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Diurnal cycle of lake methane flux
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Meteorologi, AWEP)
2014 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 119, no 3, 236-248 p.Article in journal (Refereed) Published
Abstract [en]

Air-lake methane flux (FCH4) and partial pressure of methane in the atmosphere (pCH4a) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH4 and pCH4a with high values during nighttime (FCH4 ≈ 300 nmol m−2 s−1pCH4a ≈ 2.5 µatm) and low values during day (FCH4 ≈ 0 nmol m−2 s−1pCH4a ≈ 2.0 µatm) for a large part of the data set. This diurnal cycle persist in all open water season; however, the magnitude of the diurnal cycle is largest in the spring months. Estimations of buoyancy in the water show that high nighttime fluxes coincide with convective periods. Our interpretation of these results is that the convective mixing enhances the diffusive flux, in analogy to previous studies. We also suggest that the convection may bring methane-rich water from the bottom to the surface and trigger bubble release from the sediment. A diurnal cycle is not observed for all convective occasions, indicating that the presence of convection is not sufficient for enhanced nighttime flux; other factors are also necessary. The observed diurnal cycle of pCH4a is explained with the variation of FCH4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH4 stresses the importance of making long-term continuous flux measurements. A lack of FCH4measurements during night may significantly bias estimations of total CH4 emissions from lakes to the atmosphere.

Place, publisher, year, edition, pages
2014. Vol. 119, no 3, 236-248 p.
Keyword [en]
Methane, lake, eddy correlation, eddy covariance, diurnal cycle
National Category
Meteorology and Atmospheric Sciences Climate Research Geosciences, Multidisciplinary
Research subject
URN: urn:nbn:se:uu:diva-224896DOI: 10.1002/2013JG002327ISI: 000334534500005OAI: oai:DiVA.org:uu-224896DiVA: diva2:719136
Swedish Research Council Formas, 2009-1692
Available from: 2014-05-23 Created: 2014-05-23 Last updated: 2015-03-11
In thesis
1. Lake Fluxes of Methane and Carbon Dioxide
Open this publication in new window or tab >>Lake Fluxes of Methane and Carbon Dioxide
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Methane (CH4) and carbon dioxide (CO2) are two important greenhouse gases. Recent studies have shown that lakes, although they cover a small area of the globe, can be very important natural sources of atmospheric CH4 and CO2. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange.

By using the eddy covariance method for lake flux studies, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes.

Eddy covariance measurements at a Swedish lake revealed a diel cycle in the fluxes of both CH4 and CO2, with higher fluxes during nighttime than daytime. The high nighttime CO2 fluxes could to a large extent be explained with enhanced transfer velocities due to waterside convection. For the diel cycle of CH4 flux it was suggested that waterside convection could enhance the transfer velocity, transport CH4 rich water to the surface, as well as trigger ebullition.

Simultaneous flux measurements of CH4 and CO2 have been presented using both the eddy covariance method and the floating chambers method of which the latter is the traditional measuring method for lake fluxes. For CO2 the two methods agreed well during some periods but differed considerably during others. Disagreement between the methods might be due to horizontal heterogeneity in partial pressure of CO2 in the lake. The methods agreed better for the CH4 flux measurements. However, it is clear that due to the discontinuous nature of the floating chambers, this method will likely miss important high flux events.

The main conclusions of this thesis are:

1) the two gas flux methods are not directly comparable and should be seen as supplementary to each other

2) waterside convection enhances the fluxes of both CH4 and CO2 over the water-air surface. If gas flux measurements are not conducted during nighttime, potential high flux periods might be missed and estimates of the total amount of gas released from lakes to the atmosphere may be biased.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 41 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1221
air-lake gas exchange, carbon dioxide, eddy covariance, floating chambers, methane, waterside convection
National Category
Natural Sciences Meteorology and Atmospheric Sciences
Research subject
urn:nbn:se:uu:diva-241984 (URN)978-91-554-9152-9 (ISBN)
Public defence
2015-03-20, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Available from: 2015-02-26 Created: 2015-01-19 Last updated: 2015-03-18Bibliographically approved

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