Diazotrophic Trichodesmium influence on ocean color and pigment composition in the South West tropical Pacific

We assessed the influence of the marine diazotrophic cyanobacterium Trichodesmium on the bio-optical properties of South West tropical Pacific 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, and took Underwater Vision Profiler 5 (UPV5) pictures for counting the largest Trichodesmium spp. colonies. Pigment concentrations were determined by fluorimetry and by high performance liquid chromatography and picoplankton abundance by flow cytometry. Trichome concentration was estimated from pigment algorithms and validated by surface visual counts. In result, the large colonies were well correlated to the trichome concentration estimates (though with a large factor of 600 to 900, due to aggregation processes). Large Trichodesmium abundance was always associated with particulate absorption at a peak of mycosporine-like amino acid absorption, and high particulate backscattering, but not with high fluorescence, high chlorophyll-a concentration, or blue particulate absorption in the water column. Along the West to East transect, Trichodesmium together with Prochlorococcus represented the major part of total chlorophyll and the other groups were negligible. Trichodesmium contribution to chlorophyll was the highest in the Melanesian Archipelago around New Caledonia and Vanuatu, progressively decreased to the vicinity of the Fiji Islands, and reached a minimum in the South Pacific gyre where the contribution of Prochlorococcus was maximum. At the frontal LDB, Trichodesmium and Prochlorococcus has almost same contributions. The relationship between normalized water-leaving radiance, in the ultraviolet and visible domains, nL w , and chlorophyll was generally similar to that found in the Eastern tropical at BIOSOPE. Principal component analysis (PCA) of OUTPACE data showed that nL w were strongly correlated to chlorophyll except in the green and yellow domains. These results, as well as differences in the PCA of BIOSOPE data, suggested that nL w variability in the green and yellow during OUTPACE was influenced by other variables, associated with Trichodesmium presence as the backscattering coefficient, phycoerythrin fluorescence, and/or zeaxanthin absorption. Trichodesmium detection should then involve examination of nL w at the green and yellow wavelengths.