Size-fractioned zooplankton biomass in the Barents Sea: macroecological patterns across biogeography, climate, and varying ecosystem state (1989-2020)

The zooplankton community in the Barents Sea was monitored during autumn cruises from 1989-2020, with biomass determined as the depth-integrated dry weight (g m(-2)) of 3 size fractions (>2 mm, 1-2 mm, <1 mm). A large data set of 4543 sampling stations encompassing a subdivision of the Barents Sea was spatially averaged into 15 polygons based on topography. The resulting data set is used to describe relationships between the biomass of size fractions as expressions of the size structure of the zooplankton communities. Each of the 3 size fractions was positively related 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 (R-2 approximate to 0.8). The medium fraction contained 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 in 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.