The vertical distribution of nitrate in open ocean surface water

Abstract

Near-surface nitrate concentrations in and around warm core ring 82-B were measured using a chemiluminescent technique with a precision of ±2 nM. Concentrations close to the detection limit were encountered in the Sargasso Sea and surface concentrations were below the detection limit of colorimetric techniques (± 50 nM) at all stations. Nitrate concentrations generally increased exponentially with depth. The data are consistent with a concentration-dependent uptake by phytoplankton and a diffusional supply in a one-dimensional vertical model. The constants relating nitrate uptake to nitrate concentration were evaluated from the vertical nitrate distribution, and had time scales of 14 to 140 h. These time scales are appropriate for phytoplankton-mediated processes in the sea. The model demonstrates that phytoplankton uptake of nitrate exerts a control on both the vertical nitrate flux and gradient. Temporal changes in the nitrate content of the euphotic zone at ring center show at least 0.5 gC m −2 nitrate-based productivity in mid-June. Whether or not this change was the end of the spring bloom in the ring is uncertain, but the nitrate-based productivity is high and atypical of the parent Sargasso seawater.