Net zooplankton and the biological pump: a comparison between the oligotrophic and mesotrophic equatorial Pacific

Abstract

Structure and functioning of the zooplankton community and their consequences on the export fluxes of carbon, nitrogen and phosphorus were studied in two contrasting systems of the equatorial Pacific: the oligotrophic TTS (Typical Tropical Structure) to the west, and the mesotrophic HNLC (High Nutrient-Low Chlorophyll) in the central Pacific. Data were collected during the FLUPAC cruise equatorial transect (September-October 1994) of R.V. L'Atalante and four 6–8-day long time-series stations made between 165°E and 150°W. Along the equator, a sharp 2.5-fold increase in mesozooplankton biomass (200–2000 pm) was observed between 174° and 172°W, with a simultaneous change in surface salinity and chlorophyll concentration, corresponding to the shift between the TTS and HNLC systems. No significant zonal trend was observed within the two systems. Compared with TTS, HNLC presented a significantly greater contribution of the (500–2000 μm) size class to total mesozooplankton biomass, less diel variations, and a shallower vertical distribution. Lower metabolic rates in HNLC were accounted for by different taxonomic compositions in the two areas. Microzooplankton (35–200 gm) had a rather uniform biomass in TTS and HNLC, presented no significant diel variations in the 0–100 or 0–200 m layers, and displayed a shallower vertical distribution than the mesozooplankton. Consequences of such zooplanktonic features on the “biological pump” are assessed. Dissolved nitrogen (DN) and phosphorus (DP) active fluxes, resulting from excretion of interzonal mesozooplankton migrants were 2.1 times higher in the TTS than in the HNLC. However, dissolved inorganic carbon active fluxes were equal in the TTS and HNLC systems, due to differences in C, N, P metabolisms. Combined mesozooplankton and estimated micronekton nitrogen active fluxes represented 40% of the passive flux as measured by sediment traps in the TTS, and 9% in the HNLC. Estimates of mesozooplankton fecal production in the pbotic zone lead to a 2-fold increment between the oligotrophic and mesotrophic stations, and a larger contribution of the fecal production to the sinking flux. It is, therefore, concluded that the mesozooplankton role in the biological pump is mainly passive in the HNLC system, in contrast to the TTS site.