Diazotrophic <i>Trichodesmium</i> impact on UV–Vis radiance and pigment composition in the western tropical South Pacific

Abstract

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.

Copernicus Marine Service Ocean State Report, Issue 3

“…For the entire Pacific Islands (green line, Box 3, Figure 3.1.2(d)), Chla mimics that of the western Pacific (blue line, Box 1) with a lower amplitude due to the weaker correlation with Central Pacific ENSO events (yellow line, Box 2). In the Fijian Archipelago, with the inverse, Chla concentration is increasing between 0% and 2% per year (Figure 3.1.1(c)), indicating a high positive response of phytoplankton, or/and a shift in phytoplankton composition (Dupouy et al 2018).…”

Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)

“…This has been interpreted as reflecting a successful way of living within the plankton, using filaments as a physical substrate for juvenile development and/or as a food source, and is facilitated by M. gracilis being immune to cyanobacterial toxins harmful to other species of copepods (O'Neil and Roman, 1994;Eberl and Carpenter, 2007). A relationship between Oncaea and Trichodesmium was previously suggested by Dupuy et al (2016) in the Indian Ocean around Madagascar, based on stable isotope data. However, we found no significant relationship for Pleuromamma and Euchaeta, despite their association with Trichodesmium observed by Azimuddin et al (2016) in the western Pacific, nor for Corycaeus.…”

The composition and distribution of semi-labile dissolved organic matter across the southwest Pacific

“…Samples were transferred into 40 mL plastic vials (Falcon; previously cleaned with 10 % HCl and Milli-Q water), frozen at −30 • C and freeze-dried. The obtained powder was hydrolyzed with 1 M HCl for 20 h at 100 • C and the samples were again freeze-dried to remove the HCl acid (Murrell and Hollibaugh, 2000;Engel and Handel, 2011). The dried samples were diluted in 4 mL of Milli-Q water, filtered through quartz wool and pipetted into scintillation vials for liquid chromatographic analysis.…”

“…Major polysaccharides are constituted by dissolved combined neutral sugars (DCNS), which are generally measured as their monosaccharide constituents (sum of fucose, rhamnose, arabinose, galactose, glucose, mannose and xylose) after acid hydrolysis (Mc-Carthy et al, 1996;Aluwihare et al, 1997;Skoog and Benner, 1997;Kirchman et al, 2001;Panagiotopoulos and Sempéré, 2005). Other minor carbohydrate constituents of DOC include the amino sugars (glucosamine, galactosamine and muramic acid; Benner and Kaiser, 2003), uronic acids (glucuronic and galacturonic acids; Hung et al, 2003;Engel and Handel, 2011), methylated and dimethylated sugars (Panagiotopoulos et al, 2013), heptoses (Panagiotopoulos et al, 2013), and sugar alcohols (Van Pinxteren et al, 2012).…”