• Expert MSTP/AERES

• Chimie/Matériaux
  
• Chimie/Chimie analytique
  
• Planète et Univers/Océan, Atmosphère
  

In the framework of the VAHINE project, we investigated the spectral characteristics and the variability of dissolved and particulate chromophoric materials throughout a 23-day mesocosm experiment conducted in the southwest Pacific at the mouth of the New Caledonian coral lagoon (22 • 29.073 S–166 • 26.905 E) from 13 January to 4 Febru-ary 2013. Samples were collected in a mesocosm fertilized with phosphate at depths of 1, 6 and 12 m and in the surrounding waters. Light absorption coefficients of chro-mophoric dissolved organic matter (CDOM) [a g (λ)] and particulate matter [a p (λ)] were determined using a point-source integrating-cavity absorption meter (PSICAM), while fluorescent DOM (FDOM) components were determined from excitation-emission matrices (EEMs) combined with parallel factor analysis (PARAFAC). The evolutions of a g (λ) and a p (λ) in the mesocosm were similar to those of total chlorophyll a concentration, Synechococcus spp. and picoeukary-ote abundances, bacterial production, particulate organic nitrogen and total organic carbon concentrations, with roughly a decrease from the beginning of the experiment to days 9– 10, and an increase from days 9–10 to the end of the experiment. In the surrounding waters, the same trend was observed but the increase was much less pronounced, emphasizing the effect of the phosphate fertilization on the mesocosm's plankton community. Correlations suggested that both Syne-chococcus cyanobacteria and heterotrophic bacteria were strongly involved in the production of CDOM and absorption of particulate matter. The increase in phytoplankton biomass during the second part of the experiment led to a higher contribution of particulate material in the absorption budget at 442 nm. The three FDOM components identified (tryptophan-, tyrosine-and ultraviolet C (UVC) humic-like fluorophores) did not follow the evolution of CDOM and particulate matter, suggesting they were driven by different production/degradation processes. Finally, the results of this work support the idea there is indirect coupling between the dynamics of N 2 fixation and that of chromophoric material via the stimulation of Synechococcus bloom.

In the oligotrophic ocean characterized by nitrate (NO3-) depletion in surface waters, dinitrogen (N2) fixation and dissolved organic nitrogen (DON) can represent significant nitrogen (N) sources for the ecosystem. Here we deployed in New Caledonia large in situ mesocosms in order to investigate (1) the contribution of N2 fixation and DON use to primary production (PP) and particle export and (2) the fate of the freshly produced particulate organic N (PON) i.e. whether it is preferentially accumulated and recycled in the water column or exported out of the system. The mesocosms were fertilized with phosphate (P) in order to prevent P-limitation and promote N2 fixation. The diazotrophic community was dominated by diatoms-diazotrophs associations (DDAs) during the first part of the experiment for 10 days (P1) followed by the unicellular N2-fixing cyanobacteria UCYN-C the 9 last days (P2) of the experiment. N2 fixation rates averaged 9.8 ± 4.0 and 27.7 ± 8.6 nM d−1 during P1 and P2, respectively. NO3- concentrations (< 40 nM) in the mesocosms were a negligible source of N indicating that N2 fixation was the main driver of new production all along the experiment. The contribution of v fixation to PP was not significantly different (p > 0.05) during P1 (9.0 ± 3.3%) and P2 (12.6 ± 6.1%). However, the e ratio that quantifies the efficiency of a system to export particulate organic carbon (POCexport) compared to PP (e ratio = POCexport/PP) was significantly higher (p < 0.05) during P2 (39.7 ± 24.9%) than during P1 (23.9 ± 20.2%) indicating that the production sustained by UCYN-C was more efficient at promoting C export than the production sustained by DDAs. During P1, PON was stable and the total amount of N provided by N2 fixation (0.10 ± 0.02 μM) was not significantly different (p > 0.05) from the total amount of PON exported (0.10 ± 0.04 μM), suggesting a rapid and probably direct export of the recently fixed N2 by the DDAs. During P2, both PON concentrations and PON export increased in the mesocosms by a factor 1.5–2. Unlike in P1, this PON production was not totally explained by the new N provided by N2 fixation. The use of DON, whose concentrations decreased significantly (p < 0.05) from 5.3 ± 0.5 μM to 4.4 ± 0.5 μM, appeared to be the missing N source. The DON consumption of about 0.9 μM during P2 is even higher than the total amount of new N brought by N2 fixation (about 0.25 μM) during the same period. These results suggest that while DDAs mainly rely on N2 fixation for their N requirement, both N2 fixation and DON can be significant N-sources for primary production and particulate export following UCYN-C blooms in the New Caledonia lagoon and by extension in the N-limited Ocean where similar events are likely to occur.

Abstract. The particulate matter distribution and phytoplanktoncommunity structure of the iron-fertilized Kerguelenregion were investigated in early austral spring(October–November 2011) during the KEOPS2 cruise.The iron-fertilized region was characterized by a complexmesoscale circulation resulting in a patchy distributionof particulate matter. Integrated concentrations over200m ranged from 72.2 to 317.7 mgm−2 for chlorophyll a314 to 744 mmolm−2 for biogenic silica (BSi), 1106 to2268 mmolm−2 for particulate organic carbon, 215 to436 mmolm−2 for particulate organic nitrogen, and 29.3 to39.0 mmolm−2 for particulate organic phosphorus. Threedistinct high biomass areas were identified: the coastal watersof Kerguelen Islands, the easternmost part of the studyarea in the polar front zone, and the southeastern KerguelenPlateau. As expected from previous artificial and naturaliron-fertilization experiments, the iron-fertilized areas werecharacterized by the development of large diatoms revealedby BSi size–fractionation and high performance liquid chromatography(HPLC) pigment signatures, whereas the ironlimitedreference area was associated with a low biomassdominated by a mixed (nanoflagellates and diatoms) phytoplanktonassemblage. A major difference from most previousartificial iron fertilization studies was the observationof much higher Si : C, Si : N, and Si : P ratios (0.31±0.16,1.6±0.7 and 20.5±7.9, respectively) in the iron-fertilizedareas compared to the iron-limited reference station (0.13,1.1, and 5.8, respectively). A second difference is the patchyresponse of the elemental composition of phytoplanktoncommunities to large scale natural iron fertilization. Comparisonto the previous KEOPS1 cruise also allowed to addressthe seasonal dynamics of phytoplankton bloom overthe southeastern plateau. From particulate organic carbon(POC), particulate organic nitrogen (PON), and BSi evolutions,we showed that the elemental composition of the particulatematter also varies at the seasonal scale. This temporalevolution followed changes of the phytoplankton communitystructure as well as major changes in the nutrient stocks progressivelyleading to silicic acid exhaustion at the end of theproductive season.Our observations suggest that the specific response of phytoplanktoncommunities under natural iron fertilization ismuch more diverse than what has been regularly observed inartificial iron fertilization experiments and that the elementalcomposition of the bulk particulate matter reflects phytoplanktontaxonomic structure rather than being a direct consequenceof iron availability.

Magic ink: The chemical reactivity of a bromo-terminated self-assembled monolayer immobilized on an insulating substrate is quantified from the time evolution of patterns formed during the local reduction of the SAM (see picture; bpy=2,2'-bipyridine). The potential of this lithographic strategy to determine the reactivity of the self-assembled monolayer is described and compared to surface interrogation by electrochemical microscopy.