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| Titel |
Dynamic seasonal nitrogen cycling in response to anthropogenic N loading in a tropical catchment, Athi–Galana–Sabaki River, Kenya |
| VerfasserIn |
T. R. Marwick, F. Tamooh, B. Ogwoka, C. Teodoru, A. V. Borges, F. Darchambeau, S. Bouillon |
| 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 ; 11, no. 2 ; Nr. 11, no. 2 (2014-01-29), S.443-460 |
| Datensatznummer |
250117152
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| Publikation (Nr.) |
 copernicus.org/bg-11-443-2014.pdf |
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| Zusammenfassung |
| As part of a broader study on the riverine biogeochemistry in the
Athi–Galana–Sabaki (A-G-S) River catchment (Kenya), we present data
constraining the sources, transit and transformation of multiple nitrogen
(N) species as they flow through the A-G-S catchment (~47 000 km2).
The data set was obtained in August–September 2011, November 2011, and
April–May 2012, covering the dry season, short rain season and long rain
season respectively. Release of (largely untreated) wastewater from the city
of Nairobi had a profound impact on the biogeochemistry of the upper Athi
River, leading to low dissolved oxygen (DO) saturation levels (36–67%),
high ammonium (NH4+) concentrations (123–1193 μmol L−1),
and high dissolved methane (CH4) concentrations (3765–6729 nmol L−1). Riverine dissolved inorganic nitrogen (DIN; sum of
NH4+ and nitrate (NO3−); nitrite was not measured)
concentration at the most upstream site on the Athi River was highest during
the dry season (1195 μmol L−1), while DIN concentration was an
order of magnitude lower during the short and long rain seasons (212 and 193 μmol L−1,
respectively). During the rain seasons, low water
residence time led to relatively minimal in-stream N cycling prior to
discharge to the ocean, whereas during the dry season we speculate that
prolonged residence time creates two differences comparative to wet season,
where (1) intense N cycling and removal of DIN is possible in the upper to
mid-catchment and leads to significantly lower concentrations at the outlet
during the dry season, and (2) as a result this leads to the progressive
enrichment of 15N in the particulate N (PN) pool, highlighting the
dominance of untreated wastewater as the prevailing source of riverine DIN.
The rapid removal of NH4+ in the upper reaches during the dry
season was accompanied by a quantitatively similar production of
NO3− and nitrous oxide (N2O) downstream, pointing towards
strong nitrification over this reach during the dry season. Nitrous oxide
produced was rapidly degassed downstream, while the elevated NO3−
concentrations steadily decreased to levels observed elsewhere in more
pristine African river networks. Low pelagic primary production rates over
the same reach suggest that benthic denitrification was the dominant process
controlling the removal of NO3−, although large cyanobacterial
blooms further downstream highlight the significant role of DIN assimilation
by primary producers also. Consequently, the intense nitrification and
uptake of N by algae leads to significant enrichment of 15N in the PN
pool during the dry season (mean: +16.5 ± 8.2‰ but reaching as high as +31.5‰) compared to the short
(+7.3 ± 2.6‰) and long (+7.6 ± 5.9‰) rain seasons. A strong correlation between the
seasonal N stable isotope ratios of PN (δ15NPN) and oxygen
stable isotope ratios of river water (δ18OH2O; as a proxy
of freshwater discharge) presents the possibility of employing a combination
of proxies – such as δ15NPN of sediments, bivalves and
near-shore corals – to reconstruct how historical land use changes have
influenced nitrogen cycling within the catchment, whilst potentially
providing foresight on the impacts of future land management decisions. |
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