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This work deals with the evaluation of the aqueous concentrations of dissolved reactive phosphorus (DRP), total phosphorus (TP), and ammonium nitrogen (N-NH₄) in surface water by means of direct online instrumentation. A portable, submersible, and automated analyzer designed to measure dissolved and total nutrient concentrations characterized by miniaturization of the entire analytical process was tested against laboratory methods. A total number of 36 water samples of different origin (i.e., rain, river, lake, and sewage waters) were analyzed and used in the comparison of DRP, TP, and N-NH₄ data. Raw data were distributed in a broad range of concentrations: 5–299 μg P/L for DRP, 7–97 μg P/L for TP, and 11–332 μg N/L for N-NH₄. Regression analysis underlined a high significant correlation between the measures of the probe and those of the laboratory (0.6 < R ² < 0.9; p < 0.001 …