Nitrogen isotope tracing of dissolved inorganic nitrogen behaviour in tidal estuaries

Concentration versus salinity diagrams are the most widely used method to d erive estuarine sources and sinks of nitrogen. This method can not distingu ish conservative mixing due to a lack of activity, from that due to an appr oximate balance of sources and sinks. The combined study of concentration a nd nitrogen isotopic signatures may provide this information because balanc ed sources and sinks do affect the isotopic composition of dissolved inorga nic nitrogen. This is illustrated with three cases of European tidal estuar ies: the Scheldt (Belgium/The Netherlands), the Thames (U.K.) and the Loire Estuary (France). Concentration versus salinity diagrams for nitrate showe d conservative mixing for the Thames and Loire estuaries and nitrate genera tion in the Scheldt Estuary. The delta N-15 of nitrate versus salinity diag ram confirmed the conservative behaviour for the Thames Estuary and the non -conservative behaviour for the Scheldt Estuary, but revealed active nitrat e turnover in the Loire Estuary. Concentration and delta N-15 of ammonium v ersus salinity diagrams consistently showed non-conservative mixing as a re sult of ammonium consumption. (C) 2001 Academic Press.