Physical conditions in the Barents Sea are largely determined by three main water masses: Coastal Water, (North) Atlantic Water, and Arctic Water. These three water masses are linked to three different current systems: the Norwegian Coastal Current, the Atlantic Current, and the Arctic Current. Climatic variability is determined by their properties and the activity of inflowing Atlantic Water. Variations in activity of these currents may be explained by external forcing, but may also be a result of processes
taking place in the Barents Sea itself. Year-to-year variability in sea temperatures is profoundly influenced by the relatively warm Atlantic water masses flowing in from southwest (Loeng 1991) as well as regional heat exchange with the atmosphere (Ådlandsvik and Loeng 1991; Loeng et al. 1992). The inter-annual variability is, to a large extent, determined by conditions during winter, the season when differences in temperature — between both inflowing and local water masses, and between the local atmosphere and the sea surface — are at their highest (Ottersen and Stenseth, 2001).
Ottersen and Stenseth (2001) demonstrate that climatic processes on the scale of the North Atlantic basin may profoundly influence the ecology of the highly productive Barents Sea. The impact of inter-annual and decadal shifts in regional climate — sea temperature in particular — on fish recruitment in the Barents Sea has also been well documented (Sætersdal and Loeng, 1987; Ottersen and Sundby, 1995). In the Barents Sea ‘‘warm’’ years are good years production-wise for three principal reasons: 1) a larger ice-free area allows for higher primary productivity; 2) warm years imply large influxes of zooplankton from the south into the Barents Sea, and; 3) higher temperatures lead to higher biological activity at all trophic levels (Sakshaug 1997). As a result, above-normal sea temperatures tend to have a positive impact on fish production.
Climatic fluctuations have a significant effect on the ice conditions, which in turn influence the biological production in the northern Barents Sea (Loeng, 2007). Bottom-up processes are important as changes in climate conditions (e.g. warming and reduced sea ice extent) will likely influence the timing and magnitude of phytoplankton blooms and thus influence primary productivity of the Barents Sea (Dalpadado et al., 2014). Despite high interannual variability, the ice extent in the Barents has decreased by 60% over the last 200 years (Vinje, 2001, Wassmann et al., 2006).
Productivity in the Barents Sea is responsive to loss of sea-ice, but the location of storm tracks in creating additional mixing to fuel nutrient replenishment is also an important factor (Drinkwater, 2011). Inflowing Atlantic Water largely controls nutrient concentrations in the southern and central Barents Sea. Thus, winter concentrations are typical for the North East Atlantic; these water masses have recently been exposed to biological production as surface waters. The spatial distribution of new production and phytoplankton biomass in the Barents is strongly linked to nutrient consumption during the productive period (May–early September) and vertical mixing during winter (Wassmann et al., 2006).
The microbial loop is an important pathway for channeling carbon through the food web. Studies of the microbial food web in the Barents Sea are scarce, but they are essential for a more balanced understanding of the pelagic ecosystem. Scattered investigations in the Barents Sea indicate that small planktonic forms including microbes are prevalent (e.g., Thingstad and Martinussen, 1991; Hansen et al., 1996; Hansen and Jensen, 2000; Arashkevich et al., 2002; Verity et al., 2002; Wassmann et al., 2005). Investigations close to the Barents Sea entrance (Verity et al., 1999) and in its marginal ice zone (Verity et al., 2002) suggest that often more than half of the dominant pico- and nano-plankton cell biomass is heterotrophic. Indeed, most microbes are heterotrophic, using organic compounds as both carbon and energy sources (Wassmann et al., 2006).