Here we report inline-formulaN2 fixation rates from a inline-formula∼ 4000 km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160inline-formula∘ E to 160inline-formula∘ W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. inline-formulaN2 fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631 inline-formula± 286 inline-formula
74pt15ptsvg-formulamathimg572d3011fd537fd8ff7a23225cd3a54b
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(range 196–1153 inline-formula
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) in MA waters and of 85 inline-formula± 79 inline-formula
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(range 18–172 inline-formula
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) in GY waters. Two cyanobacteria, the larger colonial filamentous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community (inline-formula> 80 %) and gene expression of the nifH gene (cDNA inline-formula> 10inline-formula5nifH copies Linline-formula−1) in MA waters. Single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry (nanoSIMS) at selected stations revealed that Trichodesmium was always the major contributor to inline-formulaN2 fixation in MA waters, accounting for 47.1–83.8 % of bulk inline-formulaN2 fixation. The most plausible environmental factors explaining such exceptionally high rates of inline-formulaN2 fixation in MA waters are discussed in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron) availability, seawater temperature and currents.