Abstract: Phosphorus is the second most important limiting nutrient for the growth of autotrophic organisms in coastal and oceanic environments and is present in a wide range of chemical forms both dissolved and particulate. Within the dissolved inorganic fraction of phosphorus, orthophosphates are the so-called soluble reactive phosphorus. This nutrient, as a central element in coastal and oceanic biogeochemical cycles, is one of the compounds measured for the physico-chemical quality indices established by the Water Framework Directive 2000/60/EC and one of the characteristics to be measured in the Marine Strategy Framework Directive 2008/56/EC. In this work, an estimation of the measurement uncertainty in the determination of orthophosphates in seawater by continuous flow analysis, based on ISO 15681-2:2003, in the range of 10–150 µg/L as P was carried out by means of a global model of uncertainty quantification from validation data. The results were coherent with previous interlaboratory comparisons of orthophosphates in seawater. Expanded uncertainty did not exceed 6.5 % in the entire range.

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