Structuring effects of bottom depth on depth-integrated and size-fractioned zooplankton biomass in the Barents Sea in autumn

Abstract The Institute of Marine Research monitors zooplankton on autumn cruises in the Barents Sea with a procedure for determination of size-fractioned dry weight (dw) biomass. We report here depth-integrated biomass (WP-2 net hauls from near the seafloor to the surface) of three size fractions (small <1 mm, medium 1–2 mm, large >2 mm) sampled over a period of 34 years (1991–2024), with a total number of 5370 sampling stations covering wide areas of Atlantic, Arctic, and mixed water masses. Bottom depth at the sampling stations had strong explanatory power for the variation of biomass for each of the three fractions and for total biomass, explaining ∼25% of the total variance for the medium-size fraction and total biomass. Biomass increased with increasing bottom depth, from ∼3 g dw m−2 at 100 m depth to ∼12 g dw m−2 at 400 m depth for total biomass. There were also temporal and spatial patterns, and depth along with year and geographical areas explained ∼40% of the total variance. A separate set of samples from the upper 100 m showed no or very slight effect of variation in the underlying bottom depth on the zooplankton biomass in the surface-near layer. The effect of bottom depth was similar when data were broken down into Atlantic, Arctic, and Mixed domains, with strong dominance of Calanus finmarchicus and Calanus glacialis in the Atlantic and Arctic domains, respectively. These two Calanus copepods dominate the mesozooplankton biomass overall, and the strong effect of bottom depth on depth-integrated biomass is interpreted to reflect the seasonal descent of the Calanus species in the autumn situation, with accumulation in deeper basins and trenches surrounding shallower bank areas.