Uptake of dissolved inorganic nitrogen in turbid, tidal estuaries

Ammonium and nitrate uptake was measured in 6 European tidal estuaries (Ems , Rhine, Scheldt, Loire, Gironde and Douro) using N-15-tracer techniques. U ptake rates of ammonium and nitrate ranged from 0.005 to 1.56 mu mol N l(-1 ) h(-1) and 0.00025 to 0.25 mu mol N l(-1) h(-1), respectively, and differe d significantly between and within estuaries. Analysis of nitrogen uptake u sing the relative preferential index (RPI) indicated ammonium to be the pre ferred 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 estuar ine-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 ammoni um and particulate nitrogen are efficiently recycled, most allochthonous or ganic matter is extensively microbially modified before export, burial, or consumption by higher trophic levels.