Polar Biol (1994)14:405-411 9 Springer-Verlag 1994 Egil Sakshaug" Arne Bjorge 9 Bjorn Gulliksen 9 Harald Loeng 9 Fritjof Mehlum Structure, biomass distribution, and energetics of the pelagic ecosystem in the Barents Sea: A synopsis Received: 27 December 1993/Accepted: 31 January 1994 Abstract Biomass distribution and energetics of trophic levels in the pelagic ecosystem of the Barents Sea are presented as averages over several years for the whole Barents Sea using data from the research programme Pro Mare in 1984-1989 and mathematical ecosystem models. Average biomasses range from more than 3 tonnes carbon km - 2 (zooplankton) to 0.1 kg C km- 2 (polar bears) and P/B ratios from 300 (bacteria) to 0.035 (minke whales). However, the Barents Sea ecosys- tem is in a far from steady state with, for instance, capelin stocks ranging from 30-700 kgCkm -2 be- tween years and cod stocks from 150-700 kg C kin- 2. As a general rule, the various fish stocks grow adequately, albeit at different rates, in "warm" years characterized by large influxes of Atlantic water and high zooplankton productivity. The skewed populations distribution which arises in "warm" years may lead to grave imbalances in "cold" years and even to the "collapses" of stocks, such as of capelin in the eighties. The food requirements of average-sized stocks of cod, seabirds and marine mammals correspond to more than twice the average productiv- ity of capelin. Thus other species of pelagic fish E. Sakshaug (t~) Trondhjem Biological Station, The Museum, University of Trondheim, Bynesveien 46, N-7018 Trondheim, Norway A. Bjorge Norwegian Institute for Nature Research, P.O. Box 1037 Blindern, N-0315 Oslo, Norway B. Gulliksen Norwegian College of Fishery Science, University of Tromso, N-9037 Tromso, Norway H. Loeng Institute of Marine Research, P.O. Box 1870 Nordnes, N-5024 Bergen, Norway F. Mehlum Norwegian Polar Institute, P.O. Box 5072, N-0301 Oslo, Norway (herring, polar cod) and zooplankton obviously play major roles as prey for these animals. Introduction Pro Mare, the Norwegian Research Programme for Marine Arctic Ecology, studied the pelagic ecosystem of the Barents Sea from 1984 to 1989. The results have in part been published internationally in symposium proceedings (Sakshaug et al. 1991) and in a book in Norwegian, "Okosystem Barentshavet" (Sakshaug et al. 1992). The latter included an epilogue in which an attempt was made to summarize the structure, the biomass at different trophic levels, and the energetics of the Barents Sea pelagic ecosystem in overall and aver, age terms for several years. Because these results have not been published internationally, they are presented here in an extended version. Admittedly, such calcu- lations are subject to large uncertainties. They do not, for instance, reflect the extremely dynamic behaviour of the pelagic ecosystem of the Barents Sea, i.e. that popu- lation levels vary tremendously over the season and from year to year, and that there are marked horizontal gradients, particularly along transects from open to ice-covered areas. Nonetheless, the various trophic levels differ so much in terms of biomass and productiv, ity that our calculations may serve as a guideline for trophic structure in spite of the large uncertainties. The Barents Sea covers 1.4 million km 2 and is a shelf sea with an average depth of 230 m (Fig. 1; Loeng 1991). Ice, virtually all seasonal, covers from 0.7-1.1 million km 2 at its maximum distribution in Febru- ary-March (Vinje and Kvambekk 1991). In some years, the Barents Sea may be entirely ice-free in late summer and early autumn. The southern half of the Barents Sea is characterized by Atlantic water with a salinity > 3~ and 3-6~ (Loeng 1991). Close to the Norwegian coast and the Kola peninsula coastal waters with salinities < 34.7 are predominant. At 74-76~ the north- and