Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment

Abstract

Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Nouméa lagoon (New Caledonia), where high N 2 fixation rates and abundant diazotroph organisms were observed, aimed to assess the role of the nitrogen input through N 2 fixation in carbon production and export and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1-D vertical biogeochemical mechanistic model was used in addition to the in situ experiment to enrich our understanding of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N) and phosphate (P) fluxes. The mesocosms were intentionally enriched with ∼ 0.8 µmol L −1 of inorganic P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N 2 fixation, primary production (PP) and C export increased by 201, 208 and 87 %, respectively, consistent with the trends observed in the mesocosms (+124, +141 and +261 % for N 2 fixation, PP and C export, respectively). In total, 5-10 days were necessary to obtain an increase in primary and export productions after the dissolved inorganic phosphate (DIP) enrichment, thereby suggesting that classical methods (short-term microcosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model enabled us to monitor the fate of fixed N 2 by providing the proportion of DDN in each compartment (inorganic and organic) of the model over time. At the end of the simulation (25 days), 43 % of the DDN was found in the non-diazotroph organisms, 33 % in diazotrophs, 16 % in the dissolved organic nitrogen pool, 3 % in the particulate detrital organic pool and 5 % in traps, indicating that N 2 fixation was of benefit to non-diazotrophic organisms and contributed to C export.

Nutrient availability and the ultimate control of the biological carbon pump in the Western Tropical South Pacific Ocean

“…In a recent mesocosms experiment, large increases in N2 fixation rates, PP rates and carbon export were obtained after a DIP enrichment of WTSP waters (Berthelot et al, 2015). Nevertheless, several days were necessary to measure significant increases indicating that regular short term experiments to establish nutrient limitation as usually operated , may not be relevant in WTSP conditions (Gimenez et al, 2016). 5…”

Dynamics of phytoplankton and heterotrophic bacterioplankton in the western tropical South Pacific Ocean along a gradient of diversity and activity of diazotrophs

“…As such, the activity of N 2 fixers is not sufficient to sustain all nitrogen needs of heterotrophic bacteria, whatever are the intermediary processes relating both fluxes, although it can reach a 520 Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-556 Manuscript under review for journal Biogeosciences 15 N fixed by Trichodesmium reaches rapidly non-fixing diatoms (Foster et al, 2011;Bonnet et al, 2016).…”

“…At sites LDA and LDB, the addition of the 3 elements NPG stimulated BP more than P alone or N alone, suggesting possible NP co-limitation of heterotrophic prokaryotes. Furthermore, if N was shown to be the first limiting nutrient during short time scale experiment, addition of P stimulates N 2 fix, PP and export at larger 555 time scales (Van Den Brock et al, 2004;Berthelot et al, 2015, Gimenez et al, 2016.…”

“…The addition of selected organic molecules such as glucose (Dekaezemacker et al, 2013) or natural organic matter such as Transparent Expolymer Particles, can influence N 2 fix rates (Benavides et al, 2015). Finally, recent experiments based on incubation with 15 N-labeled N 2 coupled to nano-SIMS analyses also demonstrated that a rapid transfer, at the scale of 24 to 48 h, can occur between N 2 fixers, non-fixing phytoplankton and heterotrophic prokaryotes 90 (Bonnet et al, 2016).…”

Diazotrophy as the main driver of planktonic production and biogeochemical C, N, P cycles in theWestern Tropical South Pacific Ocean: results from a 1DV biogeochemical-physical coupled model

“…Since experimental studies highlighted the significant contribution of N 2 fixation as a new source of N for planktonic ecosystems in the surface layer (Martínez et al, 1983;Karl et al, 1997;Capone et al, 2005), the process of diazotrophy associated (or not) with explicitly-represented diazotroph organisms has been implemented in numerous biogeochemical models in the last decades. As a result, more and more modeling studies have 30 been investigating the role of diazotrophy at global scale (Moore et al, 2001(Moore et al, , 2004Monteiro et al, 2011), at regional scale (Coles and Hood, 2007;Zamora et al, 2010), at local scale (Fennel et al, 2001;Gimenez et al, 2016) or more specifically at population scale, such as the work on Trichodesmium sp. by Rabouille et al (2006).…”