Spectrophotometric Characterisation of Surface Lakewater Samples: Implications for the Quantification of Nitrate and the Properties of Dissolved Organic Matter

Filtered lakewater samples, mainly collected in the province of Torino (Piedmont, NW Italy) were characterised from a spectrophotometric point of view. Spectral data were then used for the direct determination of nitrate by three-wavelength photometry, which should account for the spectral interference by dissolved organic matter (DOM), and the results compared with nitrate quantification by ion chromatography. The spectrophotometric method proved very suitable for nitrate measurement, with unity slope (μ±σ = 0.99±0.03) of the correlation plot (spectral vs. ion chromatography data) up to 0.1 mM nitrate, and with r² = 0.97 for 26 data points. Lakewater spectra were also used for the characterisation of DOM bymeans of the specific absorption at 285 and 254 nm (absorbance vs. NPOC, the latter to quantify the DOM amount), and the E₂/E₃ and E₃/E₄ indexes. The latter two make only use of radiation absorption data (250 vs. 365 and 300 vs. 400 nm). It could be concluded that lakewater DOM is mainly composed of autochthonous material (biologically produced aliphatic compounds and only a minor fraction of aromatic groups), with generally low molecular weight and degree of aromaticity. Some exceptions could be found in high-mountain lakes, but it should also be considered that NPOC measurement cannot be avoided if DOM origin is to be studied. From the absorption spectrum alone it is possible to get indication on the aromaticity degree of radiation-absorbing DOM, but most of the autochthonous DOM would escape spectrophotometric characterisation.