Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments

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dc.contributor.authorKnapp, Angela N
dc.contributor.authorFawcett, Sarah E
dc.contributor.authorMartínez-Garcia, Alfredo
dc.contributor.authorLeblond, Nathalie
dc.contributor.authorMoutin, Thierry
dc.contributor.authorBonnet, Sophie
dc.date.accessioned2021-10-08T06:20:16Z
dc.date.available2021-10-08T06:20:16Z
dc.date.issued2016
dc.description.abstractIn a coastal lagoon with a shallow, 25 m water column off the southwest coast of New Caledonia, large-volume ( ∼  50 m<sup>3</sup>) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8 µM dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO<sub>3</sub><sup>−</sup>) assimilated prior to the start of the 23-day experiments to N<sub>2</sub> fixation by the end of the experiments. While the <i>δ</i><sup>15</sup>N of the sinking particulate N (PN<sub>sink</sub>) flux changed during the experiments, the <i>δ</i><sup>15</sup>N of the suspended PN (PN<sub>susp</sub>) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the <i>δ</i><sup>15</sup>N of surface ocean N pools is less responsive than that of PN<sub>sink</sub> to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO<sub>3</sub><sup>−</sup> in the mesocosms, the <i>δ</i><sup>15</sup>N of PN<sub>sink</sub> indicated that NO<sub>3</sub><sup>−</sup> continued to fuel a significant fraction of export production (20 to 60 %) throughout the 23-day experiments, with N<sub>2</sub> fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO<sub>3</sub><sup>−</sup>, 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<sub>2</sub> fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a <i>Cyanothece</i>-like UCYN-C, proliferated during the last phase of the experiments when N<sub>2</sub> fixation, primary production, and the flux of PN<sub>sink</sub> increased significantly, and <i>δ</i><sup>15</sup>N 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, <i>Cylindrotheca closterium</i>, 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  ∼  10 days of the experiments both directly ( ∼  5 to 22 % of PN<sub>sink</sub>) 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 PN<sub>susp</sub> pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and <i>δ</i><sup>15</sup>N with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON.
dc.identifier.apacitationKnapp, A. N., Fawcett, S. E., Martínez-Garcia, A., Leblond, N., Moutin, T., & Bonnet, S. (2016). Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments. <i>Biogeosciences</i>, 13(16), 4645 - 4657. http://hdl.handle.net/11427/34238en_ZA
dc.identifier.chicagocitationKnapp, Angela N, Sarah E Fawcett, Alfredo Martínez-Garcia, Nathalie Leblond, Thierry Moutin, and Sophie Bonnet "Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments." <i>Biogeosciences</i> 13, 16. (2016): 4645 - 4657. http://hdl.handle.net/11427/34238en_ZA
dc.identifier.citationKnapp, A.N., Fawcett, S.E., Martínez-Garcia, A., Leblond, N., Moutin, T. & Bonnet, S. 2016. Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments. <i>Biogeosciences.</i> 13(16):4645 - 4657. http://hdl.handle.net/11427/34238en_ZA
dc.identifier.issn1726-4170
dc.identifier.issn1726-4189
dc.identifier.ris TY - Journal Article AU - Knapp, Angela N AU - Fawcett, Sarah E AU - Martínez-Garcia, Alfredo AU - Leblond, Nathalie AU - Moutin, Thierry AU - Bonnet, Sophie AB - In a coastal lagoon with a shallow, 25 m water column off the southwest coast of New Caledonia, large-volume ( ∼  50 m<sup>3</sup>) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen (N). The mesocosms were intentionally fertilized with 0.8 µM dissolved inorganic phosphorus to stimulate diazotrophy. N isotopic evidence indicates that the dominant source of N fueling export production shifted from subsurface nitrate (NO<sub>3</sub><sup>−</sup>) assimilated prior to the start of the 23-day experiments to N<sub>2</sub> fixation by the end of the experiments. While the <i>δ</i><sup>15</sup>N of the sinking particulate N (PN<sub>sink</sub>) flux changed during the experiments, the <i>δ</i><sup>15</sup>N of the suspended PN (PN<sub>susp</sub>) and dissolved organic N (DON) pools did not. This is consistent with previous observations that the <i>δ</i><sup>15</sup>N of surface ocean N pools is less responsive than that of PN<sub>sink</sub> to changes in the dominant source of new N to surface waters. In spite of the absence of detectable NO<sub>3</sub><sup>−</sup> in the mesocosms, the <i>δ</i><sup>15</sup>N of PN<sub>sink</sub> indicated that NO<sub>3</sub><sup>−</sup> continued to fuel a significant fraction of export production (20 to 60 %) throughout the 23-day experiments, with N<sub>2</sub> fixation dominating export after about 2 weeks. The low rates of organic N export during the first 14 days were largely supported by NO<sub>3</sub><sup>−</sup>, 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<sub>2</sub> fixation; they were thus not important for driving export production, either directly or indirectly. The unicellular cyanobacterial diazotroph, a <i>Cyanothece</i>-like UCYN-C, proliferated during the last phase of the experiments when N<sub>2</sub> fixation, primary production, and the flux of PN<sub>sink</sub> increased significantly, and <i>δ</i><sup>15</sup>N 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, <i>Cylindrotheca closterium</i>, 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  ∼  10 days of the experiments both directly ( ∼  5 to 22 % of PN<sub>sink</sub>) 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 PN<sub>susp</sub> pools. This inference reconciles previous observations of invariant oligotrophic surface ocean DON concentrations and <i>δ</i><sup>15</sup>N with incubation studies showing that diazotrophs can release a significant fraction of their newly fixed N as some form of DON. DA - 2016 DB - OpenUCT DP - University of Cape Town IS - 16 J1 - Biogeosciences LK - https://open.uct.ac.za PY - 2016 SM - 1726-4170 SM - 1726-4189 T1 - Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments TI - Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments UR - http://hdl.handle.net/11427/34238 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/34238
dc.identifier.vancouvercitationKnapp AN, Fawcett SE, Martínez-Garcia A, Leblond N, Moutin T, Bonnet S. Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments. Biogeosciences. 2016;13(16):4645 - 4657. http://hdl.handle.net/11427/34238.en_ZA
dc.language.isoeng
dc.publisher.departmentDepartment of Oceanography
dc.publisher.facultyFaculty of Science
dc.sourceBiogeosciences
dc.source.journalissue16
dc.source.journalvolume13
dc.source.pagination4645 - 4657
dc.source.urihttps://dx.doi.org/10.5194/bg-13-4645-2016
dc.subject.otherBurns
dc.subject.otherDisaster Planning
dc.subject.otherHumans
dc.subject.otherMass Casualty Incidents
dc.subject.otherNational Health Programs
dc.subject.otherPractice Guidelines as Topic
dc.subject.otherSocieties, Medical
dc.subject.otherSouth Africa
dc.titleNitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments
dc.typeJournal Article
uct.type.publicationResearch
uct.type.resourceJournal Article
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