PELAGIC AND BENTHIC ECOLOGY OF THE LOWER INTERFACE OF THE EASTERN TROPICAL PACIFIC OXYGEN MINIMUM ZONE

The distributions of pelagic and benthic fauna were examined in relati on to the lower boundary of the oxygen minimum zone (OMZ) on and near Volcano 7, a seamount that penetrates this feature in the Eastern Trop ical Pacific. Although the broad, pronounced OMZ in this region is an effective barrier for most zooplankton, zooplankton abundances, zoopla nkton feeding rates, and ambient suspended particulate organic carbon (POC) peaked sharply in the lower OMZ (about 740-800 m), in associatio n with the minimum oxygen concentration and the increasing oxygen leve ls just below it. Zooplankton in the lower OMZ were also larger in siz e, and the pelagic community included some very abundant, possibly opp ortunistic, species. Elevated POC and scatter in the light transmissio n data suggested the existence of a thin, particle-rich, and carbon-ri ch pelagic layer at the base of the OMZ. Gut contents of planktonic de tritivores implied opportunistic ingestion of bacterial aggregates, po ssibly from this layer. Benthic megafaunal abundances on the seamount, which extended up to 730 m, peaked at about 800 m. There was a consis tent vertical progression in the depth of first occurrence of differen t megafaunal taxa in this depth range, similar to intertidal zonation. Although the vertical gradients of temperature, salinity, and oxygen were gradual at the lower OMZ interface (in contrast to the upper OMZ interface at the thermocline), temporal variability in oxygen levels d ue to internal wave-induced vertical excursions of the OMZ may produce the distinct zonation in the benthic fauna. The characteristics of th e lower OMZ interface result from biological interactions with the che mical and organic matter gradients of the OMZ. Most zooplankton are pr obably excluded physiologically from pronounced OMZs. The zooplankton abundance peak at the lower interface of the OMZ occurs where oxygen b ecomes sufficiently high to permit the zooplankton to utilize the high concentrations of organic particles that have descended through the O MZ relatively unaltered because of low metazoan abundance. A similar s cenario applies to megabenthic distributions. Plankton layers and a po tential short food chain (bacteria to zooplankton) at OMZ interfaces s uggest intense utilization and modification of organic material, local ized within a thin midwater depth zone. This could be a potentially si gnificant filter for organic material sinking to the deep-sea floor.