Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments
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 inorga...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2016-08-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/4645/2016/bg-13-4645-2016.pdf |
| _version_ | 1811191835814526976 |
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| author | A. N. Knapp S. E. Fawcett A. Martínez-Garcia N. Leblond T. Moutin S. Bonnet |
| author_facet | A. N. Knapp S. E. Fawcett A. Martínez-Garcia N. Leblond T. Moutin S. Bonnet |
| author_sort | A. N. Knapp |
| collection | DOAJ |
| description | 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. |
| first_indexed | 2024-04-11T23:42:36Z |
| format | Article |
| id | doaj.art-ba9d8471c57c4d78a2aefc4adc44ab00 |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-04-11T23:42:36Z |
| publishDate | 2016-08-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-ba9d8471c57c4d78a2aefc4adc44ab002022-12-22T03:56:45ZengCopernicus PublicationsBiogeosciences1726-41701726-41892016-08-0113164645465710.5194/bg-13-4645-2016Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experimentsA. N. Knapp0S. E. Fawcett1A. Martínez-Garcia2N. Leblond3T. Moutin4S. Bonnet5Earth, Ocean, and Atmospheric Science Department, Florida State University, 117 N Woodward AVE, Tallahassee, FL, 32306, USADepartment of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544, USAMax Plank Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, GermanyMediterranean Institute of Oceanography (MIO), IRD/CNRS/Aix-Marseille University, IRD Noumea, 101 Promenade R. Laroque, BPA5, 98848 Noumea Cedex, FranceMediterranean Institute of Oceanography (MIO), IRD/CNRS/Aix-Marseille University, IRD Noumea, 101 Promenade R. Laroque, BPA5, 98848 Noumea Cedex, FranceMediterranean Institute of Oceanography (MIO), IRD/CNRS/Aix-Marseille University, IRD Noumea, 101 Promenade R. Laroque, BPA5, 98848 Noumea Cedex, FranceIn 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.http://www.biogeosciences.net/13/4645/2016/bg-13-4645-2016.pdf |
| spellingShingle | A. N. Knapp S. E. Fawcett A. Martínez-Garcia N. Leblond T. Moutin S. Bonnet Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments Biogeosciences |
| title | Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments |
| title_full | Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments |
| title_fullStr | Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments |
| title_full_unstemmed | Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments |
| title_short | Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments |
| title_sort | nitrogen isotopic evidence for a shift from nitrate to diazotroph fueled export production in the vahine mesocosm experiments |
| url | http://www.biogeosciences.net/13/4645/2016/bg-13-4645-2016.pdf |
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