Fecal pellet production by mesozooplankton in the subarctic Northeast Pacific Ocean

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AbstractMesozooplankton fecal pellets sinking from the sea‐surface constitute an important component of the biological carbon pump, which is variable in part due to differences in zooplankton community composition on various scales. Particulate organic carbon production as fecal pellets by the entire mesozooplankton community has, until now, not been directly measured. We measured fecal pellet carbon (FPC) production rates of the mesozooplankton community separated into five size classes (0.2 to > 5.0 mm), and rates of key taxa, in the upper 100 m of the subarctic Northeast Pacific Ocean, August–September, 2018. We scaled up FPC production rates to total FPC production in the epipelagic zone, which was 2.8‐fold higher at night than day due to diel vertical migration. Despite its lower total biomass, the smallest size class (0.2–0.5 mm) contributed more FPC than other size classes (up to 81.6%), due to high weight‐specific FPC production rates (3.0 μgFPC mgDW−1 h−1). The pelagic tunicate Salpa aspera had the highest individual FPC production rate (7.5 μgFPC ind−1 h−1) and when present, contributed most to community FPC production (up to 82.3%). While the large copepod Neocalanus cristatus dominated the 2.0–5.0 mm fraction (up to ~ 100% by biomass), its FPC production rate was low (0.4 μgFPC mgDW−1 h−1), likely due to minimal feeding and gut shrinkage preceding diapause. Comparison of FPC production with export recorded in sediment traps suggests high attenuation of smaller pellet classes in the epipelagic (86%). Our results provide direct measurements of mesozooplankton FPC production in a “low‐production, low‐export” region.