ROLE OF MESOPELAGIC ZOOPLANKTON IN THE COMMUNITY METABOLISM OF GIANT LARVACEAN HOUSE DETRITUS IN MONTEREY BAY, CALIFORNIA, USA

The mucous feeding structures or 'houses' of the giant larvacean Batho chordaeus spp. provide a useful detrital system to study biological pr ocesses that mediate remineralization of particulate organic carbon in the mesopelagic zone: degradation by bacteria and grazing by zooplank ton. The role of particle-associated zooplankton in remineralization i n the mesopelagic zone has not previously been studied, mostly due to sampling difficulties. We collected houses between 100 and 500 m in Mo nterey Bay, California, USA, using a submersible ROV (remotely operate d Vehicle) and measured community metabolism on houses using oxygen el ectrodes. Houses were sites of elevated metabolic activity compared to surrounding waters. The average daily oxygen utilization indicates th at approximately 1% of house C is used daily to sustain community resp iration, although the rate is higher (8%) when large numbers of zoopla nkton are present. Estimated rates of zooplankton remineralization of houses are similar to bacterial remineralization rates reported for ot her types of detritus. Respiration rates provide minimal estimates of carbon transformations by communities on detritus, especially when met azoans are present. Based on published estimates of the relationship b etween zooplankton carbon consumption and respiration rates, and our m easurement of zooplankton abundance on houses, we calculate that a mea n of 6% and up to 43% of house C is ingested by zooplankton each day. Thus, a substantial part of the house could be consumed by detritus-fe eding zooplankton before sinking out of the mesopelagic zone. Particle -associated zooplankton are important in recycling carbon on these hou ses and potentially on other aggregates at depth, not only by consumin g and remineralizing detritus, but also by altering detritus through r epackaging it in fecal pellets, releasing it as DOC, and fragmenting i ts fragile structure into smaller particulate matter.