We assessed the influence of the marine dia-zotrophic cyanobacterium Trichodesmium on the bio-optical properties of western tropical South Pacific (WTSP) waters (18–22 • S, 160 • E–160 • W) during the February–March 2015 OUTPACE cruise. We performed measurements of backscattering and absorption coefficients, irradiance, and radiance in the euphotic zone with a Satlantic MicroPro free-fall profiler and took Underwater Vision Profiler 5 (UPV5) pictures for counting the largest Trichodesmium spp. colonies. Pigment concentrations were determined by fluorimetry and high-performance liquid chromatography and picoplankton abundance by flow cytometry. Tri-chome concentration was estimated from pigment algorithms and validated by surface visual counts. The abundance of large colonies counted by the UVP5 (maximum 7093 colonies m −3) was well correlated to the trichome concentrations (maximum 2093 trichomes L −1) with an ag-gregation factor of 600. In the Melanesian archipelago, a maximum of 4715 trichomes L −1 was enumerated in pump samples (3.2 m) at 20 • S, 167 30 • E. High Trichodesmium abundance was always associated with absorption peaks of mycosporine-like amino acids (330, 360 nm) and high particulate backscattering, but not with high Chl a fluorescence or blue particulate absorption (440 nm). Along the west-to-east transect, Trichodesmium together with Prochlorococ-cus represented the major part of total chlorophyll concentration ; the contribution of other groups were relatively small or negligible. The Trichodesmium contribution to total chlorophyll concentration was the highest in the Melane-sian archipelago around New Caledonia and Vanuatu (60 %), progressively decreased to the vicinity of the islands of Fiji (30 %), and reached a minimum in the South Pacific Gyre where Prochlorococcus dominated chlorophyll concentration. The contribution of Trichodesmium to zeaxanthin was respectively 50, 40 and 20 % for these regions. During the OUTPACE cruise, the relationship between normalized water-leaving radiance (nL w) in the ultraviolet and visible and chlorophyll concentration was similar to that found during the BIOSOPE cruise in the eastern tropical Pacific. Principal component analysis (PCA) of OUTPACE data showed that nL w at 305, 325, 340, 380, 412 and 440 nm was strongly correlated to chlorophyll and zeaxanthin, while nL w at 490 Published by Copernicus Publications on behalf of the European Geosciences Union. 5250 C. Dupouy et al.: Trichodesmium impact on UV–Vis radiance and pigments and 565 nm exhibited lower correlations. These results, as well as differences in the PCA of BIOSOPE data, indicated that nL w variability in the greenish blue and yellowish green during OUTPACE was influenced by other variables associated with Trichodesmium presence, such as backscattering coefficient, phycoerythrin fluorescence and/or zeaxanthin absorption , suggesting that Trichodesmium detection should involve examination of nL w in this spectral domain.