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

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.