15NO3− Aquatic ecosystems 1 Chlorine compounds 1 Cost effectiveness 1 Decision making 1 Enrichment experiments 1 Gross NO3− immobilization 1 Gross nitrification 1 Isotope dilution method 1 Isotope dilution methods 1 Isotope enrichments 1 Isotopes 1 Mass spectrometry 1 Membrane inlet mass spectrometry 1 Nitrates 1 Nitrification 1 Potassium compounds 1 REOX/MIMS 1 REOX/membrane inlet mass spectrometry 1 Stable isotopes 1 ammonium 1 concentration (composition) 1 decision making 1 immobilization 1 mass spectrometry 1 nitrate 1 nitrogen cycle 1 stable isotope 1