Water samples were collected using a 12 x 10 L Niskin bottle rosette sampler equipped with a conductivity, temperature, and pressure instrument package (SBE9, Sea-Bird Electronics, Bellevue, Washington, U.S.A.) and a sensor for dissolved oxygen (SBE43, Sea-Bird).
Water for ammonia oxidation rate incubations was collected directly from the rosette into 250 mL polycarbonate bottles.
Rates of water column ammonia oxidation were measured from bottle incubations with 15NH4Cl (Dugdale and Goering 1967; Sigman et al. 2001; Damashek et al. 2016). Isotope samples were analyzed at the University of California Davis Stable Isotope Facility or the Central Appalachians Stable Isotope Facility at the University of Maryland Center for Environmental Science with a ThermoFinnigan GasBench + PreCon trace gas concentration system interfaced to a ThermoScientific Delta V Plus isotope-ratio mass spectrometer.