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Nitrifier activity and growth were measured in the Scheldt estuary over a salinity gradient. Measurements were made during all 4 seasons using 15 N enriched ammonium and 14 C labeled carbon incorporation. Established conversion ratios are often used to convert the growth of nitrifiers (measured as the incorporation of carbon) to nitrifying activity (ie oxidation of ammonium to nitrate). Our study demonstrated that the conversion of growth rates to nitrifying activity induces uncertainty because activity and growth of nitrifiers may be uncoupled. The C: N conversion ratio appears to be oxygen and temperature dependent. We advocate the use of 15 N stable isotope techniques to study nitrification: this technique measures the actual activity of nitrifiers without the disadvantages involved in using inhibitors, and thus allows light inhibition to be measured.