Abstract. We assessed the influence of the marine diazotrophic 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. Trichome 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 aggregation 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 Prochlorococcus 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 Melanesian 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
(nLw) 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 nLw at 305, 325, 340, 380, 412 and 440 nm was
strongly correlated to chlorophyll and zeaxanthin, while nLw at
490 and 565 nm exhibited lower correlations. These results, as well as
differences in the PCA of BIOSOPE data, indicated that nLw
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 nLw in this spectral
domain.
Selenium supply affects chlorophyll concentration and biomass production of maize (Zea mays L.)
