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| Titel |
Photoproduction of ammonium in the southeastern Beaufort Sea and its biogeochemical implications |
| VerfasserIn |
H. Xie, S. Bélanger, G. Song, R. Benner, A. Taalba, M. Blais, J.-É. Tremblay, M. Babin |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 8 ; Nr. 9, no. 8 (2012-08-10), S.3047-3061 |
| Datensatznummer |
250007235
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| Publikation (Nr.) |
 copernicus.org/bg-9-3047-2012.pdf |
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| Zusammenfassung |
| Photochemistry of dissolved organic matter (DOM) plays an important role in
marine biogeochemical cycles, including the regeneration of inorganic
nutrients. DOM photochemistry affects nitrogen cycling by converting
bio-refractory dissolved organic nitrogen to labile inorganic nitrogen,
mainly ammonium (NH4+). During the August 2009 Mackenzie Light and
Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of
NH4+ photoproduction (i.e. photoammonification) were determined using
water samples from the SE Beaufort Sea, including the Mackenzie River
estuary, shelf, and Canada Basin. The photoammonification efficiency
decreased with increasing wavelength across the ultraviolet and visible
regimes and was higher in offshore waters than in shelf and estuarine
waters. The efficiency was positively correlated with the molar
nitrogen:carbon ratio of DOM and negatively correlated with the absorption
coefficient of chromophoric DOM (CDOM). Combined with collateral
measurements of CO2 and CO photoproduction, this study revealed a
stoichiometry of DOM photochemistry with a CO2 : CO : NH4+ molar ratio
of 165 : 11 : 1 in the estuary, 60 : 3 : 1 on the shelf, and 18 : 2 : 1 in the Canada
Basin. The NH4+ efficiency spectra, along with solar photon fluxes,
CDOM absorption coefficients and sea ice concentrations, were used to model
the monthly surface and depth-integrated photoammonification rates in 2009.
The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1
on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore;
the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1
and 11.3 μmol m−2 d−1. The offshore
depth-integrated rate in August (8.0 μmol m−2 d−1) was
comparable to the missing dissolved inorganic nitrogen (DIN) source required
to support the observed primary production in the upper 10-m layer of that
area. The yearly NH4+ photoproduction in the entire study area was
estimated to be 1.4 × 108 moles, with 85% of it being
generated in summer when riverine DIN input is low. Photoammonification
could mineralize 4% of the annual dissolved organic nitrogen (DON)
exported from the Mackenzie River and provide a DIN source corresponding to
7% of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities
could increase correspondingly to 6%, 11%, and 2100 times.
Photoammonification is thus a significant nitrogen cycling term and may fuel
previously unrecognized autotrophic and heterotrophic production pathways in
the surface SE Beaufort Sea. |
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