Diurnal cycle of lake methane flux. Issue 3 (13th March 2014)

• Record Type: Journal Article
• Authors: Podgrajsek, E.; Sahlée, E.; Rutgersson, A.
• Is Part Of: Journal of geophysical research. Volume 119:Issue 3(2014:Sep.)
• Page Start: 236
• Page End: 248
• Publication Date: 2014-03-13
• Subjects: Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1002/2013JG002327
• Languages: English
• ISSNs: 2169-8953

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>Air‐lake methane flux (<italic>FCH<sub>4</sub></italic>) and partial pressure of methane in the atmosphere (<italic>pCH<sub>4a</sub></italic>) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both <italic>FCH<sub>4</sub></italic> and <italic>pCH<sub>4a</sub></italic> with high values during nighttime (<italic>FCH<sub>4</sub></italic> ≈ 300 nmol m<sup>−2</sup> s<sup>−1</sup>, <italic>pCH<sub>4a</sub></italic> ≈ 2.5 µatm) and low values during day (<italic>FCH<sub>4</sub></italic> ≈ 0 nmol m<sup>−2</sup> s<sup>−1</sup>, <italic>pCH<sub>4a</sub></italic> ≈ 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 <italic>pCH<sub>4a</sub></italic> is explained with the variation of <italic>FCH<sub>4</sub></italic> and a changing internal boundary layer above the lake. The presence of a diurnal cycle of <italic>FCH<sub>4</sub></italic> stresses the importance of making long‐term continuous flux measurements. A lack of <italic>FCH<sub>4</sub></italic> measurements during night may significantly bias estimations of total CH<sub>4</sub> emissions from lakes to the atmosphere.</p> </abstract>