15N-labeling experiments to dissect the contributions of heterotrophic denitrification and anammox to nitrogen removal in the OMZ waters of the ocean.

In recent years, (15)N-labeling experiments have become a powerful tool investigating rates and regulations of microbially mediated nitrogen loss processes in the ocean. This chapter introduces the theoretical and practical aspects of (15)N-labeling experiments to dissect the contribution of denitrification and anammox to nitrogen removal in oxygen minimum zones (OMZs). We provide a detailed description of the preparation and realization of the experiments on board. Subsequent measurements of N(2) isotopes using gas chromatography mass spectrometry as well as processing of data and calculation of anammox and denitrification rates are explained. Important supplementary measurements are specified, such as the measurement of nanomolar concentrations of ammonium, nitrite, and nitrate. Nutrient profiles and (15)N-experiments from the Peruvian OMZ are presented and discussed as an example.