Biomass and productivity distributions and their variability in the Barents Sea

Abstract

On the basis of results that largely have been derived from the Norwegian research programme Pro Mare (1984–1989), an overview of the distribution of biomass and productivity at different trophic levels of the pelagic Barents Sea ecosystem is presented, with comments on year-to-year and horizontal variations. Average biomasses for the whole Barents Sea and several years range from 2000 kg of carbon km⁻² for each of Calanus spp. and phytoplankton stocks, down to 0.1 kg carbon km⁻² for the polar bear population.

Phytoplankton blooms that deplete the winter nutrients give rise locally to a “new” productivity of 40–50 g C m⁻². Areal differences, however, are pronounced in terms of annual productivity: the “new” fraction is more than twice as high in the southern (Atlantic) part of the Barents Sea as in the areas north of the oceanic Polar Front (90 vs. <40 g C m⁻² yr⁻¹ ), Wind-generated vertical mixing (in association with passing atmospheric depressions) and turbulence generated on and around banks in the southern part are crucial in accounting for this difference. In the northern half of the Barents Sea a pronounced upper layer stability caused by the supply of meltwater from seasonal ice retards mixing so that “new” production is small following the depletion of winter nutrients – high productivity is restricted to the 20–50 km wide ice edge bloom.

Year-to-year variations in fish stocks are pronounced and have always been so – there exists no “ecological balance” in any meaningful sense. These variations are clearly related to the influx of Atlantic water to the Barents Sea (i.e. “warm” years are high-productive years) and, thus, ultimately to the same forcing factors that determine climate variations in coastal Europe. Thus, sound management may diminish the annual fluctuations in fish stock sizes but certainly cannot eradicate them.