Latitudinal comparisons of equatorial Pacific zooplankton

Zooplankton biomass and rates of ingestion, egestion and production in the equatorial Pacific Ocean along 140degreesW and 180degrees exhibit maximum values in the High-Nutrient Low-Chlorophyll (HNLC) zone associated with equatorial upwelling (5degreesS-5degreesN) as compared to the more oligotrophic regions to the north and south. Zooplankton biomass and rates are not usually highest on the equator, but increase downstream of the upwelling center as the zooplankton populations exhibit a delayed response to enhanced phytoplankton production. The vertical distribution of zooplankton biomass in the equatorial HNLC area tends to be concentrated in surface waters and is more uniform with depth in oligotrophic regions to the north and south of the equatorial upwelling zone. In general, the amount of mesozooplankton (> 200 mum) carbon biomass is approximately 25% of estimated phytoplankton biomass and 30% of bacterial biomass in the HNLC area of the equatorial Pacific Ocean. Zooplankton grazing on phytoplankton is low in the equatorial Pacific Ocean, generally <5% of the total chlorophyll-a standing stock grazed per day. Based on estimates of metabolic demand, it is apparent that zooplankton in the equatorial Pacific Ocean are omnivores, consuming primarily microzooplankton and detritus. Estimated zooplankton growth rates in the warm waters of the HNLC equatorial Pacific Ocean are high, ranging from 0.58 d(-1) for 64-200 mum zooplankton to 0.08 d(-1) for 1000-2000 mum zooplankton. Thus, the numerical and functional response of equatorial zooplankton to increases in phytoplankton production are more rapid than normally occurs in sub-tropical and temperate waters. Potential zooplankton fecal pellet production, estimated from metabolic demand, is approximately 1.6 times the estimated gravitational carbon flux at 150 m in the zone of equatorial upwelling (5degreesS-5degreesN) and 1.1 times the export flux in the more oligotrophic regions to the north and south. The active flux of carbon by diel migrant zooplankton in the HNLC zone is a minor fraction of the gravitational flux (2% at 140degreesW, 4% at 180degrees) but increases in the more oligotrophic regions to the north and south where there is a deeper mixed layer and a greater relative proportion of diel migrant zooplankton. (C) 2002 Elsevier Science Ltd. All rights reserved.