MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14485

ABSTRACT: Zooplankton was monitored with a standard procedure on autumn cruises in the Barents Sea with biomass determined as depth-integrated dry weight (g m^-2) in three size fractions (>2 mm, 1-2 mm, <1 mm). A large data set of 4543 sampling stations for the period 1989-2020 was spatially averaged for a subdivision of the Barents Sea into 15 polygons based on topography. The resulting data set is used to describe relationships between biomass of size fractions as expressions of the size structure of the zooplankton communities. Each of the three size fractions was related positively to the total zooplankton biomass (sum of fractions). The medium size fraction, which contained on average 48% of the total biomass, showed a strong linear correlation with total biomass (R² ~0.8). The medium fraction contains the older copepodite stages of Calanus species (C. finmarchicus and C. glacialis), which are dominant species interpreted to drive the overall changes in zooplankton biomass. The biomass of the small size fraction decreased with decreasing total biomass, but less so than the medium fraction. Thus, the small/medium biomass ratio increased as the total biomass decreased. This trend was most pronounced for shallow and central polygons, which coincide with the core feeding area of the large Barents Sea stock of capelin (Mallotus villosus). The change of the small/medium biomass ratio is interpreted to reflect a shift from Calanus to smaller zooplankton in response to predation by capelin and other planktivorous predators, and possibly also by warming due to climate change.