is separated from Arctic Water by the ocean Polar Front, which is characterised by strong horizontal gradients in temperature, salinity and concomitant differences in biodiversity supported within the various regions. The system is also dominated by seasonally occurring sea ice, particularly in the eastern and northern parts. A distinct assemblage of species is associated with sea ice.
The Barents Sea is home to one of the largest concentrations of seabirds in the world, a diverse assemblage of marine mammals, including polar bears, and several commercially important fish stocks, the largest of which are Northeast Arctic cod, capelin and haddock. In addition, the Barents Sea is a nursery area for Norwegian spring spawning herring, one of the largest fish stocks in the world. There is also a rich community of benthic animals in the Barents Sea, numbering more than 3000 species, as well as a diverse community of zooplankton.
Planktonic algae and algae attached to the sea ice both contribute to primary production in the region. Infectious organisms and free-living bacteria and virus may be important groups, but their role for the overall dynamics of the system has received little research attention. The ecosystem has been invaded by several alien species, such as the red king crab; the influence of which is being studied currently, but is still largely unknown.
Capelin - a key species
Capelin is a key species in the Barents Sea ecosystem. This fish species feeds in the marginal ice zone and spawns near the coast in the southern part of the Barents Sea and thus transports large amounts of energy from the north to the south. It is important as prey for several species of seabirds, mammals and commercially important fish stocks, in particular Northeast Arctic cod and juvenile herring. Capelin is an important predator of zooplankton that can actually suppress the biomass of zooplankton in the Barents Sea. Capelin stock size has varied considerably in recent decades and has undergone three population collapses during the last 25 years. There is at present no consensus among scientists about the causes of the observed capelin recruitment failures leading to capelin stock collapses. While no one holds the view that the causes are all known, some suggest that the collapses are mainly a consequence of predation on capelin larvae from increased amounts of juvenile herring, others suggest several factors as likely to cause capelin collapses, including climatic fluctuations, predation from fish and marine mammals and fisheries. What-ever the cause, these collapses have had far reaching consequences for other species in the ecosystem, including a severe food shortage for the Northeast Arctic cod stock, collapses of seabird populations and food shortage and massive migrations in seal populations. It should, however, be noted that the ecosystem consequences of the first collapse (late 1980s) was much more severe than during the two later collapses, probably because more alternative prey were available for the predators during the latter collapses.
Variations in water temperature have important effects on the Barents Sea ecosystem. In particular, periods of high temperature tend to stimulate recruitment of Northeast Arctic cod and Norwegian spring spawning herring and other fish stocks. Indirectly, recruitment of capelin may be impaired by high temperatures because of increased predation from larger amounts of juvenile herring drifting into the area from spawning grounds along the Norwegian coast. Higher water temperatures, or changes in the characteristics of the Polar Front, are often accompanied by a decrease in sea ice cover and thereby a negative impact on ice-dependent species. Predicting the response of primary productivity to temperature variation is associated with uncertainty because the amount of light reaching the water column and supply of nutrients necessary for primary production may respond in opposite directions. When it gets warmer, amount of light will increase because more sea ice is melted away. At the same time, nutrient supplies may decrease because warming and increased input of freshwater from sea ice melting can lead to increased stratification of the water column, thus reducing the mixing of nutrient rich deepwater with the layers higher in the water column where primary production occurs.
Fisheries - the main anthropogenic driver
The anthropogenic driver with the largest documented effects on the Barents Sea ecosystem is currently fisheries. Negative impacts of fisheries include overfishing of several of the smaller stocks and damage to benthic communities caused by bottom trawling.
Barents Sea - a relatively clean ocean
The Barents Sea is presently a relatively clean ocean with respect to pollution, however, it receives long-range transboundary transported pollution through advection, in particular PCBs and other persistent organic pollutants as well as some inorganic contaminants (e.g., Hg and Pb). These substances are detectable in biota, but to date significant effects are limited to top predators, such as polar bears and glaucous gulls.