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Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as aglobal hot spot of dinitrogen (N 2) fixation. Here, as in other marineenvironments across the oceans, N 2  fixation studies have focused onthe sunlit layer. However, studies have confirmed the importance of aphoticN 2  fixation activity, although until now only one had been performedin the WTSP. In order to increase our knowledge of aphotic N 2  fixationin the WTSP, we measured N 2  fixation rates and identified diazotrophicphylotypes in the mesopelagic layer along a transect spanning from NewCaledonia to French Polynesia. Because non-cyanobacterial diazotrophspresumably need external dissolved organic matter (DOM) sources for theirnutrition, we also identified DOM compounds using Fourier transform ioncyclotron resonance mass spectrometry (FTICRMS) with the aim of searching forrelationships between the composition of DOM and non-cyanobacterial N 2 fixation in the aphotic ocean. N 2  fixation rates were low (average0.63  ±  0.07 nmol N L −1  d −1) but consistently detected acrossall depths and stations, representing ∼  6–88 % of photicN 2  fixation. N 2  fixation rates were not significantlycorrelated with DOM compounds. The analysis of nifH gene ampliconsrevealed a wide diversity of non-cyanobacterial diazotrophs, mostly matchingclusters 1 and 3. Interestingly, a distinct phylotype from the majornifH subcluster 1G dominated at 650 dbar, coinciding with theoxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggeststhat the distribution of aphotic diazotroph communities is to some extentcontrolled by water mass structure. While the data available are still tooscarce to elucidate the distribution and controls of mesopelagicnon-cyanobacterial diazotrophs in the WTSP, their prevalence in themesopelagic layer and the consistent detection of active N 2  fixationactivity at all depths sampled during our study suggest that aphotic N 2  fixation may contribute significantly to fixed nitrogen inputs in this areaand/or areas downstream of water mass circulation.

biogeosciences

Aphotic N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean