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Diurnal cycle of lake methane flux
Author(s) -
Podgrajsek E.,
Sahlée E.,
Rutgersson A.
Publication year - 2014
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/2013jg002327
Subject(s) - diurnal cycle , flux (metallurgy) , atmospheric sciences , eddy covariance , diurnal temperature variation , convection , methane , buoyancy , convective mixing , convective boundary layer , environmental science , planetary boundary layer , boundary layer , climatology , chemistry , geology , meteorology , thermodynamics , physics , ecology , organic chemistry , ecosystem , biology
Air‐lake methane flux ( FCH 4 ) and partial pressure of methane in the atmosphere ( pCH 4a ) were measured using the eddy covariance method over a Swedish lake for an extended period. The measurements show a diurnal cycle in both FCH 4 and pCH 4a with high values during nighttime ( FCH 4 ≈ 300 nmol m −2 s −1 , pCH 4a ≈ 2.5 µatm) and low values during day ( FCH 4 ≈ 0 nmol m −2 s −1 , pCH 4a ≈ 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 pCH 4a is explained with the variation of FCH 4 and a changing internal boundary layer above the lake. The presence of a diurnal cycle of FCH 4 stresses the importance of making long‐term continuous flux measurements. A lack of FCH 4 measurements during night may significantly bias estimations of total CH 4 emissions from lakes to the atmosphere.