2 Comparison of nitrifier activity versus growth in the Scheldt estuary - a turbid, tidal estuary in northern Europe

Nitrification, the microbial oxidation of ammonium (NH4 ), is one of the key nitrogen transformation processes. It is performed under oxic conditions by 2 groups of bacteria, one group that oxidizes ammonium to nitrite (NO2 ) and a second group that further oxidizes nitrite to nitrate (NO − 3 ). Nitrification links organic matter mineralization (ammonification) and nitrogen removal through denitrification (conversion of nitrate to nitrogen gas). Nitrifying bacteria are chemolithoautotrophic organisms which use energy from the oxidation of NH4 or NO − 2 to fix inorganic carbon in their biomass. Accordingly, nitrification links the flows of nitrogen with those of oxygen and carbon. In ammonium-rich systems, nitrification may make significant contributions to, or even dominate, total oxygen consumption and carbon fixation. Nitrifiers were responsible for up to 60 % of the total O2 consumption in the Seine estuary, France (Garnier et al., 2001). Nitrification is the most important carbon fixation process in the upper part of the Scheldt estuary (Soetaert and Herman, 1995), where it accounts for 12 to 78 % of oxygen consumption (Gazeau et al., 2005). One by-product formed during nitrification is nitrous oxide gas (N2O), which contributes to global warming and ozone depletion. de Wilde and de Bie