Elevated NH4+ in a neutrally buoyant hydrothermal plume

Anomalously high concentrations of NH4+ in hydrothermal vent discharge fluids were tracked in the resulting neutrally buoyant plume over the unsedimented Endeavour Segment, Juan de Fuca Ridge. Vertical profiles of NH4+ concentrations showed plume depth (similar to 2025 m) maxima of nearly 400 nM over the Endeavour Segment's main vent field, on the same order as most subsurface NH4+ maxima. Plume NH4+ values decreased to just above background deep water values (50-70 nM) in older plume water at a station 15 km southwest of the vent held, although most of the plume NH4+ appears to be transported beyond the ridge axial valley. Preliminary net specific NH4+ removal rates determined within the plume maximum (0.04-0.13 d(-1)) and corresponding turnover times (8-28 d) are similar to values for CH, oxidation rates. The data suggest that NH4+ may be a significant substrate for in situ chemolithotrophic production of organic matter in the Endeavour Segment hydrothermal plume. (C) 1998 Elsevier Science Ltd. All rights reserved.