Impact of aggregate-colonizing copepods on the biological carbon pump in a high-latitude fjord

Zooplankton consumption of sinking aggregates affects the quality and quantity of organic carbon exported to the deep ocean. Increasing laboratory evidence shows that small particle-associated copepods impact the flux attenuation by feeding on sinking particles, but this has not been quantified in situ. We investigated the impact of an abundant particle-colonizing copepod, Microsetella norvegica, on the attenuation of the vertical carbon flux in a sub-Arctic fjord. This study combines field measurements of vertical carbon flux, abundance, and size-distribution of marine snow and degradation rates of fecal pellets and algal aggregates. Female M. norvegica altered their feeding behavior when exposed to aggregates, and ingestion rates were 0.20 μg C ind.⁻¹ d⁻¹ on marine snow and 0.11 μg C ind.⁻¹ d⁻¹ on intact krill fecal pellets, corresponding to 48% and 26% of the females' body carbon mass. Due to high sea surface abundance of up to ~ 50 ind. L⁻¹, the population of M. norvegica had the potential to account for almost all the carbon removal in the upper 50 m of the water column, depending on the type of the aggregate. Our observations highlight the potential importance of abundant small-sized copepods for biogeochemical cycles through their impact on export flux and its attenuation in the twilight zone.