Fringing marshes are generally believed to act as a filter for the estuarine and riverine water because they can trap nutrients both in inorganic and organic form from the floodwater. This perception is based on classical exchange studies, where budgets for the flood and ebb water are compared. However, these studies are rather inaccurate and reveal only the overall changes. In a Flemish-Dutch co-operation project, we used the stable isotope 15N as a tracer in a large-scale experiment to study the nitrogen cycling in a freshwater marsh fringing the Scheldt River. In the field, we added 15N-ammonium to the flood water of a tidal creek feeding a marsh area (3500 m2) and subsequently traced the fate of this labelled N through the entire ecosystem. We analysed the different pools of 15N in the flood and ebb water as well as in the sediment, plants and fauna within the marsh. In addition, discharge characteristics of the creek such as tidal height, water movement and concentrations of the different N pools was measured. This whole ecosystem experiment revealed that nitrification, the oxidation of ammonium to nitrate, is one of the fundamental processes regulating the N-budget in this freshwater marsh (Fig. 1). Moreover, mass balance calculations showed that whole-system nitrification rates were much (up to 10 times) higher than obtained from measurements of nitrification in the floodwater. This suggests that this process mainly takes place on the sediment surface of the marsh. Combined with a number of other experiments dealing with the short and long term fate of N, the overall goal is to construct an integrated view of the role of tidal marshes in the nitrogen retention and processing of ammonium and nitrate.
