Contributions of mesozooplankton to vertical carbon export in a coastal upwelling system

Mesozooplankton can directly impact global biogeochemical cycles by repackaging particulate organic carbon (POC) into dense, rapidly sinking fecal pellets and by undertaking vertical migrations that transport carbon and nutrients to depth. We assessed these contributions of mesozooplankton to vertical flux in the California Current Ecosystem, a productive but spatiotemporally variable coastal upwelling system, during cruises in April 2007 and October 2008. Sediment traps and Thorium-234 (234Th) disequilibrium measurements were used to assess the total passive flux of sinking POC, while pigment analyses and microscopic enumeration of sediment trap samples provided estimates of total fecal carbon transport. Identification of mesozooplankton in paired day-night, vertically stratified plankton tows allowed calculation of the active transport of carbon by the dominant taxa of vertically migrating mesozooplankton (particularly copepods and euphausiids). Across the range of 9 ecosystem conditions encountered on the cruises, recognizable fecal pellet mass flux varied from 3.5 to 135 mg C m−2 d−1 (3 to 94% of total passive flux) at the 100 m depth horizon. The active transport of carbon by migratory mesozooplankton taxa contributed an additional 2.4 to 47.1 mg C m−2 d−1 (1.9 to 40.5% of total passive flux). Inter-cruise comparisons suggest that fecal pellets contributed a higher portion of passive export during the productive spring cruise, when fecal material may have been responsible for close to 100% of sinking material. During the fall cruise, a gradient was observed with carbon export in productive water parcels driven by a large contribution of fecal pellets. In the less productive regions, fall vertical fluxes contained a higher proportion of marine snow and unidentifiable particles.