Correcting Nitrate Profiles Measured by the In Situ Ultraviolet Spectrophotometer in Arctic Ocean Waters

Abstract

A recently commercialized sensor, the In Situ Ultraviolet Spectrophotometer (ISUS), can determine seawater nitrate concentrations rapidly. Procedures for incorporating ISUS data within routine conductivity, temperature, and depth (CTD) profiling studies have not been described. Using a CTD system with an ISUS, vertical hydrographic profiles with nutrient concentrations were determined at 42 sites across the shelf break on the Beaufort Sea, Alaska. Key data processing steps included elimination of redundant nitrate values, shifting the ISUS data so as to be simultaneous with the CTD data, and correcting the ISUS output voltages for warm-up drift. After regressing the corrected ISUS output voltages against nitrate concentrations determined on bottle samples, the ISUS-estimated nitrate concentrations reproduced the bottle results within +/- 0.28 μM. The highly detailed ISUS profiles showed mesoscale intrusions of low nutrient water on the upper continental slope and strong nutrient upwelling near the shelf break. Based on conductivity, temperature, depth (CTD) and ISUS data collected in the Beaufort Sea north of Alaska, we developed the processing procedures for the ISUS data collected during routine CTD vertical profiling. Specifically, our routines eliminated redundant measurement values, corrected for time-delay and for the warm-up drift found in the ISUS data. The results were compared and normalized to measured nitrate concentrations from bottle collections of seawater, which showed that ISUS data can correctly estimate seawater nitrate concentrations to within 0.28 μM.