Zooplankton fecal pellet flux drives the biological carbon pump during the winter–spring transition in a high-Arctic system

Recent research highlighted significant marine biological activity during the Arctic winter, with poorly known implications for the biological carbon pump. We used moored instruments to (1) track the development of the pelagic food web of a high-Arctic marine ecosystem from winter to spring, and (2) assess the role of zooplankton-mediated processes in the sinking export of particulate organic carbon (POC). Zooplankton collected by a sediment trap at 40 m depth in Kongsfjorden showed a shift in species composition in February coinciding with an inflow of Atlantic water and the return of sunlight. The Atlantic copepod Calanus finmarchicus and the Arctic Calanus glacialis became dominant in the post-inflow assemblage of large mesozooplankton. However, large copepods were never abundant (0.3–4.6 ind m⁻³) in January–April in the upper 40 m. Despite the low chlorophyll fluorescence, POC export increased substantially, from 2–13 mg C m⁻² d⁻¹ in January–February to 13–35 mg C m⁻² d⁻¹ in March–April 2014. By late March, zooplankton fecal pellets contributed largely (23–100%) to this significant POC export before the phytoplankton bloom. The lack of change in copepod and euphausiid population sizes suggests that enhanced feeding activity in the surface layer supported the increasing fecal pellet export. Our results revealed the swift response of active zooplankton in winter, evidenced by increased carbon export, to improved food availability.