Isotopic composition of hydrothermal epiplume zooplankton: evidence of enhanced carbon recycling in the water column

We compare stable carbon and nitrogen isotope ratios in zooplankton living immediately above the Endeavour Ridge hydrothermal vent plume with similar data from the upper ocean and deep off-axis regions. Results over 5 years indicate that unusual recycling of organic material is occurring within the vicinity of the upwelling plume. We hypothesize that the epiplume zooplankton at the Endeavour vent fields are trophically complex and opportunistic, consuming a mixture of settling pelagic organic material, as well as upwelled organic material from the bottom and newly synthesized organic material from the plume. Observational support for this hypothesis includes: (1) delta(15)N ratios of epiplume zooplankton of all trophic groups are heavier than those from non-vent deep-sea fauna, indicating greater fractionation due to increased recycling of organic material. This recycling decreases with increasing distance from the upwelling plume, but re-occurs 200 km to the south over a recent venting site; and (2) delta(13)C values in epiplume particle feeders and predators above the upwelling plume are unusually heavy and similar to values in some vent organisms, and in non-vent benthic communities found within a few kilometers of active vents. Atypically low delta(15) N ratios downstream in the direction of the advecting plume appear to be evidence of utilization of vent-derived chemosynthetic organic (de novo) material. In particular the dominant particle feeders Neocalanus spp. may be ingesting particles with delta(15)N ratios similar to those found within vent bacteria and consumers. This pattern intensifies away from the upwelling plume to a distance of at least 26 km, where the deep scattering layer associated with the plume is still evident. There was a statistically significant difference in delta15 N between near-vent and off-axis organisms. Specifically, deep-sea Neocalanus spp. and radiolarians (mixed assemblage) were found to have significantly different delta15 N values between areas, whereas the predaceous fish Cyclothone microdon did not. Other taxa were too inconsistently measured to test independently. The enhanced organic recycling in the vicinity of the plume is presumably facilitated by a combination of higher (plume-induced) turbulent kinetic energy in the water column and current-induced entrainment of bottom particulate material. Higher vent-induced near-bottom flow increases overall particle suspension while buoyant plumes carry mixed upper ocean and vent-derived organic material from the seafloor upwards to the epiplume community 200-300 m above the venting region. Further downstream, where an enhanced zooplankton scattering layer is still evident at plume depth, the upwelled bottom material has settled, revealing a clearer delta(15)N signal in near-plume zooplankton from the de novo vent-derived chemosynthetic organic material advected in the drifting plume. (C) 2002 Elsevier Science Ltd. All rights reserved.