Elevated NH+4 in a neutrally buoyant hydrothermal plume

Anomalously high concentrations of NH+4 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 NH+4 concentrations showed plume depth (∼2025 m) maxima of nearly 400 nM over the Endeavour Segment’s main vent field, on the same order as most subsurface NH+4 maxima. Plume NH+4 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 field, although most of the plume NH+4 appears to be transported beyond the ridge axial valley. Preliminary net specific NH+4 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 CH4 oxidation rates. The data suggest that NH+4 may be a significant substrate for in situ chemolithotrophic production of organic matter in the Endeavour Segment hydrothermal plume.