@article{fdi:010068148,
  title = {{N}itrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the {VAHINE} mesocosm experiments},
  author = {{K}napp, {A}. {N}. and {F}awcett, {S}. {E}. and {M}artinez-{G}arcia, {A}. and {L}eblond, {N}. and {M}outin, {T}. and {B}onnet, {S}ophie},
  editor = {},
  language = {{ENG}},
  abstract = {{I}n a coastal lagoon with a shallow, 25 m water column off the southwest coast of {N}ew {C}aledonia, large-volume (similar to 50 m(3)) mesocosm experiments were undertaken to track the fate of newly fixed nitrogen ({N}). {T}he 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 ({NO}3-) assimilated prior to the start of the 23-day experiments to {N}-2 fixation by the end of the experiments. {W}hile the delta {N}-15 of the sinking particulate {N} ({PN}sink) flux changed during the experiments, the delta {N}-15 of the suspended {PN} ({PN}susp) and dissolved organic {N} ({DON}) pools did not. {T}his is consistent with previous observations that the delta {N}-15 of surface ocean {N} pools is less responsive than that of {PN}sink to changes in the dominant source of new {N} to surface waters. {I}n spite of the absence of detectable {NO}3- in the mesocosms, the delta {N}-15 of {PN}sink indicated that {NO}3- 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. {T}he low rates of organic {N} export during the first 14 days were largely supported by {NO}3-, and phytoplankton abundance data suggest that sinking material primarily comprised large diatoms. {C}oncurrent molecular and taxonomic studies indicate that the diazotroph community was dominated by diatom-diazotroph assemblages ({DDA}s) at this time. {H}owever, these {DDA}s 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. {T}he unicellular cyanobacterial diazotroph, a {C}yanothece-like {UCYN}-{C}, proliferated during the last phase of the experiments when {N}-2 fixation, primary production, and the flux of {PN}sink increased significantly, and delta {N}-15 budgets reflected a predominantly diazotrophic source of {N} fueling export. {A}t this time, the export flux itself was likely dominated by the non-diazotrophic diatom, {C}ylindrotheca closterium, along with lesser contributions from other eukaryotic phytoplankton and aggregated {UCYN}-{C} cells, as well as fecal pellets from zooplankton. {D}espite 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 {PN}sink) 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}susp pools. {T}his 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}.},
  keywords = {{NOUVELLE} {CALEDONIE} ; {NOUMEA} {LAGON}},
  booktitle = {},
  journal = {{B}iogeosciences},
  volume = {13},
  numero = {16},
  pages = {4645--4657},
  ISSN = {1726-4170},
  year = {2016},
  DOI = {10.5194/bg-13-4645-2016},
  URL = {https://www.documentation.ird.fr/hor/fdi:010068148},
}