Downward export of carbon by diel migrant mesozooplankton in the central equatorial Pacific

Using data collected during cruises of the JGOFS equatorial Pacific Study in March/April and October of 1992 at the equator (140°W), we examine the downward transport of carbon by three size classes of die] migrant mesozooplankton (200–500 gm, 500–1000 μm and 1000–2000 gm). In addition to respiratory carbon flux, we consider the flux due to mortality of migrators below the euphotic zone. Diel migrant mesozooplankton biomass was estimated from the difference between nighttime and daytime biomass within the euphotic zone. Except for a four-day period early in the March/April cruise, mesozooplankton nighttime biomass was significantly larger than daytime biomass within the euphotic zone during both cruises. We estimate that the downward flux of carbon from the euphotic zone due to mesozooplankton die] vertical migrators was an average of 0.6 mmol Cm⁻² d⁻¹ and 1.1 mmol C m⁻² d⁻¹ during the March/April and October cruises, respectively. Addition of this flux to the gravitational particle sinking flux estimated from²³⁴Th measurements during the same period results in a 31 % increase in the carbon export flux from the euphotic zone in the equatorial Pacific during the March/April cruise and a 44% increase in the October cruise. The migratory flux is strongly dependent on whether feeding takes place below the euphoric zone, the length of time migrators spend in the deep waters, and the mortality rate of migrators.