Carbon dynamics of the planktonic food web in the central equatorial Pacific

Mesozooplankton biomass and grazing within the euphotic zone were measured in two field seasons in 1992 during the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) in the central equatorial Pacific (EqPac). The first season was characterized by an El Nino event whereas the second one was characterized by typical climatological conditions. Mesozooplankton grazing was similar during the El Nino and typical conditions. In both situations, mesozooplankton grazing in the equatorial Pacific was too low to keep the growth of phytoplankton in check. Results also suggest that neither grazing nor ammonium excretion of mesozooplankton may give rise to the high-nutrient low-chlorophyll conditions of the equatorial Pacific.^ Diel vertical migration did not take place during El Nino. However, on average 38% of the mesozooplankton biomass performed diel vertical migration during typical conditions. The amount of organic carbon transported from the euphotic zone to the deep water by the migrators was 1.1 mmol C $\rm m\sp{-2}\ d\sp{-1},$ which was equivalent to 44% of that from gravitational particle sinking estimated from $\sp{234}$Th measurements during the same period.^ The role of advection and the biota in carbon export from the euphotic zone of the equatorial Pacific are examined by modeling. Results suggest that (1) total organic carbon exported from the euphotic zone of the equatorial Pacific is 8.7 (El Nino) and 20.3 mmol C $\rm m\sp{-2}\ d\sp{-1}$ (typical conditions); (2) export of organic carbon by advection accounts for 78-88% of the organic carbon exported from the area, 67-72% of the export of organic carbon due to advection is dissolved organic carbon; (3) 76-83% of the primary production is respired by bacteria and microzooplankton. Assuming that microzooplankton allocate 40% of their carbon consumption to respiration, the condition of steady state requires that bacteria allocate 68-78% of their carbon consumption to respiration. Sensitivity analysis indicates that relatively small departures ($\u3c$25%) from the values in (3) above would determine whether the equatorial Pacific, in the model, is a local source or sink of organic carbon. Contrary to common statements in the literature, the equatorial Pacific may not always be a source of organic carbon.