Water samples were collected as part of a 32-day cruise in the Eastern Tropical South Pacific, aiming to study nitrification in this oxygen minimum region of the ocean. In the Event Log for RR2311, transect stations are designated "T" and process stations are designated "PS". Nutrients were collected on the first cast upon arrival at a process station, and in one case on the last day, of a 2-5 day station occupation. Ammonium concentrations were initially planned to be measured in high-resolution depth profiles at each major process and transect station. However, due to the exceedingly low concentrations detected by the shipboard method, only the data from the first process station were included, with remaining samples frozen for later analysis ashore. Ammonium concentration was measured manually using fluorometric methods (Holmes et al. 1999), with water collected using Niskin bottles. Five ml volumes were analyzed immediately upon retrieval without filtration. High-resolution profiles were also obtained for nitrite measurements at both process and transect stations. Nitrite concentration was determined manually using colorimetric methods (Hansen and Koroleff 1999) on duplicate samples, measured immediately after collection without filtration.