Vertical flux regulation by zooplankton in the northern Barents Sea during Arctic spring

The influence of zooplankton on vertical carbon export was investigated during three field investigations conducted on and off the northern Barents Sea shelf during bloom conditions in July 2003, 2004 and May 2005. Short-term patterns in vertical flux of organic matter were measured at high vertical resolution (eight depths) in the upper 200 m at 11 stations. Vertical flux of particulate organic carbon (POC) was highly variable between stations and depths, depending on water mass characteristics such as stratification, phytoplankton bloom development, and zooplankton-related activities. Detailed investigations of the vertical flux composition revealed that faecal pellets (FPs) produced by larger meso- and macrozooplankton comprised on average 20% of the vertical POC flux (average for all depths and stations). The relative importance of FP carbon (FPC) increased with depth, and at depths >60 m FPC comprised ∼30% of the vertical POC flux. The main contributors to the FPC flux varied depending on the prevailing water masses and the phytoplankton bloom stage. FPs produced by older copepodite stages of Calanus spp. dominated at most stations, while FP produced by appendicularians and euphausiids dominated at certain depths and stations. A conservative grazing estimate obtained by calculating the ingestion necessary to support the measured FPC at 50 m depth, suggests that average zooplankton community carbon ingestion equals the vertical POC export. This study clearly shows that zooplankton is important for vertical flux regulation. Zooplankton ingest POC in the range of 22–44% of the daily primary production, but accelerate the vertical flux through production of fast-sinking FP.