Impacts of ontogenetically migrating copepods on downward carbon flux in the western subarctic Pacific Ocean

To evaluate the impacts of ontogenetically (seasonally) migrating copepods on carbon transport to the mesopelagic zone, we investigated depth distribution, population structure, and feeding activity of the ontogentic copepod community in the western subarctic Pacific Ocean from day–night pairs of zooplankton samples down to 1000 m during the VERtical Transport In the Global Ocean (VERTIGO) program. Over the 31 July–16 August 2005 study period, the biomass of Neocalanus cristatus and Neocalanus plumchrus predominated in the near surface waters, while Neocalanus flemingeri was already dormant at depth. We observed a strong diel migration for Metridia pacifica, and a seasonal downward migration for Eucalanus bungii. Based on gut pigment analysis, ingestion rate of the copepod community was 214–375 mg C m−2 day−1, which was equal to 26–37% of the concurrent primary production. However, comparison of grazing estimated from gut pigments to calculated carbon demand of the copepod community indicates that phytoplankton comprised 37–59% of the ingested carbon. Thus, the copepod community appears to have also relied on detritus and microzooplankton for their nutrition, likely because primary production during this time was dominated by picophytoplankton too small to be grazed by these large copepods. Fecal pellet flux by the copepod community was estimated to account for 141–223% of the sedimentary particulate organic carbon (POC) flux at 150 m, suggesting considerable fragmentation and consumption of pellets in the upper layers. Fecal pellets alone were adequate to meet copepod carbon demand in the surface 0–150 m layer. Active carbon flux by diel migration of M. pacifica (respiration, egestion, and mortality) was 4–17 mg C m−2 day−1, equal to 6–44% of sedimentary POC flux at 150 m. Active carbon flux by N. flemingeri ontogenetic migration (i.e., respiration and mortality at depth) contributed 246 mg C m−2 year−1, equal to 9% of sedimentary POC flux at 1000 m. The imminent downward migration of N. cristatus and N. plumchrus would lead to an additional ontogenetic carbon flux on the order of 1719 mg C m−2 year−1. Copepod fecal pellet transport and active transport by diel and ontogenetic migration are thus important carbon fluxes during a season dominated by small phytoplankton, and ontogenetic migrants in the subarctic Pacific Ocean play a relatively more important role in active carbon flux compared with other open-ocean regions.