Improved algorithm for the computation of nitrate concentrations in seawater using an in situ ultraviolet spectrophotometer

Improvements in the data processing algorithm and calibration procedures have greatly increased the accuracy of nitrate measurements using an in situ ultraviolet spectrophotometer (ISUS). Two major changes in the algorithm involve application of a temperature-dependent correction to the bromide spectrum and then using the observed temperature and salinity to subtract the bromide component before fitting nitrate. By reducing the degrees of freedom in calculating nitrate concentrations, the accuracy of the ISUS instrument is substantially improved. The new algorithm was tested in environments ranging from the Southern Ocean to oligotrophic eastern Pacific seawater and found to be applicable at all temperatures and depths. The standard error of the estimate for regression between ISUS nitrate values and discrete samples measured by standard wet chemistry methods for the combined data set is reduced by greater than 2-fold (1.4 down to 0.65 mu M) using the new algorithm. This corresponds to a 5-fold reduction in variance (2.0 down to 0.4 mu M-2). Although biofouling and calibration drift remain issues for any instrument deployed in situ for long periods of time, using the measured salinity and temperature to correct the ultraviolet spectra before the nitrate calculations will reduce the impacts of these confounding processes.