|Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments |
(Article) Publié: Biogeosciences, vol. 13 p.4645-4657 (2016)
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In a coastal lagoon with a shallow, 25 m water column off the southwest coast of New Caledonia, large-volume (similar to 50 m(3)) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8 mu M dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO3-) assimilated prior to the start of the 23-day experiments to N-2 fixation by the end of the experiments. While the delta N-15 of the sinking particulate N (PNsink) flux changed during the experiments, the delta N-15 of the suspended PN (PNsusp) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the delta N-15 of surface ocean N pools is less responsive than that of PNsink to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO3- in the mesocosms, the delta N-15 of PNsink indicated that NO3- continued to fuel a significant fraction of export production (20 to 60 %) throughout the 23-day experiments, with N-2 fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO3-, and phytoplankton abundance data suggest that sinking material primarily comprised large diatoms. Concurrent molecular and taxonomic studies indicate that the diazotroph community was dominated by diatom-diazotroph assemblages (DDAs) at this time. However, these DDAs represented a minor fraction (<5 %) of the total diatom community and contributed very little new N via N-2 fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a Cyanothece-like UCYN-C, proliferated during the last phase of the experiments when N-2 fixation, primary production, and the flux of PNsink increased significantly, and delta N-15 budgets reflected a predominantly diazotrophic source of N fueling export. At this time, the export flux itself was likely dominated by the non-diazotrophic diatom, Cylindrotheca closterium, along with lesser contributions from other eukaryotic phytoplankton and aggregated UCYN-C cells, as well as fecal pellets from zooplankton. Despite comprising a small fraction of the total biomass, UCYN-C was largely responsible for driving export production during the last similar to 10 days of the experiments both directly (similar to 5 to 22% of PNsink) and through the rapid transfer of its newly fixed N to other phytoplankton; we infer that this newly fixed N was transferred rapidly through the dissolved N (including DON) and PNsusp pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and delta N-15 with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON.