Abundance and biomass of copepods and cladocerans in Atlantic and Arctic domains of the Barents Sea ecosystem

Zooplankton in the Barents Sea have been monitored annually with a standard procedure with determination of size-fractioned biomass since the mid-1980s. Biomass of copepods and cladocerans was estimated based on measured abundance and individual weights taken from literature. Calanus species were dominant, making up similar to 85% of the estimated biomass of copepods. The second most important taxon was Oithona spp. (similar to 0.5 g dry weight (dw) m(-2), similar to 10%), followed by Metridia spp. (similar to 0.15 g dw m(-2), 2-3%) and Pseudocalanus spp. (0.10-0.15 g dw m(-2), 1-5%). Estimated biomass of cladoceran taxa (Evadne and Podon) was low (0.01 g dw m(-2)). Calanus spp. contributed most of the biomass of the medium size fraction (1-2 mm), whereas small copepod species (Oithona, Pseudocalanus and others) contributed to the small size fraction (<1 mm). Estimated biomass of Calanus spp. and of the sum of small copepod species were both positively correlated with measured total zooplankton biomass (R-2 = 0.72 and 0.34, respectively). The biomass ratio of small copepod species to Calanus was similar in Atlantic and Arctic water masses (similar to 0.15-0.2) but tended to increase with decreasing total biomass. This suggests a shift to relatively larger roles of small copepods as Calanus and total biomass decrease.