ZOOPLANKTON ECOLOGY IN THE EASTERN TROPICAL PACIFIC OXYGEN MINIMUM ZONE ABOVE A SEAMOUNT - 2 - VERTICAL-DISTRIBUTION OF COPEPODS

The abundance and vertical distribution (0-1230 m) of copepods were st udied in the eastern tropical Pacific near the seamount Volcano 7 to e xamine the influence of the extreme oxygen minimum zone (OMZ). Maximum zooplankton biomass and copepod abundance were in the thermocline zon e. A secondary peak in biomass and copepod abundance was evident betwe en 600 and 1000 m, which included the depth of the lower interface of the OMZ. This prominent secondary peak in zooplankton is a feature uni que to OMZ regions. There were four general trends of vertical distrib ution of copepod abundance. These trends appeared to be related to the oxygen concentration and gradients. The most common vertical distribu tion was a pattern of maximum abundance in the mixed layer and thermoc line zones, with a secondary maximum in the zone of the lower OMZ inte rface (600-1000 m). Clausocalanus spp., Oncaea, spp., Euchaeta spp., O ithona spp. and Corycaeus spp. showed this trend. Low oxygen concentra tion did not appear to restrict these groups, since they were present throughout the OMZ. The second vertical distributional pattern was ver tical migration between the thermocline and the OMZ. Pleuromamma robus ta showed this pattern, with maximum abundance al night in the thermoc line zone and during the day in the core of the OMZ. In addition, ther e was a secondary maximum of abundance at the lower OMZ interface zone . The third type of distribution was shown by copepods that were abund ant in the upper OMZ and at the lower OMZ interface zones. Eucalanus i nermis, Haloptilus paralongicirrus and Heterostylites longicornis were dominant copepod species that exhibited this pattern. They were eithe r absent from the mixed layer or at similar abundances in the mixed la yer and upper OMZ. The fourth pattern was shown by copepod species tha t live primarily above the OMZ day and night. The majority of the spec ies appeared to be tolerant of the extreme low oxygen concentrations. Rhincalanus spp. was the dominant copepod that was excluded by low oxy gen concentrations. In general, species also found in other OMZ region s showed similar distributions in this study, indicating that low oxyg en is a major controlling factor. Some vertical niche separation among congeneric species was indicated for Eucalanidae, Metridiidae and Aug aptilidae. Published by Elsevier Science Ltd.