Abstract
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 mouth 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 phosphate at depths of 1, 6 and 12 m and in the surrounding waters. Light absorption coefficients of chromophoric dissolved organic matter (CDOM) [ag(λ)] and particulate matter [ap(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 ag(λ) and ap(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 phosphate 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 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 N2 fixation and that of chromophoric material via the stimulation of Synechococcus bloom.
Highlights
Besides water itself, light absorption in the marine environment is due to three main biogeochemical constituents: (1) chromophoric dissolved organic matter (CDOM), known as gelbstoff, gilvin and yellow substances, and chromophoric particulate matter, subdivided into (2) phytoplankton, composed of both prokaryotic and eukaryotic species, and (3) non algal particles (NAP), comprising organic and minerogenic detritus, and heterotrophic organisms
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 mouth of the New Caledonian coral lagoon (22◦29.073 S–166◦26.905 E) from 13 January to 4 February 2013
Light absorption coefficients of chromophoric dissolved organic matter (CDOM) [ag(λ)] and particulate matter [ap(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)
Summary
Light absorption in the marine environment is due to three main biogeochemical constituents: (1) chromophoric dissolved organic matter (CDOM), known as gelbstoff, gilvin and yellow substances, and chromophoric particulate matter, subdivided into (2) phytoplankton (photoautotrophic microorganisms), composed of both prokaryotic (cyanobacteria) and eukaryotic species (diatoms, dinoflagellates, coccolithophores, . . . ), and (3) non algal particles (NAP), comprising organic and minerogenic detritus, and heterotrophic organisms. In addition to their key role in the oceanic carbon cycle, these three constituents strongly influence the underwater light field and the apparent optical properties of seawater The knowledge of their absorption spectra is essential for bio-optical modelling and remote sensing applications but can be used to investigate biological processes in the ocean. Even though CDOM absorption spectra are usually featureless, some “shoulders” have been observed sporadically in the UV and visible spectral domains and attributed to the presence of dissolved absorbing pigments released by phytoplankton cells: mycosporine-like amino acids (MAAs) at 310–320 or at 330–360 nm, and phaeopigments or non-chlorin metal-free porphyrins at 410–420 nm (Whitehead and Vernet, 2000; Röttgers and Koch, 2012; Organelli et al, 2014; Pavlov et al, 2014).
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