The role of mesozooplankton community structure in fecal pellet carbon production in the subarctic northeast Pacific Ocean

Production of sinking fecal pellets is an important process by which zooplankton mediate particulate organic carbon (POC) export from the sunlit surface ocean to depth. Zooplankton fecal pellets usually comprise a high proportion of total sinking POC; thus quantifying fecal pellet production is important to understanding the biological carbon pump. We performed fecal pellet production experiments near Ocean Station Papa, in the NE subarctic Pacific Ocean, in late summer 2018, as part of the NASA EXPORTS program. We separately measured fecal pellet production (FPP) by eight dominant mesozooplankton taxa, including abundant, large seasonal vertically migrating copepods (e.g., Neocalanus spp.). Whole zooplankton community FPP experiments were also performed by live size-fractionating zooplankton into five size classes. FPP was measured in both types of experiments with animals collected in the epipelagic zone during day and night. The amphipod Themisto pacifica had the highest weight-specific FPP rate (2.9 x 10^-3 mgC mgDW^-1 h^-1), while the pelagic tunicate Salpa aspera had the highest individual FPP rate (1.4 x 10^-2 mgC individual^-1 h^-1). Surprisingly, fecal pellet production rates for large, Neocalanus cristatus copepods were low compared to other species, and thus while abundant, their contribution to community FPP was minor. When present, Salpa aspera contributed more to total fecal pellet carbon in the surface waters than any other species. Using whole community FPP experiments combined with size fractionated biomass data from MOCNESS tows, we determined that production of fecal pellets by epipelagic mesozooplankton at night was ~3 times higher than that during the day due to diel vertical migration. We discuss how variations in community structure influence total FPP, and the relative contribution of this pathway to flux in the region.