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| Titel |
Modeling benthic–pelagic nutrient exchange processes and porewater distributions in a seasonally hypoxic sediment: evidence for massive phosphate release by Beggiatoa? |
| VerfasserIn |
A. W. Dale, V. J. Bertics, T. Treude, S. Sommer, K. Wallmann |
| 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. 2 ; Nr. 10, no. 2 (2013-02-01), S.629-651 |
| Datensatznummer |
250017493
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| Publikation (Nr.) |
 copernicus.org/bg-10-629-2013.pdf |
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| Zusammenfassung |
| This study presents benthic data from 12 samplings from February to December 2010 in a 28 m deep channel in the southwest Baltic Sea. In winter, the
distribution of solutes in the porewater was strongly modulated by
bioirrigation which efficiently flushed the upper 10 cm of sediment, leading
to concentrations which varied little from bottom water values. Solute
pumping by bioirrigation fell sharply in the summer as the bottom waters became
severely hypoxic (< 2 μM O2). At this point the giant
sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an
increase in O2 following mixing of the water column in November,
macrofauna remained absent until the end of the sampling. Contrary to
expectations, metabolites such as dissolved inorganic carbon, ammonium and
hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic
period when bioirrigation was absent, but instead tended toward bottom water
values. This was taken as evidence for episodic bubbling of methane gas out
of the sediment acting as an abiogenic irrigation process.
Porewater–seawater mixing by escaping bubbles provides a pathway for
enhanced nutrient release to the bottom water and may exacerbate the
feedback with hypoxia. Subsurface dissolved phosphate (TPO4) peaks in
excess of 400 μM developed in autumn, resulting in a very large
diffusive TPO4 flux to the water column of 0.7 ± 0.2 mmol m−2 d−1. The model was not able to simulate this TPO4 source
as release of iron-bound P (Fe–P) or organic P. As an alternative
hypothesis, the TPO4 peak was reproduced using new kinetic expressions
that allow Beggiatoa to take up porewater TPO4 and accumulate an intracellular P
pool during periods with oxic bottom waters. TPO4 is then released
during hypoxia, as previous published results with sulfide-oxidizing
bacteria indicate. The TPO4 added to the porewater over the year by
organic P and Fe–P is recycled through Beggiatoa, meaning that no additional source
of TPO4 is needed to explain the TPO4 peak. Further experimental
studies are needed to strengthen this conclusion and rule out Fe–P and
organic P as candidate sources of ephemeral TPO4 release. A measured
C/P ratio of < 20 for the diffusive flux to the water column during
hypoxia directly demonstrates preferential release of P relative to C under
oxygen-deficient bottom waters. This coincides with a strong decrease in
dissolved inorganic N/P ratios in the water column to ~ 1. Our
results suggest that sulfide oxidizing bacteria could act as phosphorus
capacitors in systems with oscillating redox conditions, releasing massive
amounts of TPO4 in a short space of time and dramatically increasing
the internal loading of TPO4 to the overlying water. |
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