Diurnal cycle of lake methane flux

Air-lake methane flux (FCH4) 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 FCH4 and pCH(4a) with high values during nighttime (FCH4 approximate to 300 nmol m(-2) s(-1), pCH(4a) approximate to 2.5 mu atm) and low values during day (FCH4 approximate to 0 nmol m(-2) s(-1), pCH(4a) approximate to 2.0 mu atm) 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 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 FCH4 measurements during night may significantly bias estimations of total CH4 emissions from lakes to the atmosphere.