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| Titel |
Isotopic composition of dissolved inorganic nitrogen in high mountain lakes: variation with altitude in the Pyrenees |
| VerfasserIn |
M. Bartrons, L. Camarero, J. Catalán |
| 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 ; 7, no. 5 ; Nr. 7, no. 5 (2010-05-10), S.1469-1479 |
| Datensatznummer |
250004765
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| Publikation (Nr.) |
 copernicus.org/bg-7-1469-2010.pdf |
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| Zusammenfassung |
Nitrogen deposition in remote areas has increased, but the effect on
ecosystems is still poorly understood. For aquatic systems, knowledge of the
main processes driving the observed variation is limited, as is knowledge of
how changes in nitrogen supply affect lake biogeochemical and food web
processes. Differences in dissolved inorganic nitrogen (DIN) between lakes
cannot be understood without considering catchment characteristics. In
mountains, catchment features (e.g., thermal conditions, land cover) vary
considerably with elevation. The isotopic composition of nitrogen (δ15N)
is increasingly used to study aquatic ecosystem dynamics. Here we
explore the variability of δ15N in DIN in high mountain lakes
and show that environmental conditions that change with altitude can affect
the isotopic ratio.
We measured ammonium and nitrate δ15N values in atmospheric
deposition, epilimnetic water, deep chlorophyll maximum water (DCMW) and
sediment pore water (SPW) from eight mountain lakes in the Pyrenees, both
above and below the treeline. Lakes showed relatively uniform δ15N-NH4+
values in SPW (2.2±1.6‰), with no variation
corresponding to catchment or lake characteristics. We suggest that organic
matter diagenesis under similar sediment conditions is responsible for the
low variation between the lakes.
In the water column, the range of δ15N values was larger for
ammonium (−9.4‰ to 7.4‰) than for nitrate (−11.4‰ to −3.4‰), as a result of
higher variation both between and within lakes (epilimnetic vs. DCM water).
For both compounds part of the difference correlated with altitude or
catchment features (e.g., scree proportion). Based on concentration,
chemical and isotopic tendencies, we suggest that patterns arise from the
distinct relative contributions of two types of water flow paths to the
lakes: one from snowpack melting, with little soil interaction; and another
highly influenced by soil conditions. The snow-type flow path contributes
low DIN concentrations depleted in 15N, whereas the soil-type flow path
contributes high nitrate concentrations with higher δ15N. The
proportion of these two types of source correlates with average catchment
features when there is extensive snow cover during spring and early summer
and probably becomes more dependent on local characteristics around the lake
as summer advances. Lake depth and pore water ammonium concentrations, among
other features, introduce secondary variation. In the context of nitrogen
deposition studies, lakes with higher snow-type influence will probably
register changes in N deposition and pollution sources better, whereas lakes
with higher soil-type influence may reflect long-term effects of vegetation
and soil dynamics. |
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