Quantification of multiple simultaneously occurring nitrogen flows in the euphotic ocean

Abstract. The general features of the N cycle in the sunlit region of the ocean are well known, but methodological difficulties have previously confounded simultaneous quantification of transformation rates among the many different forms of N, e.g., ammonium (NH₄⁺), nitrite (NO₂⁻), nitrate (NO₃⁻), and particulate/dissolved organic nitrogen (PN/DON). However, recent advances in analytical methodology have made it possible to employ a convenient isotope labeling technique to quantify in situ fluxes among oft-measured nitrogen species within the euphotic zone. Addition of a single ¹⁵N-labeled NH₄⁺ tracer and monitoring of the changes in the concentrations and isotopic compositions of the total dissolved nitrogen (TDN), PN, NH₄⁺, NO₂⁻, and NO₃⁻ pools allowed us to quantify the ¹⁵N and ¹⁴N fluxes simultaneously. Constraints expressing the balance of ¹⁵N and ¹⁴N fluxes between the different N pools were expressed in the form of simultaneous equations, the unique solution of which via matrix inversion yielded the relevant N fluxes, including rates of NH₄⁺, NO₂⁻, and NO₃⁻ uptake; ammonia oxidation; nitrite oxidation; DON release; and NH₄⁺ uptake by bacteria. The matrix inversion methodology that we used was designed specifically to analyze the results of incubations under simulated in situ conditions in the euphotic zone. By taking into consideration simultaneous fluxes among multiple N pools, we minimized potential artifacts caused by non-targeted processes in traditional source–product methods. The proposed isotope matrix method facilitates post hoc analysis of data from on-deck incubation experiments and can be used to probe effects of environmental factors (e.g., pH, temperature, and light) on multiple processes under controlled conditions.