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Evolution of dissolved and particulate chromophoric materials during the VAHINE mesocosm experiment in the New Caledonian coral lagoon (South West Pacific)

Abstract : 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 South West Pacific at the exit of the New Caledonian coral lagoon (22°29.073 S-166°26.905 E) from 13 January to 4 February 2013. Samples were collected in a mesocosm fertilized with phosphorus at 1, 6 and 12 m depth and in the surrounding waters. Light absorption coefficients of chromophoric dissolved organic matter (CDOM) (ag(λ)), particulate matter (ap(λ)) and CDOM + particulate matter (ag+p(λ)) were measured 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 ag(λ), ap(λ) and ag+p(λ) in the mesocosm were similar to those of total chlorophyll a concentration, Synechococcus spp. and picoeukaryote 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 phosphorus fertilization on the mesocosm's plankton community. Correlations suggested that both Synechococcus cyanobacteria and heterotrophic bacteria were strongly involved in the production of CDOM and absorption of particulate matter. The increase in phytoplankton activities 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 UVC humic-like fluorophores) did not follow the evolution of CDOM and particulate matter, proving that these were driven by different production/degradation processes. Finally, this works indicates a coupling between the dynamics of the N2 fixation and that of chromophoric material in the South West Pacific through Synechococcus bloom.
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M. Tedetti, L. Marie, R. Röttgers, M. Rodier, F. van Wambeke, et al.. Evolution of dissolved and particulate chromophoric materials during the VAHINE mesocosm experiment in the New Caledonian coral lagoon (South West Pacific). Biogeosciences Discussions, European Geosciences Union, 2015, 12, pp.17453-17505. ⟨10.5194/bgd-12-17453-2015⟩. ⟨insu-03634050⟩



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