Top‐Down Control of Ammonia Oxidizers by Grazing in the North Pacific

Abstract The impact of grazing pressure on ammonia oxidizers (AO) has never been quantified in the field. Here we develop a new method to quantify the grazing rates on AO in aquatic systems. We introduce 15NH4+ tracer into the traditional dilution experiments to measure AO's apparent growth rates by using the end‐product of 15NH4+ oxidation, that is, 15NOx−. Field studies in the North Pacific revealed that 15NOx− in the end‐product was sensitive enough to detect AO's grazing rates. Experiments from the lower euphotic zone showed NH4+ replete growth rates of 0.40 d−1 and 0.77 d−1 of AO and in situ grazing rates on AO of 0.41 d−1 and 0.45 d−1, respectively, indicating a strong top‐down control by grazing on AO. Compiled data show a vertical decoupling between ammonia oxidation rates and AO abundance within the euphotic zone, indicating that strong grazing may have affected the distribution of AO in the global ocean.