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| Titel |
Surface water methane super-saturation and emission in Lake Lugano, southern Switzerland |
| VerfasserIn |
Jan Blees, Helge Niemann, Markus Erne, Marco Simona, Moritz F. Lehmann |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098806
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| Publikation (Nr.) |
 EGU/EGU2014-14517.pdf |
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| Zusammenfassung |
| Large amounts of greenhouse gases are produced in anoxic aquatic environments. Emission
of these greenhouse gases to the atmosphere depends on their turbulent diffusion across the
water – air boundary, which in turn depends on the concentrations within the boundary layers.
We measured methane concentrations in the surface water of the northern basin of Lake
Lugano in spring and autumn, and calculated diffusive fluxes to the atmosphere, using three
different relationships for the parameterisation of the transfer velocity, taking into account
temperature and wind effects. Surface water concentrations always exceeded atmospheric
equilibrium concentration, and increased from 16 nmol L-1 in May to 45 nmol L-1 in
October, indicating CH4 accumulation in the surface mixed layer during summer. Calculated
CH4 fluxes were highly variable in space and time. As a result of the higher surface
water CH4 concentration and cooling of the surface boundary layer, resulting in
increased buoyancy turbulence, the diffusive flux was highest in October (97 μmol m-2
d-1, compared to 7 μmol m-2 d-1 in May). The observed concentration profiles
indicate that mixed layer CH4 accumulation derives from a near-surface source,
and cannot be explained by the diffusive supply of CH4 from the large deep-water
CH4 pool in the anoxic hypolimnion, where CH4 oxidation at the redox transition
zone consumes CH4 effectively. Our study confirms that lake environments can
act as a significant terrestrial source for atmospheric methane, despite efficient
microbial CH4 oxidation in the hypolimnion. The fact that the magnitude of the
observed total CH4 flux increases during the productive period between spring
(409 mol d-1) and autumn (968 mol d-1) suggests links between methane evasion
and the annual biological cycle, yet wind and temperature forcing of the surface
mixed layer must play an equally important role for lacustrine methane emission. |
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