Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount: 1. General trends

The distribution of zooplankton in the oxygen minimum zone (OMZ) of the eastern tropical Pacific and near a seamount was investigated. The oxygen minimum layer appeared to be an important factor influencing the vertical distribution of zooplankton taxa below the thermocline. The maximum zooplankton in terms of biomass and numbers was in the thermocline zone, with a secondary maximum in the depth zone that included the lower OMZ interface (600-1000 m). Most taxonomic groups showed this secondary peak in abundance in the lower OMZ interface depth zone and minimum abundances above it. The secondary maximum in zooplankton at the lower OMZ interface seems to be unique to OMZ regions. Only larvaceans and mollusks did not appear to be strongly influenced by the minimum concentrations of oxygen. Four vertical distribution patterns characteristic of different taxonomic groups may be indicative of their different tolerances to minimum oxygen concentrations. The physical intrusion of the seamount (summit at 730 m depth) did not cause major changes in the distribution of pelagic zooplankton. Significant biological and physical differences between locations above and away from the seamount were found only within the upper OMZ (100-300 m).