A system for determining a level of nitrate in a water sample, comprising: a. an optical flow cell which is at least partially transparent and which is configured to contain a sample of water; b. a first illuminator for illuminating the sample within the cell by light in a first wavelength, and a first photodetector for collecting the first-wavelength illumination, following the light passage through the sample; characterized by further comprising: c. a second illuminator for illuminating the sample within the cell by light in a second, fluorescence-excitation wavelength, and a second photodetector for collecting illumination in a third, fluorescence-emission wavelength from the sample; and d. an analysis unit for: d.1. determining a combined effect of nitrate+Dissolved Organic Carbon, referred to as nitrate+DOC, within the sample on absorbance, said combined effect of nitrate+DOC being proportional to a rate of absorbance of light due to said illumination by said first illuminator, said absorbance being determined from a difference between a level of illumination by said first illuminator and a level of collected illumination by said first photodetector; d.2. given (a) an intensity level by said second illuminator in said second, fluorescence-excitation wavelength, (b) a measured emission intensity from the sample in said third, fluorescence-emission wavelength, and (c) a predetermined calibration, determining a concentration of DOC within the sample, said DOC concentration being proportional to the intensity of said fluorescence emission from the sample; and d.3. deducting said effect of DOC based on the determined DOC concentration by subtracting said effect of DOC from said effect of nitrate+DOC on the absorbance, thereby to determine the concentration of nitrate within the sample. a. an optical flow cell which is at least partially transparent and which is configured to contain a sample of water; b. a first illuminator for illuminating the sample within the cell by light in a first wavelength, and a first photodetector for collecting the first-wavelength illumination, following the light passage through the sample; c. a second illuminator for illuminating the sample within the cell by light in a second, fluorescence-excitation wavelength, and a second photodetector for collecting illumination in a third, fluorescence-emission wavelength from the sample; and d. an analysis unit for: d.1. determining a combined effect of nitrate+Dissolved Organic Carbon, referred to as nitrate+DOC, within the sample on absorbance, said combined effect of nitrate+DOC being proportional to a rate of absorbance of light due to said illumination by said first illuminator, said absorbance being determined from a difference between a level of illumination by said first illuminator and a level of collected illumination by said first photodetector; d.2. given (a) an intensity level by said second illuminator in said second, fluorescence-excitation wavelength, (b) a measured emission intensity from the sample in said third, fluorescence-emission wavelength, and (c) a predetermined calibration, determining a concentration of DOC within the sample, said DOC concentration being proportional to the intensity of said fluorescence emission from the sample; and d.3. deducting said effect of DOC based on the determined DOC concentration by subtracting said effect of DOC from said effect of nitrate+DOC on the absorbance, thereby to determine the concentration of nitrate within the sample. d.1. determining a combined effect of nitrate+Dissolved Organic Carbon, referred to as nitrate+DOC, within the sample on absorbance, said combined effect of nitrate+DOC being proportional to a rate of absorbance of light due to said illumination by said first illuminator, said absorbance being determined from a difference between a level of illumination by said first illuminator and a level of collected illumination by said first photodetector; d.2. given (a) an intensity level by said second illuminator in said second, fluorescence-excitation wavelength, (b) a measured emission intensity from the sample in said third, fluorescence-emission wavelength, and (c) a predetermined calibration, determining a concentration of DOC within the sample, said DOC concentration being proportional to the intensity of said fluorescence emission from the sample; and d.3. deducting said effect of DOC based on the determined DOC concentration by subtracting said effect of DOC from said effect of nitrate+DOC on the absorbance, thereby to determine the concentration of nitrate within the sample.