Ammonium and nitrate uptake was measured in 6 European tidal estuaries (Ems, Rhine, Scheldt, Loire, Gironde and Douro) using 15 N-tracer techniques. Uptake rates of ammonium and nitrate ranged from 0.005 to 1.56 µmol N l -1 h -1 and 0.00025 to 0.25 µmol N l -1 h -1 , respectively, and differed significantly between and within estuaries. Analysis of nitrogen uptake using the relative preferential index (RPI) indicated ammonium to be the preferred substrate. The turnover times of particulate nitrogen (0.7 to 31 d) and dissolved ammonium (0.1 to 27 d) were similar to or shorter than estuarine-water residence times, whereas turnover times of dissolved nitrate (19 to 2160 d) were longer than residence times. Assimilation of nitrate in the water column of estuaries consequently does not influence its distribution, and most nitrate entering or produced in estuaries flushes through unless significant denitrification and/or burial in the sediment occur. As ammonium and particulate nitrogen are efficiently recycled, most allochthonous organic matter is extensively microbially modified before export, burial, or consumption by higher trophic levels.