The monitoring of nitrates using the second derivative of UV–vis spectra has been tested for a large set of samples (≈900) covering a large range of freshwater environments in the Moselle River drainage basin (headwater streams in the Vosges Mountains; tributaries, including the Madon River and Vologne River; and waters of various origins exposed to anthropogenic activities in villages). Satisfactory linear correlations (coefficient of determination > 0.74) were obtained between nitrate concentrations (measured by ion chromatography with a conductivity detector) and the maxima of the second derivative in the UV range for sets of mesotrophic and eutrophic samples. The meaningfulness of the linear regressions was verified by applying a Fisher-Snedecor test using a level of confidence of 0.05. The quality of the correlation decreased when the samples came from oligotrophic environments due to the limited sensitivity of the ion chromatography technique. UV–vis spectroscopy combined with the second derivative method appears to be a good alternative for monitoring nitrate-related eutrophication in the laboratory as well as in situ using submersible spectrophotometers.