15N tracers were used in sub-tropical and Antarctic waters to study the production of nitrite from nitrate and ammonia, and the uptake of nitrate and ammonia. It was found that ammonia was the predominant source of nitrite in the primary nitrite maximum in sub-tropical waters and in the Ross Sea in summer, while nitrate and ammonia were both important in the Scotia Sea in early spring....
Olson, Robert J.. 1981. "15N tracer studies of the primary nitrite maximum." Journal of Marine Research 39, (2). https://elischolar.library.yale.edu/journal_of_marine_research/1540