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

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) [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 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 N 2 fixation and that of chromophoric material via the stimulation of Synechococcus bloom.

Dynamics of transparent exopolymer particles (TEP) during the VAHINE mesocosm experiment in the New Caledonian lagoon

“…Samples were collected during the early morning of each day for 23 days with a clean Teflon pumping system from three selected depths (1, 6, 12 m) in each mesocosm (M1, M2, and M3) and outside (hereafter called "lagoon waters" -O). Based on the results of different biogeochemical and biological parameters during VAHINE Bonnet et al, 2016a;Turk-Kubo et al, 2015), three specific periods were discerned (see detailed description in Sect. 3.1) within which we have also investigated TEP dynamics: days 2-4 (P0) are the pre-fertilization days when the DIP concentrations were 0.02-0.05 µmol L −1 PO 3− 4 and combined dissolved inorganic nitrogen concentrations were extremely low; days 5-14 (P1); after fertilization on day 5, the PO 3− 4 concentrations were ∼ 0.8 µmol L −1 and diazotrophic populations were dominated by diatom-diazotroph associations.…”

“…Samples were spiked with 99 % 15 N 2 -enriched seawater (Mohr et al, 2010), incubated in situ under ambient light and seawater temperatures as detailed in Berthelot et al (2015) and Bonnet et al (2016a). Data and protocols of sampling for diazotrophic abundance and calculation of their respective growth rates are detailed fully in Turk-Kubo et al (2015). Briefly, samples (from 6 m only) were collected every other day from the mesocosms, and from the lagoon waters.…”

“…Abundances are reported as nifH copies L −1 as the number of nifH copies per genome in these diazotrophs is uncertain. Growth and mortality rates were calculated for individual diazotrophs inside the mesocosms when abundances were higher than the limit of quantification for two consecutive sampling days as detailed in Turk-Kubo et al (2015).…”

“…At this initial stage the total Chl a concentrations averaged around 0.2 µg L −1 in the lagoon wa-ter and in the mesocosms and the phytoplankton consisted of diverse representatives from the cyanobacteria (Prochlorococcus, Synechococcus), and diatoms such as Pseudosolenia calcar-avis (Leblanc et al, 2016). During P0, the most abundant members of the diazotrophic community in the lagoon waters were Richelia-Rhizosolenia (Het-1), the unicellular UCYN-A1, UCYN-A2, UCYN-C, and the filamentous Trichodesmium (Turk-Kubo et al, 2015). Fertilization of the mesocosms with DIP on day 4 stimulated a two-stage response by the diazotrophic community that was further reflected by many of the measured chemical and biological parameters Bonnet et al, 2016a, b;Turk-Kubo et al, 2015).…”

“…During P0, the most abundant members of the diazotrophic community in the lagoon waters were Richelia-Rhizosolenia (Het-1), the unicellular UCYN-A1, UCYN-A2, UCYN-C, and the filamentous Trichodesmium (Turk-Kubo et al, 2015). Fertilization of the mesocosms with DIP on day 4 stimulated a two-stage response by the diazotrophic community that was further reflected by many of the measured chemical and biological parameters Bonnet et al, 2016a, b;Turk-Kubo et al, 2015). After fertilization, from day 5 through day 14 (P1), excluding a significant increase in N 2 fixation rates, the functional community-wide biological responses (Chl a, PP, BP, BA) remained relatively low and similar to the values for P0 and for P1 in the outside lagoon waters Leblanc et al, 2016;.…”