In situ ultraviolet spectrophotometry for high resolution and long-term monitoring of nitrate, bromide and bisulfide in the ocean

The design for an in situ ultraviolet spectrophotometer (ISUS) that can operate while submerged to depths of at least 2000 m is reported. We show that the ISUS can be used to make high resolution (∼1/s and 0.5 cm) and long-term (>3 months) measurements of the concentration of nitrate, bisulfide and bromide in seawater using the distinctive, ultraviolet absorption spectra of these chemical species. The precision, accuracy and stability of the chemical concentrations derived with the ISUS are sufficient for many biogeochemical studies. One standard deviation of the nitrate concentration in seawater is ∼0.5 μM and the limit of detection (3 SD) for one observation would be ∼1.5 μM. However, the noise is nearly random and significant reductions in the detection limit are possible by averaging multiple observations. The 95% confidence interval for a 30 s scan is 0.2 μM. Low temperatures appear to produce a bias (∼10% at 400 m depth in the ocean) in the nitrate concentration and in the salinity estimated from the bromide concentration. If an independent estimate of salinity is available, then the bias in nitrate can be eliminated by correcting nitrate concentrations by the same amount that the optical estimate of salinity is in error. The instrument has been deployed on a mooring in the equatorial Pacific for a 6-month period with no apparent degradation in performance during the first 3 months. Measurements of UV spectra at a height of 1 cm over the bottom in a cold seep at 960 m depth demonstrate the capability to detect bisulfide ion within the benthic boundary layer.