Zooplankton ecology in the eastern tropical Pacific oxygen minimum zone above a seamount: 2. Vertical distribution of copepods

Abstract

The abundance and vertical distribution (0-1230 m) of copepods were studied in the eastern tropical Pacific near the seamount Volcano 7 to examine the influence of the extreme oxygen minimum zone (OMZ). Maximum zooplankton biomass and copepod abundance were in the thermocline zone. A secondary peak in biomass and copepod abundance was evident between 600 and 1000 m, which included the depth of the lower interface of the OMZ. This prominent secondary peak in zooplankton is a feature unique to OMZ regions. There were four general trends of vertical distribution of copepod abundance. These trends appeared to be related to the oxygen concentration and gradients. The most common vertical distribution was a pattern of maximum abundance in the mixed layer and thermocline zones, with a secondary maximum in the zone of the lower OMZ interface (600-1000 m). Clausocalanus spp., Oncaea, spp., Euchaeta spp., Oithona spp. and Corycaeus spp. showed this trend. Low oxygen concentration did not appear to restrict these groups, since they were present throughout the OMZ. The second vertical distributional pattern was vertical migration between the thermocline and the OMZ. Pleuromamma robusta showed this pattern, with maximum abundance at night in the thermocline zone and during the day in the core of the OMZ. In addition, there was a secondary maximum of abundance at the lower OMZ interface zone. The third type of distribution was shown by copepods that were abundant in the upper OMZ and at the lower OMZ interface zones. Eucalanus inermis, Haloptilusparalongicirrus and Heterostylites longicornis were dominant copepod species that exhibited this pattern. They were either absent from the mixed layer or at similar abundances in the mixed layer and upper OMZ. The fourth pattern was shown by copepod species that live primarily above the OMZ day and night. The majority of the species appeared to be tolerant of the extreme low oxygen concentrations. Rhincalanus spp. was the dominant copepod that was excluded by low oxygen concentrations. In general, species also found in other OMZ regions showed similar distributions in this study, indicating that low oxygen is a major controlling factor. Some vertical niche separation among congeneric species was indicated for Eucalanidae, Metridiidae and Augaptilidae.