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| Titel |
Benthic fluxes of dissolved organic nitrogen in the lower St. Lawrence estuary and implications for selective organic matter degradation |
| VerfasserIn |
M. Alkhatib, P. A. del Giorgio, Y. Gelinas, M. F. Lehmann |
| 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 ; 10, no. 11 ; Nr. 10, no. 11 (2013-11-22), S.7609-7622 |
| Datensatznummer |
250085437
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| Publikation (Nr.) |
 copernicus.org/bg-10-7609-2013.pdf |
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| Zusammenfassung |
| The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in
sediment porewaters was determined at nine locations along the St. Lawrence
estuary and in the gulf of St. Lawrence. In a previous manuscript (Alkhatib
et al., 2012a), we have shown that this study area is characterized by
gradients in the sedimentary particulate organic matter (POM) reactivity,
bottom water oxygen concentrations, and benthic respiration rates. Based on
the porewater profiles, we estimated the benthic diffusive fluxes of DON and
DOC in the same area. Our results show that DON fluxed out of the sediments
at significant rates (110 to 430 μmol m−2 d−1). DON
fluxes were positively correlated with sedimentary POM reactivity and varied
inversely with sediment oxygen exposure time (OET), suggesting direct links
between POM quality, aerobic remineralization and the release of DON to the
water column. DON fluxes were on the order of 30 to 64% of the total
benthic inorganic fixed N loss due to denitrification, and often exceeded the
diffusive nitrate fluxes into the sediments. Hence they represented a large
fraction of the total benthic N exchange, a result that is particularly
important in light of the fact that DON fluxes are usually not accounted for
in estuarine and coastal zone nutrient budgets. In contrast to DON, DOC
fluxes out of the sediments did not show any significant spatial variation
along the Laurentian Channel (LC) between the estuary and the gulf
(2100 ± 100 μmol m−2 d−1). The molar C / N
ratio of dissolved organic matter (DOM) in porewater and the overlying bottom
water varied significantly along the transect, with lowest C / N in the
lower estuary (5–6) and highest C / N (> 10) in the gulf.
Large differences between the C / N ratios of porewater DOM and POM are
mainly attributed to a combination of selective POM hydrolysis and elemental
fractionation during subsequent DOM mineralization, but selective adsorption
of DOM to mineral phases could not be excluded as a potential C / N
fractionating process. The extent of this C- versus N- element partitioning
seems to be linked to POM reactivity and redox conditions in the sediment
porewaters. Our results thus highlight the variable effects selective organic
matter (OM) preservation can have on bulk sedimentary C / N ratios,
decoupling the primary source C / N signatures from those in sedimentary
paleoenvironmental archives. Our study further underscores that the role of
estuarine sediments as efficient sinks of bioavailable nitrogen is strongly
influenced by the release of DON during early diagenetic reactions, and that
DON fluxes from continental margin sediments represent an important internal
source of N to the ocean. |
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