Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Noumea lagoon (New Caledonia) aimed to assess the role of the nitrogen input through N2 fixation on carbon production and export, and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1D-vertical biogeochemical mechanistic model was used in addition to the in situ experiment to complement our comprehension of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N), phosphate (P) fluxes. The mesocosms were intentionally enriched with 0.8 μmol.L−1 of 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, N2 fixation, primary production and C export increased by 201, 208 and 87 %, respectively, consistent with the observed trends observed in the mesocosms (+ 124 %, + 141 %, + 261 % for N2 fixation, PP and C export, respectively). The increase in primary and export productions became significant 10 days after the DIP enrichment, indicating that i) several days were necessary to obtain a significant response at the population scale, and ii) classical methods (short-term microscosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model allowed to follow the fate of fixed N2 by providing over time the proportion of DDN in each compartment (mineral and organic) of the model. 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 N2 fixation efficiently benefitted to non-diazotrophic organisms and contributed to C export.
The Role of Tidal Marsh Restoration in Fish Management in the San Francisco Estuary